@article{peper_role_2023,
	title = {Role of {Coulomb} antiblockade in the photoassociation of long-range {Rydberg} molecules},
	volume = {107},
	url = {https://link.aps.org/doi/10.1103/PhysRevA.107.012812},
	doi = {10.1103/PhysRevA.107.012812},
	abstract = {We present a mechanism contributing to the detection of photoassociated long-range Rydberg molecules via pulsed-field ionization: ionic products, created by the decay of a long-range Rydberg molecule, modify the excitation spectrum of surrounding ground-state atoms and facilitate the excitation of further atoms into Rydberg states by the photoassociation light. Such an ion-mediated excitation mechanism has been previously called a Coulomb antiblockade. Pulsed-field ionization typically does not discriminate between the ionization of a long-range Rydberg molecule and an isolated Rydberg atom, and thus the number of atomic ions detected by this mechanism is not proportional to the number of long-range Rydberg molecules present in the probe volume. By combining high-resolution UV and rf spectroscopy of a dense, ultracold gas of cesium atoms, theoretical modeling of the molecular-level structures of long-range Rydberg molecules bound below n2P3/2 Rydberg states of cesium, and a rate model of the photoassociation and decay processes, we unambiguously identify the signatures of this detection mechanism in the photoassociation of long-range Rydberg molecules bound below atomic asymptotes with negative Stark shifts.},
	number = {1},
	urldate = {2023-01-20},
	journal = {Physical Review A},
	author = {Peper, Michael and Trautmann, Martin and Deiglmayr, Johannes},
	month = jan,
	year = {2023},
	note = {Publisher: American Physical Society},
	keywords = {Ultracold Rydberg gases},
	pages = {012812},
}

We present a mechanism contributing to the detection of photoassociated long-range Rydberg molecules via pulsed-field ionization: ionic products, created by the decay of a long-range Rydberg molecule, modify the excitation spectrum of surrounding ground-state atoms and facilitate the excitation of further atoms into Rydberg states by the photoassociation light. Such an ion-mediated excitation mechanism has been previously called a Coulomb antiblockade. Pulsed-field ionization typically does not discriminate between the ionization of a long-range Rydberg molecule and an isolated Rydberg atom, and thus the number of atomic ions detected by this mechanism is not proportional to the number of long-range Rydberg molecules present in the probe volume. By combining high-resolution UV and rf spectroscopy of a dense, ultracold gas of cesium atoms, theoretical modeling of the molecular-level structures of long-range Rydberg molecules bound below n2P3/2 Rydberg states of cesium, and a rate model of the photoassociation and decay processes, we unambiguously identify the signatures of this detection mechanism in the photoassociation of long-range Rydberg molecules bound below atomic asymptotes with negative Stark shifts.

@article{hoveler_observation_2022,
	title = {Observation of quantum capture in an ion-molecule reaction},
	volume = {106},
	url = {https://link.aps.org/doi/10.1103/PhysRevA.106.052806},
	doi = {10.1103/PhysRevA.106.052806},
	abstract = {Vogt and Wannier [Phys. Rev. 95, 1190 (1954)] predicted that the capture rate of a polarizable neutral atom or molecule by an ion should increase by a factor of 2 compared to the classical Langevin rate as the collision energy approaches zero. This prediction has not been verified experimentally. The H+2 + H2 reaction is ideally suited to observe this effect because the small reduced mass makes quantum effects related to s-wave scattering observable at higher collision energies than in other systems. Moreover, the reaction rate for this barrierless, strongly exothermic reaction follows the classical Langevin capture model down to cold-collision conditions (about kB×1 K) and is not affected by short-range interactions. Below this temperature, a strong enhancement of the reaction rate resulting from charge–quadrupole interaction between H+2 and ground-state ortho H2 (J=1) was observed. Here we present an experimental study of the reaction of H+2 and para H2 (J=0), which has no dipole and no quadrupole moments, at collision energies below kB×1 K. We observe an enhancement at the lowest collision energies which is attributed to the quantum enhancement predicted by Vogt and Wannier. Measurements of the reaction of HD+ with HD support this conclusion.},
	number = {5},
	urldate = {2022-11-15},
	journal = {Physical Review A},
	author = {Höveler, Katharina and Deiglmayr, Johannes and Agner, Josef A. and Hahn, Raphaël and Zhelyazkova, Valentina and Merkt, Frédéric},
	month = nov,
	year = {2022},
	note = {Publisher: American Physical Society},
	keywords = {Cold chemistry, Cold molecules},
	pages = {052806},
}

Vogt and Wannier [Phys. Rev. 95, 1190 (1954)] predicted that the capture rate of a polarizable neutral atom or molecule by an ion should increase by a factor of 2 compared to the classical Langevin rate as the collision energy approaches zero. This prediction has not been verified experimentally. The H+2 + H2 reaction is ideally suited to observe this effect because the small reduced mass makes quantum effects related to s-wave scattering observable at higher collision energies than in other systems. Moreover, the reaction rate for this barrierless, strongly exothermic reaction follows the classical Langevin capture model down to cold-collision conditions (about kB×1 K) and is not affected by short-range interactions. Below this temperature, a strong enhancement of the reaction rate resulting from charge–quadrupole interaction between H+2 and ground-state ortho H2 (J=1) was observed. Here we present an experimental study of the reaction of H+2 and para H2 (J=0), which has no dipole and no quadrupole moments, at collision energies below kB×1 K. We observe an enhancement at the lowest collision energies which is attributed to the quantum enhancement predicted by Vogt and Wannier. Measurements of the reaction of HD+ with HD support this conclusion.

@article{merkt_reactions_2022,
	title = {Reactions of {H2}, {HD}, and {D2} with {H2}+, {HD}+, and {D2}+: {Product}-{Channel} {Branching} {Ratios} and {Simple} {Models}},
	shorttitle = {Reactions of {H2}, {HD}, and {D2} with {H2}+, {HD}+, and {D2}+},
	url = {https://doi.org/10.1021/acs.jpclett.1c03374},
	doi = {10.1021/acs.jpclett.1c03374},
	abstract = {We present measurements of the product-channel branching ratios of the reactions (i) HD+ + HD forming H2D+ + D (38.1(30)\%) and HD2+ + H (61.9(30)\%), (ii) HD+ + D2 forming HD2+ + D (61.4(35)\%) and D3+ + H (38.6(35)\%), and (iii) D2+ + HD forming HD2++ D (60.5(20)\%) and D3+ + H (39.5(20)\%) at collision energies Ecoll near zero, i.e., below kB × 1 K. These branching ratios are compared with branching ratios predicted using three simple models: a combinatorial model (M1), a model (M2) describing the reactions as H-, H+-, D-, and D+-transfer processes, and a statistical model (M3) that relates the reaction rate coefficients to the translational and rovibrational state densities of the HnD3–n+ + H/D (n = 0, 1, 2 or 3) product channels. The experimental data are incompatible with the predictions of models M1 and M2 and reveal that the branching ratios exhibit clear correlations with the product state densities.},
	urldate = {2022-01-21},
	journal = {The Journal of Physical Chemistry Letters},
	author = {Merkt, Frédéric and Höveler, Katharina and Deiglmayr, Johannes},
	month = jan,
	year = {2022},
	note = {Publisher: American Chemical Society},
	keywords = {Cold chemistry, Cold molecules},
	pages = {864--871},
}

We present measurements of the product-channel branching ratios of the reactions (i) HD+ + HD forming H2D+ + D (38.1(30)%) and HD2+ + H (61.9(30)%), (ii) HD+ + D2 forming HD2+ + D (61.4(35)%) and D3+ + H (38.6(35)%), and (iii) D2+ + HD forming HD2++ D (60.5(20)%) and D3+ + H (39.5(20)%) at collision energies Ecoll near zero, i.e., below kB × 1 K. These branching ratios are compared with branching ratios predicted using three simple models: a combinatorial model (M1), a model (M2) describing the reactions as H-, H+-, D-, and D+-transfer processes, and a statistical model (M3) that relates the reaction rate coefficients to the translational and rovibrational state densities of the HnD3–n+ + H/D (n = 0, 1, 2 or 3) product channels. The experimental data are incompatible with the predictions of models M1 and M2 and reveal that the branching ratios exhibit clear correlations with the product state densities.

@article{hoveler_deviation_2021,
	title = {Deviation of the rate of the reaction from {Langevin} behaviour below 1 {K}, branching ratios for the and product channels, and product-kinetic-energy distributions},
	volume = {0},
	issn = {0026-8976},
	url = {https://www.tandfonline.com/doi/abs/10.1080/00268976.2021.1954708},
	doi = {10.1080/00268976.2021.1954708},
	abstract = {The reactions between ground-state H2+ (X 2Σg+(v=0,J=0)) and D2 forming HD2++H and H2D++D were investigated in the range of collision energies Ecoll between Ecoll/kB=0 and 10 K using a merged-beam approach. The reaction rates measured experimentally are compared to those obtained for the reaction between H2+ and H2 forming H3++H under similar experimental conditions. Below 1 K, a clear enhancement of the reaction rate coefficient compared to the Langevin rate measured at higher collision energies was observed in both reaction systems. This enhancement is interpreted as originating from the interaction between the charge of H2+ and the quadrupole of para D2 and ortho H2 molecules in the J = 1 rotational level. The enhancement of the reaction with D2 was found to be significantly less than that of the reaction with H2, reflecting the relative population of the J = 1 rotational level of H2 (75\%) and D2 (33\%) in natural samples at low temperatures. Simulations of the experimental results based on the theoretical predictions of the reaction cross sections by Dashevskaya et al. [J. Chem. Phys. 145, 244315 (2016)] reveal agreement within the experimental uncertainties. The branching ratio η of the reaction involving H2+ and D2 and forming H2D+ and D (η=[H2D+][H2D+]+[HD2+]) near Ecoll=0 was determined to be 0.341(15). Time-of-flight measurements of the velocity distributions of the reaction products are compatible with an isotropic product emission with an average total kinetic energy of 0.45(5) eV for both channels, representing about 30\% of the total energy released by the reaction.},
	number = {0},
	urldate = {2021-07-21},
	journal = {Molecular Physics},
	author = {Höveler, Katharina and Deiglmayr, Johannes and Merkt, Frédéric},
	month = jul,
	year = {2021},
	note = {Publisher: Taylor \& Francis
\_eprint: https://www.tandfonline.com/doi/pdf/10.1080/00268976.2021.1954708},
	keywords = {Cold chemistry, Cold molecules},
	pages = {e1954708},
}

The reactions between ground-state H2+ (X 2Σg+(v=0,J=0)) and D2 forming HD2++H and H2D++D were investigated in the range of collision energies Ecoll between Ecoll/kB=0 and 10 K using a merged-beam approach. The reaction rates measured experimentally are compared to those obtained for the reaction between H2+ and H2 forming H3++H under similar experimental conditions. Below 1 K, a clear enhancement of the reaction rate coefficient compared to the Langevin rate measured at higher collision energies was observed in both reaction systems. This enhancement is interpreted as originating from the interaction between the charge of H2+ and the quadrupole of para D2 and ortho H2 molecules in the J = 1 rotational level. The enhancement of the reaction with D2 was found to be significantly less than that of the reaction with H2, reflecting the relative population of the J = 1 rotational level of H2 (75%) and D2 (33%) in natural samples at low temperatures. Simulations of the experimental results based on the theoretical predictions of the reaction cross sections by Dashevskaya et al. [J. Chem. Phys. 145, 244315 (2016)] reveal agreement within the experimental uncertainties. The branching ratio η of the reaction involving H2+ and D2 and forming H2D+ and D (η=[H2D+][H2D+]+[HD2+]) near Ecoll=0 was determined to be 0.341(15). Time-of-flight measurements of the velocity distributions of the reaction products are compatible with an isotropic product emission with an average total kinetic energy of 0.45(5) eV for both channels, representing about 30% of the total energy released by the reaction.

@article{hoveler_h2_2021,
	title = {The {H2}+ + {HD} reaction at low collision energies: {H3}+/{H2D}+ branching ratio and product-kinetic-energy distributions},
	issn = {1463-9084},
	shorttitle = {The {H2}+ + {HD} reaction at low collision energies},
	url = {https://pubs.rsc.org/en/content/articlelanding/2021/cp/d0cp06107g},
	doi = {10.1039/D0CP06107G},
	abstract = {The fully state-selected reactions between H2+ molecules in the X+ 2Σg+(v+ = 0, N+ = 0) state and HD molecules in the X 1Σg+(v = 0, J = 0) state forming H3+ + D and H2D+ + H have been studied at collision energies Ecoll between 0 and kB·30 K with a resolution of about 75 mK at the lowest energies. H2 molecules in a supersonic beam were prepared in Rydberg-Stark states with principal quantum number n = 27 and merged with a supersonic beam of ground-state HD molecules using a curved surface-electrode Rydberg-Stark decelerator and deflector. The reaction between H2+ and HD was studied within the orbit of the Rydberg electron to avoid heating of the ions by stray electric fields. The reaction was observed for well-defined and adjustable time intervals, called reaction-observation windows, between two electric-field pulses. The first pulse swept all ions away from the reaction volume and its falling edge defined the beginning of the reaction-observation window. The second pulse extracted the product ions toward a charged-particle detector located at the end of a time-of-flight tube and its rising edge defined the end of the reaction-observation window. Monitoring and analysing the time-of-flight distributions of the H3+ and H2D+ products in dependence of the duration of the reaction-observation window enabled us to obtain information on the kinetic-energy distribution of the product ions and determine branching ratios of the H3+ + D and H2D+ + H reaction channels. The mean product-kinetic-energy release is 0.46(5) eV, representing 27(3)\% of the available energy, and the H3+ + D product branching ratio is 0.225(20). The relative reaction rates correspond closely to Langevin capture rates down to the lowest energies probed experimentally (≈kB·50 mK).},
	language = {en},
	urldate = {2021-01-22},
	journal = {Physical Chemistry Chemical Physics},
	author = {Höveler, Katharina and Deiglmayr, Johannes and Agner, Josef A. and Schmutz, Hansjürg and Merkt, Frédéric},
	month = jan,
	year = {2021},
	note = {Publisher: The Royal Society of Chemistry},
	keywords = {Cold chemistry, Cold molecules},
}

The fully state-selected reactions between H2+ molecules in the X+ 2Σg+(v+ = 0, N+ = 0) state and HD molecules in the X 1Σg+(v = 0, J = 0) state forming H3+ + D and H2D+ + H have been studied at collision energies Ecoll between 0 and kB·30 K with a resolution of about 75 mK at the lowest energies. H2 molecules in a supersonic beam were prepared in Rydberg-Stark states with principal quantum number n = 27 and merged with a supersonic beam of ground-state HD molecules using a curved surface-electrode Rydberg-Stark decelerator and deflector. The reaction between H2+ and HD was studied within the orbit of the Rydberg electron to avoid heating of the ions by stray electric fields. The reaction was observed for well-defined and adjustable time intervals, called reaction-observation windows, between two electric-field pulses. The first pulse swept all ions away from the reaction volume and its falling edge defined the beginning of the reaction-observation window. The second pulse extracted the product ions toward a charged-particle detector located at the end of a time-of-flight tube and its rising edge defined the end of the reaction-observation window. Monitoring and analysing the time-of-flight distributions of the H3+ and H2D+ products in dependence of the duration of the reaction-observation window enabled us to obtain information on the kinetic-energy distribution of the product ions and determine branching ratios of the H3+ + D and H2D+ + H reaction channels. The mean product-kinetic-energy release is 0.46(5) eV, representing 27(3)% of the available energy, and the H3+ + D product branching ratio is 0.225(20). The relative reaction rates correspond closely to Langevin capture rates down to the lowest energies probed experimentally (≈kB·50 mK).

@article{peper_heteronuclear_2021,
	title = {Heteronuclear {Long}-{Range} {Rydberg} {Molecules}},
	volume = {126},
	url = {https://link.aps.org/doi/10.1103/PhysRevLett.126.013001},
	doi = {10.1103/PhysRevLett.126.013001},
	abstract = {We present the formation of homonuclear Cs2, K2, and heteronuclear CsK long-range Rydberg molecules in a dual-species magneto-optical trap for 39K and 133Cs by one-photon UV photoassociation. The different ground-state-density dependence of homo- and heteronuclear photoassociation rates and the detection of stable molecular ions resulting from autoionization provide an unambiguous assignment. We perform bound-bound millimeter-wave spectroscopy of long-range Rydberg molecules to access molecular states not accessible by one-photon photoassociation. Calculations based on the most recent theoretical model and atomic parameters do not reproduce the full set of data from homo- and heteronuclear long-range Rydberg molecules consistently. This shows that photoassociation and millimeter-wave spectroscopy of heteronuclear long-range Rydberg molecules provide a benchmark for the development of theoretical models.},
	number = {1},
	urldate = {2021-01-06},
	journal = {Physical Review Letters},
	author = {Peper, Michael and Deiglmayr, Johannes},
	month = jan,
	year = {2021},
	note = {Publisher: American Physical Society},
	keywords = {Ultracold Rydberg gases},
	pages = {013001},
}

We present the formation of homonuclear Cs2, K2, and heteronuclear CsK long-range Rydberg molecules in a dual-species magneto-optical trap for 39K and 133Cs by one-photon UV photoassociation. The different ground-state-density dependence of homo- and heteronuclear photoassociation rates and the detection of stable molecular ions resulting from autoionization provide an unambiguous assignment. We perform bound-bound millimeter-wave spectroscopy of long-range Rydberg molecules to access molecular states not accessible by one-photon photoassociation. Calculations based on the most recent theoretical model and atomic parameters do not reproduce the full set of data from homo- and heteronuclear long-range Rydberg molecules consistently. This shows that photoassociation and millimeter-wave spectroscopy of heteronuclear long-range Rydberg molecules provide a benchmark for the development of theoretical models.

@article{peper_photodissociation_2020,
	title = {Photodissociation of long-range {Rydberg} molecules},
	volume = {102},
	url = {https://link.aps.org/doi/10.1103/PhysRevA.102.062819},
	doi = {10.1103/PhysRevA.102.062819},
	abstract = {We present photodissociation of ultracold long-range Rydberg molecules as a tool to characterize their electronic properties. We photoassociate 39K2 372P molecules with highly entangled electronic and nuclear spins of the two bound atoms and quantify the entanglement by projection of the molecular state onto noninteracting atoms using radio-frequency photodissociation. By comparison of experimental photodissociation rates with theoretical predictions, we further characterize the electronic and nuclear wave function of the photoassociated molecules. Based on the complete characterization of the formed long-range Rydberg molecules, we demonstrate a full hyperfine-spin flip of a free ground-state atom through the interaction with a Rydberg atom.},
	number = {6},
	urldate = {2020-12-22},
	journal = {Physical Review A},
	author = {Peper, Michael and Deiglmayr, Johannes},
	month = dec,
	year = {2020},
	note = {Publisher: American Physical Society},
	keywords = {Cold chemistry, Ultracold Rydberg gases},
	pages = {062819},
}

We present photodissociation of ultracold long-range Rydberg molecules as a tool to characterize their electronic properties. We photoassociate 39K2 372P molecules with highly entangled electronic and nuclear spins of the two bound atoms and quantify the entanglement by projection of the molecular state onto noninteracting atoms using radio-frequency photodissociation. By comparison of experimental photodissociation rates with theoretical predictions, we further characterize the electronic and nuclear wave function of the photoassociated molecules. Based on the complete characterization of the formed long-range Rydberg molecules, we demonstrate a full hyperfine-spin flip of a free ground-state atom through the interaction with a Rydberg atom.

@article{peper_formation_2020,
	title = {Formation of ultracold ion pairs through long-range {Rydberg} molecules},
	volume = {53},
	issn = {0953-4075},
	url = {https://doi.org/10.1088%2F1361-6455%2Fab63ac},
	doi = {10.1088/1361-6455/ab63ac},
	abstract = {We propose a new approach to excite ion-pair states of ultracold dimers. The central idea is a two-step process where first long-range Rydberg molecules are formed by photoassociation, which are then driven by stimulated emission towards the ion-pair state, a process bearing features of a photo-induced harpooning reaction. We assess the feasibility of this approach through a detailed experimental and theoretical study on a specific system, p-wave-scattering dominated long-range Rydberg molecules in caesium, and discuss potential applications for the study of strongly correlated plasmas consisting of oppositely charged particles of equal or similar mass.},
	language = {en},
	number = {6},
	urldate = {2020-02-24},
	journal = {Journal of Physics B: Atomic, Molecular and Optical Physics},
	author = {Peper, Michael and Deiglmayr, Johannes},
	month = feb,
	year = {2020},
	keywords = {Cold molecules, Quantum chemistry, Ultracold Rydberg gases},
	pages = {064001},
}

We propose a new approach to excite ion-pair states of ultracold dimers. The central idea is a two-step process where first long-range Rydberg molecules are formed by photoassociation, which are then driven by stimulated emission towards the ion-pair state, a process bearing features of a photo-induced harpooning reaction. We assess the feasibility of this approach through a detailed experimental and theoretical study on a specific system, p-wave-scattering dominated long-range Rydberg molecules in caesium, and discuss potential applications for the study of strongly correlated plasmas consisting of oppositely charged particles of equal or similar mass.

@article{garcia_single-shot_2019,
	title = {Single-{Shot} {Nondestructive} {Detection} of {Rydberg}-{Atom} {Ensembles} by {Transmission} {Measurement} of a {Microwave} {Cavity}},
	volume = {123},
	url = {https://link.aps.org/doi/10.1103/PhysRevLett.123.193201},
	doi = {10.1103/PhysRevLett.123.193201},
	abstract = {We present an experimental realization of single-shot nondestructive detection of ensembles of helium Rydberg atoms. We use the dispersive frequency shift of a superconducting microwave cavity interacting with the ensemble. By probing the transmission of the cavity, we determine the number of Rydberg atoms or the populations of Rydberg quantum states when the ensemble is prepared in a superposition. At the optimal microwave probe power, determined by the critical photon number, we reach single-shot detection of the atom number with 13\% relative precision for ensembles of about 500 Rydberg atoms with a measurement backaction characterized by approximately 2\% population transfer.},
	number = {19},
	urldate = {2019-11-12},
	journal = {Physical Review Letters},
	author = {Garcia, S. and Stammeier, M. and Deiglmayr, J. and Merkt, F. and Wallraff, A.},
	month = nov,
	year = {2019},
	keywords = {Cavity QED},
	pages = {193201},
}

We present an experimental realization of single-shot nondestructive detection of ensembles of helium Rydberg atoms. We use the dispersive frequency shift of a superconducting microwave cavity interacting with the ensemble. By probing the transmission of the cavity, we determine the number of Rydberg atoms or the populations of Rydberg quantum states when the ensemble is prepared in a superposition. At the optimal microwave probe power, determined by the critical photon number, we reach single-shot detection of the atom number with 13% relative precision for ensembles of about 500 Rydberg atoms with a measurement backaction characterized by approximately 2% population transfer.

@article{peper_precision_2019,
	title = {Precision measurement of the ionization energy and quantum defects of {39K}},
	volume = {100},
	url = {https://link.aps.org/doi/10.1103/PhysRevA.100.012501},
	doi = {10.1103/PhysRevA.100.012501},
	abstract = {We present absolute-frequency measurements in ultracold 39K samples of the transitions from the 4s1/2 ground state to np1/2 and np3/2 Rydberg states. A global nonlinear regression of the np1/2 and np3/2 term values yields an improved wave number of 35009.8139710(22)sys(3)statcm−1 for the first ionization threshold of 39K and the quantum defects of the np1/2 and np3/2 series. In addition, we report the frequencies of selected one-photon transitions n′s1/2←np3/2, n′dj←np3/2, n′fj′←ndj, and n′gj′←nfj and two-photon transitions nfj′←npj determined by millimeter-wave spectroscopy, where j is the total angular-momentum quantum number. By combining the results from the laser and millimeter-wave spectroscopic experiments, we obtain improved values for the quantum defects of the s1/2, d3/2, d5/2, fj, and gj states. For the dj series, the inverted fine structure was confirmed for n≥32. The fine-structure splitting of the f series is less than 100 kHz at n=31, significantly smaller than the hydrogenic splitting, and the fine structure of the g series is regular for n≥30, with a fine-structure splitting compatible with the hydrogenic prediction. From the measured quantum defects of the f and g series we derive an estimate for the static dipole αd and quadrupole αq polarizabilities of the K+ ion core. Additionally, the hyperfine splitting of the 4s1/2 ground state of 39K was determined to be 461.719700(5) MHz using radio-frequency spectroscopy and Ramsey-type interferometry.},
	number = {1},
	urldate = {2019-07-05},
	journal = {Physical Review A},
	author = {Peper, Michael and Helmrich, Felix and Butscher, Jonas and Agner, Josef Anton and Schmutz, Hansjürg and Merkt, Frédéric and Deiglmayr, Johannes},
	month = jul,
	year = {2019},
	keywords = {Precision Measurements, Ultracold Rydberg gases},
	pages = {012501},
}

We present absolute-frequency measurements in ultracold 39K samples of the transitions from the 4s1/2 ground state to np1/2 and np3/2 Rydberg states. A global nonlinear regression of the np1/2 and np3/2 term values yields an improved wave number of 35009.8139710(22)sys(3)statcm−1 for the first ionization threshold of 39K and the quantum defects of the np1/2 and np3/2 series. In addition, we report the frequencies of selected one-photon transitions n′s1/2←np3/2, n′dj←np3/2, n′fj′←ndj, and n′gj′←nfj and two-photon transitions nfj′←npj determined by millimeter-wave spectroscopy, where j is the total angular-momentum quantum number. By combining the results from the laser and millimeter-wave spectroscopic experiments, we obtain improved values for the quantum defects of the s1/2, d3/2, d5/2, fj, and gj states. For the dj series, the inverted fine structure was confirmed for n≥32. The fine-structure splitting of the f series is less than 100 kHz at n=31, significantly smaller than the hydrogenic splitting, and the fine structure of the g series is regular for n≥30, with a fine-structure splitting compatible with the hydrogenic prediction. From the measured quantum defects of the f and g series we derive an estimate for the static dipole αd and quadrupole αq polarizabilities of the K+ ion core. Additionally, the hyperfine splitting of the 4s1/2 ground state of 39K was determined to be 461.719700(5) MHz using radio-frequency spectroscopy and Ramsey-type interferometry.

@article{peper_magic_2019,
	title = {Magic {Rydberg}-{Rydberg} transitions in electric fields},
	volume = {100},
	url = {https://link.aps.org/doi/10.1103/PhysRevA.100.032512},
	doi = {10.1103/PhysRevA.100.032512},
	abstract = {Rydberg states of atoms and molecules are very sensitive to electric fields. This property makes them ideal electric-field sensors but is detrimental to applications of Rydberg states in quantum optics, quantum-information processing, and quantum simulation because of inhomogeneous Stark broadening and the resulting loss of quantum coherence. We demonstrate, with the example of Rydberg states of 39K, the existence of Rydberg-Rydberg transitions with extremely small differential dc Stark shifts, which we call dc-field-magic Rydberg-Rydberg transitions. These transitions hardly exhibit any Stark broadening, even when the electric-field strength varies by as much as 10Vcm−1 over the experimental volume. We present a systematic study of dc-field-magic Rydberg-Rydberg transitions combining experiment and calculations and classify them in three main types, which should also be encountered in the other alkali-metal atoms, in alkaline-earth-metal atoms, and even in molecules. The observed insensitivity to dc electric fields does not reduce the interactions between Rydberg atoms, even if they are dominantly electric dipole-dipole in nature. Rydberg states coupled by dc-field-magic Rydberg-Rydberg transitions, therefore, have great potential as qubits.},
	number = {3},
	urldate = {2019-09-17},
	journal = {Physical Review A},
	author = {Peper, Michael and Deiglmayr, Johannes and Merkt, Frédéric and Sanna, Carla and van den Heuvell, H. B. van Linden},
	month = sep,
	year = {2019},
	keywords = {Ultracold Rydberg gases},
	pages = {032512},
}

Rydberg states of atoms and molecules are very sensitive to electric fields. This property makes them ideal electric-field sensors but is detrimental to applications of Rydberg states in quantum optics, quantum-information processing, and quantum simulation because of inhomogeneous Stark broadening and the resulting loss of quantum coherence. We demonstrate, with the example of Rydberg states of 39K, the existence of Rydberg-Rydberg transitions with extremely small differential dc Stark shifts, which we call dc-field-magic Rydberg-Rydberg transitions. These transitions hardly exhibit any Stark broadening, even when the electric-field strength varies by as much as 10Vcm−1 over the experimental volume. We present a systematic study of dc-field-magic Rydberg-Rydberg transitions combining experiment and calculations and classify them in three main types, which should also be encountered in the other alkali-metal atoms, in alkaline-earth-metal atoms, and even in molecules. The observed insensitivity to dc electric fields does not reduce the interactions between Rydberg atoms, even if they are dominantly electric dipole-dipole in nature. Rydberg states coupled by dc-field-magic Rydberg-Rydberg transitions, therefore, have great potential as qubits.

@article{beyer_metrology_2018,
	title = {Metrology of high-n {Rydberg} states of molecular hydrogen with 2·10⁻¹⁰ accuracy},
	volume = {97},
	url = {https://link.aps.org/doi/10.1103/PhysRevA.97.012501},
	doi = {10.1103/PhysRevA.97.012501},
	abstract = {Precision spectroscopy of molecular Rydberg states of high principal quantum number (n≥50) followed by Rydberg series extrapolation currently represents the most accurate method of determining the ionization energies of neutral molecules. Until recently, such determinations relied on the use of pulsed laser radiation and supersonic molecular beams, which limited the spectral linewidths and induced ac Stark shifts, thus causing undesirable statistical and systematic uncertainties. We report on the combination of single-mode continuous-wave laser radiation, calibrated with a frequency comb, with slow supersonic beams expanding from low-temperature reservoirs, to record transitions to high-n molecular Rydberg states with unprecedented accuracy. As illustration, we present the result of a measurement, with a relative frequency accuracy of Δν/ν=2×10−10 and an absolute accuracy of 64 kHz, of the transition from the GK 1Σ+g(v=0,N=2) state of H2 to the n=50 f Rydberg state belonging to a series converging on the X+2Σ+g(v+=0,N+=0) ground state of H2+. The accuracy of the result [νNIR=380132832.236(0.035)stat(0.054)sysMHz] approaches the accuracy that can be achieved in similar measurements on ultracold alkali-metal-atom samples, although the dominant sources of uncertainties are different. The implications of our result for precision measurements of the adiabatic ionization energy of H2 and of the energy-level structure of H2+ are discussed.},
	number = {1},
	urldate = {2018-01-07},
	journal = {Physical Review A},
	author = {Beyer, Maximilian and Hölsch, Nicolas and Agner, Josef A. and Deiglmayr, Johannes and Schmutz, Hansjürg and Merkt, Frédéric},
	month = jan,
	year = {2018},
	keywords = {Cold molecules, Precision Measurements},
	pages = {012501},
}

Precision spectroscopy of molecular Rydberg states of high principal quantum number (n≥50) followed by Rydberg series extrapolation currently represents the most accurate method of determining the ionization energies of neutral molecules. Until recently, such determinations relied on the use of pulsed laser radiation and supersonic molecular beams, which limited the spectral linewidths and induced ac Stark shifts, thus causing undesirable statistical and systematic uncertainties. We report on the combination of single-mode continuous-wave laser radiation, calibrated with a frequency comb, with slow supersonic beams expanding from low-temperature reservoirs, to record transitions to high-n molecular Rydberg states with unprecedented accuracy. As illustration, we present the result of a measurement, with a relative frequency accuracy of Δν/ν=2×10−10 and an absolute accuracy of 64 kHz, of the transition from the GK 1Σ+g(v=0,N=2) state of H2 to the n=50 f Rydberg state belonging to a series converging on the X+2Σ+g(v+=0,N+=0) ground state of H2+. The accuracy of the result [νNIR=380132832.236(0.035)stat(0.054)sysMHz] approaches the accuracy that can be achieved in similar measurements on ultracold alkali-metal-atom samples, although the dominant sources of uncertainties are different. The implications of our result for precision measurements of the adiabatic ionization energy of H2 and of the energy-level structure of H2+ are discussed.

@article{peper_trendbericht_2018,
	title = {Trendbericht {Physikalische} {Chemie} 2017: {Elektronisch} höchst angeregte {Moleküle}},
	volume = {66},
	issn = {1868-0054},
	shorttitle = {Trendbericht {Physikalische} {Chemie} 2017},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/nadc.20184071831},
	doi = {10.1002/nadc.20184071831},
	language = {en},
	number = {3},
	urldate = {2018-06-22},
	journal = {Nachrichten aus der Chemie},
	author = {Peper, Michael and Deiglmayr, Johannes},
	year = {2018},
	keywords = {Ultracold Rydberg gases},
	pages = {303--306},
}

@article{stammeier_measuring_2017,
	title = {Measuring the dispersive frequency shift of a rectangular microwave cavity induced by an ensemble of {Rydberg} atoms},
	volume = {95},
	url = {https://link.aps.org/doi/10.1103/PhysRevA.95.053855},
	doi = {10.1103/PhysRevA.95.053855},
	abstract = {In recent years the interest in studying interactions of Rydberg atoms or ensembles thereof with optical and microwave frequency fields has steadily increased, both in the context of basic research and for potential applications in quantum information processing. We present measurements of the dispersive interaction between an ensemble of helium atoms in the 37s Rydberg state and a single resonator mode by extracting the amplitude and phase change of a weak microwave probe tone transmitted through the cavity. The results are in quantitative agreement with predictions made on the basis of the dispersive Tavis-Cummings Hamiltonian. We study this system with the goal of realizing a hybrid between superconducting circuits and Rydberg atoms. We measure maximal collective coupling strengths of 1 MHz, corresponding to 3×103 Rydberg atoms coupled to the cavity. As expected, the dispersive shift is found to be inversely proportional to the atom-cavity detuning and proportional to the number of Rydberg atoms. This possibility of measuring the number of Rydberg atoms in a nondestructive manner is relevant for quantitatively evaluating scattering cross sections in experiments with Rydberg atoms.},
	number = {5},
	urldate = {2017-06-13},
	journal = {Physical Review A},
	author = {Stammeier, M. and Garcia, S. and Thiele, T. and Deiglmayr, J. and Agner, J. A. and Schmutz, H. and Merkt, F. and Wallraff, A.},
	month = may,
	year = {2017},
	keywords = {Cavity QED},
	pages = {053855},
}

In recent years the interest in studying interactions of Rydberg atoms or ensembles thereof with optical and microwave frequency fields has steadily increased, both in the context of basic research and for potential applications in quantum information processing. We present measurements of the dispersive interaction between an ensemble of helium atoms in the 37s Rydberg state and a single resonator mode by extracting the amplitude and phase change of a weak microwave probe tone transmitted through the cavity. The results are in quantitative agreement with predictions made on the basis of the dispersive Tavis-Cummings Hamiltonian. We study this system with the goal of realizing a hybrid between superconducting circuits and Rydberg atoms. We measure maximal collective coupling strengths of 1 MHz, corresponding to 3×103 Rydberg atoms coupled to the cavity. As expected, the dispersive shift is found to be inversely proportional to the atom-cavity detuning and proportional to the number of Rydberg atoms. This possibility of measuring the number of Rydberg atoms in a nondestructive manner is relevant for quantitatively evaluating scattering cross sections in experiments with Rydberg atoms.

@article{koepsell_measuring_2017,
	title = {Measuring the polarization of electromagnetic fields using {Rabi}-rate measurements with spatial resolution: {Experiment} and theory},
	volume = {95},
	shorttitle = {Measuring the polarization of electromagnetic fields using {Rabi}-rate measurements with spatial resolution},
	url = {https://link.aps.org/doi/10.1103/PhysRevA.95.053860},
	doi = {10.1103/PhysRevA.95.053860},
	abstract = {When internal states of atoms are manipulated using coherent optical or radio-frequency (rf) radiation, it is essential to know the polarization of the radiation with respect to the quantization axis of the atom. We first present a measurement of the two-dimensional spatial distribution of the electric-field amplitude of a linearly polarized pulsed rf electric field at ∼25.6GHz and its angle with respect to a static electric field. The measurements exploit coherent population transfer between the 35s and 35p Rydberg states of helium atoms in a pulsed supersonic beam. Based on this experimental result, we develop a general framework in the form of a set of equations relating the five independent polarization parameters of a coherently oscillating field in a fixed laboratory frame to Rabi rates of transitions between a ground and three excited states of an atom with arbitrary quantization axis. We then explain how these equations can be used to fully characterize the polarization in a minimum of five Rabi-rate measurements by rotation of an external bias field, or, knowing the polarization of the driving field, to determine the orientation of the static field using two measurements. The presented technique is not limited to Rydberg atoms and rf fields but can also be applied to characterize optical fields. The technique has the potential for sensing the spatiotemporal properties of electromagnetic fields, e.g., in metrology devices or in hybrid experiments involving atoms close to surfaces.},
	number = {5},
	urldate = {2017-06-13},
	journal = {Physical Review A},
	author = {Koepsell, J. and Thiele, T. and Deiglmayr, J. and Wallraff, A. and Merkt, F.},
	month = may,
	year = {2017},
	keywords = {Cavity QED},
	pages = {053860},
}

When internal states of atoms are manipulated using coherent optical or radio-frequency (rf) radiation, it is essential to know the polarization of the radiation with respect to the quantization axis of the atom. We first present a measurement of the two-dimensional spatial distribution of the electric-field amplitude of a linearly polarized pulsed rf electric field at ∼25.6GHz and its angle with respect to a static electric field. The measurements exploit coherent population transfer between the 35s and 35p Rydberg states of helium atoms in a pulsed supersonic beam. Based on this experimental result, we develop a general framework in the form of a set of equations relating the five independent polarization parameters of a coherently oscillating field in a fixed laboratory frame to Rabi rates of transitions between a ground and three excited states of an atom with arbitrary quantization axis. We then explain how these equations can be used to fully characterize the polarization in a minimum of five Rabi-rate measurements by rotation of an external bias field, or, knowing the polarization of the driving field, to determine the orientation of the static field using two measurements. The presented technique is not limited to Rydberg atoms and rf fields but can also be applied to characterize optical fields. The technique has the potential for sensing the spatiotemporal properties of electromagnetic fields, e.g., in metrology devices or in hybrid experiments involving atoms close to surfaces.

@article{deiglmayr_precision_2016,
	title = {Precision measurement of the ionization energy of {Cs} {I}},
	volume = {93},
	url = {http://link.aps.org/doi/10.1103/PhysRevA.93.013424},
	doi = {10.1103/PhysRevA.93.013424},
	abstract = {We present absolute-frequency measurements for the transitions from the 6s1/2 ground state of Cs133 to np1/2 and np3/2 Rydberg states. The transition frequencies are determined by one-photon UV spectroscopy in ultracold samples of Cs atoms using a narrow-band laser system referenced to a frequency comb. From a global fit of the ionization energy EI and the quantum defects of the two series we determine an improved value of EI/hc=31406.4677325(14)cm−1 for the ionization energy of Cs with a relative uncertainty of 5×10−11. We also report improved values for the quantum defects of the np1/2, np3/2, ns1/2, and nd5/2 series.},
	number = {1},
	urldate = {2016-01-29},
	journal = {Physical Review A},
	author = {Deiglmayr, Johannes and Herburger, Holger and Saßmannshausen, Heiner and Jansen, Paul and Schmutz, Hansjürg and Merkt, Frédéric},
	month = jan,
	year = {2016},
	keywords = {Precision Measurements, Ultracold Rydberg gases},
	pages = {013424},
}

We present absolute-frequency measurements for the transitions from the 6s1/2 ground state of Cs133 to np1/2 and np3/2 Rydberg states. The transition frequencies are determined by one-photon UV spectroscopy in ultracold samples of Cs atoms using a narrow-band laser system referenced to a frequency comb. From a global fit of the ionization energy EI and the quantum defects of the two series we determine an improved value of EI/hc=31406.4677325(14)cm−1 for the ionization energy of Cs with a relative uncertainty of 5×10−11. We also report improved values for the quantum defects of the np1/2, np3/2, ns1/2, and nd5/2 series.

@article{allmendinger_new_2016,
	title = {New {Method} to {Study} {Ion}–{Molecule} {Reactions} at {Low} {Temperatures} and {Application} to the {H}₂⁺+{H}₂→{H}₃⁺+{H} {Reaction}},
	volume = {17},
	issn = {1439-7641},
	url = {http://onlinelibrary.wiley.com/doi/10.1002/cphc.201600828/abstract},
	doi = {10.1002/cphc.201600828},
	abstract = {Studies of ion–molecule reactions at low temperatures are difficult because stray electric fields in the reaction volume affect the kinetic energy of charged reaction partners. We describe a new experimental approach to study ion–molecule reactions at low temperatures and present, as example, a measurement of the H2++H2→H3++H
reaction with the H2+
ion prepared in a single rovibrational state at collision energies in the range Ecol/kB=5–60 K. To reach such low-collision energies, we use a merged-beam approach and observe the reaction within the orbit of a Rydberg electron, which shields the ions from stray fields. The first beam is a supersonic beam of pure ground-state H2 molecules and the second is a supersonic beam of H2 molecules excited to Rydberg–Stark states of principal quantum number n selected in the range 20–40. Initially, the two beams propagate along axes separated by an angle of 10°.  To merge the two beams, the Rydberg molecules in the latter beam are deflected using a surface-electrode Rydberg–Stark deflector. The collision energies of the merged beams are determined by measuring the velocity distributions of the two beams and they are adjusted by changing the temperature of the pulsed valve used to generate the ground-state H2 beam and by adapting the electric-potential functions applied to the electrodes of the deflector. The collision energy is varied down to below Ecol/kB=10 K, that is, below Ecol≈1 meV, with an energy resolution of 100 μeV. We demonstrate that the Rydberg electron acts as a spectator and does not affect the cross sections, which are found to closely follow a classical Langevin-capture model in the collision energy range investigated. Because all neutral atoms and molecules can be excited to Rydberg states, this method of studying ion–molecule reactions is applicable to other reactions involving singly charged cations.},
	language = {en},
	number = {22},
	urldate = {2016-11-23},
	journal = {ChemPhysChem},
	author = {Allmendinger, Pitt and Deiglmayr, Johannes and Schullian, Otto and Höveler, Katharina and Agner, Josef A. and Schmutz, Hansjürg and Merkt, Frédéric},
	month = nov,
	year = {2016},
	keywords = {Cold chemistry, Cold molecules},
	pages = {3596--3608},
}

Studies of ion–molecule reactions at low temperatures are difficult because stray electric fields in the reaction volume affect the kinetic energy of charged reaction partners. We describe a new experimental approach to study ion–molecule reactions at low temperatures and present, as example, a measurement of the H2++H2→H3++H reaction with the H2+ ion prepared in a single rovibrational state at collision energies in the range Ecol/kB=5–60 K. To reach such low-collision energies, we use a merged-beam approach and observe the reaction within the orbit of a Rydberg electron, which shields the ions from stray fields. The first beam is a supersonic beam of pure ground-state H2 molecules and the second is a supersonic beam of H2 molecules excited to Rydberg–Stark states of principal quantum number n selected in the range 20–40. Initially, the two beams propagate along axes separated by an angle of 10°. To merge the two beams, the Rydberg molecules in the latter beam are deflected using a surface-electrode Rydberg–Stark deflector. The collision energies of the merged beams are determined by measuring the velocity distributions of the two beams and they are adjusted by changing the temperature of the pulsed valve used to generate the ground-state H2 beam and by adapting the electric-potential functions applied to the electrodes of the deflector. The collision energy is varied down to below Ecol/kB=10 K, that is, below Ecol≈1 meV, with an energy resolution of 100 μeV. We demonstrate that the Rydberg electron acts as a spectator and does not affect the cross sections, which are found to closely follow a classical Langevin-capture model in the collision energy range investigated. Because all neutral atoms and molecules can be excited to Rydberg states, this method of studying ion–molecule reactions is applicable to other reactions involving singly charged cations.

@article{allmendinger_observation_2016,
	title = {Observation of enhanced rate coefficients in the {H}₂⁺+{H}₂→{H}₃⁺+{H} reaction at low collision enrgies},
	volume = {145},
	issn = {0021-9606},
	url = {http://aip.scitation.org/doi/abs/10.1063/1.4972130},
	doi = {10.1063/1.4972130},
	abstract = {The energy dependence of the rate coefficient of the H2++H2→H3++H reaction has been measured in the range of collision energies between kB 10K and kB⋅300mK. A clear deviation of the rate coefficient from the value expected on the basis of the classical Langevin-capture behavior has been observed at collision energies below kB⋅1K, which is attributed to the joint effects of the ion-quadrupole and Coriolis interactions in collisions involving ortho-H2 molecules in the  j = 1 rotational level, which make up 75\% of the population of the neutral H2 molecules in the experiments. The experimental results are compared to very recent predictions by Dashevskaya et al. [J. Chem. Phys. 145, 244315 (2016)], with which they are in agreement.},
	number = {24},
	urldate = {2017-01-18},
	journal = {The Journal of Chemical Physics},
	author = {Allmendinger, Pitt and Deiglmayr, Johannes and Höveler, Katharina and Schullian, Otto and Merkt, Frédéric},
	year = {2016},
	keywords = {Cold chemistry, Cold molecules},
	pages = {244316},
}

The energy dependence of the rate coefficient of the H2++H2→H3++H reaction has been measured in the range of collision energies between kB 10K and kB⋅300mK. A clear deviation of the rate coefficient from the value expected on the basis of the classical Langevin-capture behavior has been observed at collision energies below kB⋅1K, which is attributed to the joint effects of the ion-quadrupole and Coriolis interactions in collisions involving ortho-H2 molecules in the  j = 1 rotational level, which make up 75% of the population of the neutral H2 molecules in the experiments. The experimental results are compared to very recent predictions by Dashevskaya et al. [J. Chem. Phys. 145, 244315 (2016)], with which they are in agreement.

@article{deiglmayr_long-range_2016,
	title = {Long-range interactions between {Rydberg} atoms},
	volume = {91},
	issn = {1402-4896},
	url = {http://stacks.iop.org/1402-4896/91/i=10/a=104007},
	doi = {10.1088/0031-8949/91/10/104007},
	abstract = {We present an overview over theoretical models to describe adiabatic potential-energy curves, experimental excitation spectra, and electronic and nuclear dynamics in interacting Rydberg-atom pairs at large internuclear separations. The potential-energy curves and molecular wavefunctions are determined from the multipole expansion of the static Coulomb interaction which is evaluated numerically in a product basis of atomic orbitals. The convergence of this approach both in the truncation of the multipole expansion as well as in the size of the product basis is discussed, and the comparison of simulated excitation spectra is established as a useful criterium to test the convergence of the calculation. We finally discuss the dynamics of electronic and nuclear motions of pairs of Rydberg atoms, focusing on the stability of ultralong range Rydberg molecules with respect to autoionization.},
	language = {en},
	number = {10},
	urldate = {2016-10-05},
	journal = {Physica Scripta},
	author = {Deiglmayr, Johannes},
	year = {2016},
	keywords = {Review, Ultracold Rydberg gases},
	pages = {104007},
}

We present an overview over theoretical models to describe adiabatic potential-energy curves, experimental excitation spectra, and electronic and nuclear dynamics in interacting Rydberg-atom pairs at large internuclear separations. The potential-energy curves and molecular wavefunctions are determined from the multipole expansion of the static Coulomb interaction which is evaluated numerically in a product basis of atomic orbitals. The convergence of this approach both in the truncation of the multipole expansion as well as in the size of the product basis is discussed, and the comparison of simulated excitation spectra is established as a useful criterium to test the convergence of the calculation. We finally discuss the dynamics of electronic and nuclear motions of pairs of Rydberg atoms, focusing on the stability of ultralong range Rydberg molecules with respect to autoionization.

@article{jacovella_infrared_2016,
	title = {Infrared spectroscopy of molecular ions in selected rotational and spin-orbit states},
	volume = {145},
	issn = {0021-9606, 1089-7690},
	url = {http://scitation.aip.org/content/aip/journal/jcp/145/1/10.1063/1.4954701},
	doi = {10.1063/1.4954701},
	abstract = {First results are presented obtained with an experimental setup developed to record IR spectra of rotationally state-selected ions. The method we use is a state-selective version of a method developed by Schlemmer et al. [Int. J. Mass Spectrom. 185, 589 (1999); J. Chem. Phys. 117, 2068 (2002)] to record IR spectra of ions. Ions are produced in specific rotational levels using mass-analyzed-threshold-ionization spectroscopy. The state-selected ions generated by pulsed-field ionization of Rydberg states of high principal quantum number (n ≈ 200) are extracted toward an octupole ion guide containing a neutral target gas. Prior to entering the octupole, the ions are excited by an IR laser. The target gas is chosen so that only excited ions react to form product ions. These product ions are detected mass selectively as a function of the IR laser wavenumber. To illustrate this method, we present IR spectra of C2H2+ in selected rotational levels of the 2Πu,3/2 and 2Πu,1/2 spin-orbit components of the vibronic ground state.},
	number = {1},
	urldate = {2016-08-18},
	journal = {The Journal of Chemical Physics},
	author = {Jacovella, U. and Agner, J. A. and Schmutz, H. and Deiglmayr, J. and Merkt, F.},
	month = jul,
	year = {2016},
	keywords = {Cold molecules, Precision Measurements},
	pages = {014301},
}

First results are presented obtained with an experimental setup developed to record IR spectra of rotationally state-selected ions. The method we use is a state-selective version of a method developed by Schlemmer et al. [Int. J. Mass Spectrom. 185, 589 (1999); J. Chem. Phys. 117, 2068 (2002)] to record IR spectra of ions. Ions are produced in specific rotational levels using mass-analyzed-threshold-ionization spectroscopy. The state-selected ions generated by pulsed-field ionization of Rydberg states of high principal quantum number (n ≈ 200) are extracted toward an octupole ion guide containing a neutral target gas. Prior to entering the octupole, the ions are excited by an IR laser. The target gas is chosen so that only excited ions react to form product ions. These product ions are detected mass selectively as a function of the IR laser wavenumber. To illustrate this method, we present IR spectra of C2H2+ in selected rotational levels of the 2Πu,3/2 and 2Πu,1/2 spin-orbit components of the vibronic ground state.

@article{sasmannshausen_observation_2016,
	title = {Observation of {Rydberg}-{Atom} {Macrodimers}: {Micrometer}-{Sized} {Diatomic} {Molecules}},
	volume = {117},
	shorttitle = {Observation of {Rydberg}-{Atom} {Macrodimers}},
	url = {http://link.aps.org/doi/10.1103/PhysRevLett.117.083401},
	doi = {10.1103/PhysRevLett.117.083401},
	abstract = {Long-range metastable molecules consisting of two cesium atoms in high Rydberg states have been observed in an ultracold gas. A sequential three-photon two-color photoassociation scheme is employed to form these molecules in states, which correlate to np(n+1)s dissociation asymptotes. Spectral signatures of bound molecular states are clearly resolved at the positions of avoided crossings between long-range van der Waals potential curves. The experimental results are in agreement with simulations based on a detailed model of the long-range multipole-multipole interactions of Rydberg-atom pair states. We show that a full model is required to accurately predict the occurrence of bound Rydberg macrodimers. The macrodimers are distinguished from repulsive molecular states by their behavior with respect to spontaneous ionization and possible decay channels are discussed.},
	number = {8},
	urldate = {2016-08-22},
	journal = {Physical Review Letters},
	author = {Saßmannshausen, Heiner and Deiglmayr, Johannes},
	month = aug,
	year = {2016},
	keywords = {Ultracold Rydberg gases},
	pages = {083401},
}

Long-range metastable molecules consisting of two cesium atoms in high Rydberg states have been observed in an ultracold gas. A sequential three-photon two-color photoassociation scheme is employed to form these molecules in states, which correlate to np(n+1)s dissociation asymptotes. Spectral signatures of bound molecular states are clearly resolved at the positions of avoided crossings between long-range van der Waals potential curves. The experimental results are in agreement with simulations based on a detailed model of the long-range multipole-multipole interactions of Rydberg-atom pair states. We show that a full model is required to accurately predict the occurrence of bound Rydberg macrodimers. The macrodimers are distinguished from repulsive molecular states by their behavior with respect to spontaneous ionization and possible decay channels are discussed.

@article{sasmannshausen_exotic_2016,
	title = {Exotic {Chemistry} with {Ultracold} {Rydberg} {Atoms}},
	volume = {70},
	doi = {10.2533/chimia.2016.263},
	abstract = {We review recent experiments carried out with dense (1012 cm–3) ultracold (T = 40 μK) samples of Cs atoms which have the goal to characterize, by high-resolution spectroscopy, the interactions between Cs atoms, Cs+ ions and electrons that lead to the formation of metastable long-range molecules. The types of molecules observed in these experiments and the mechanisms leading to the aggregation of atoms in weakly bound molecules are very different from those encountered in warmer samples. In particular, we present results on molecules with binding energies of less than 0.05 J/mol and discuss their properties in the context of a new category of molecular states arising from slow-electron–atom scattering and their relation to atomic and molecular Rydberg states. One of the astonishing aspects of these types of molecules is that they can still be treated in good approximation in the realm of the Born-Oppenheimer approximation despite a huge electronic-state density. Non-Born-Oppenheimer effects are revealed by the decay of the molecules into neutral and charged fragments.},
	number = {4},
	journal = {CHIMIA International Journal for Chemistry},
	author = {Saßmannshausen, Heiner and Deiglmayr, Johannes and Merkt, Frédéric},
	month = apr,
	year = {2016},
	keywords = {Review, Ultracold Rydberg gases},
	pages = {263--267},
}

We review recent experiments carried out with dense (1012 cm–3) ultracold (T = 40 μK) samples of Cs atoms which have the goal to characterize, by high-resolution spectroscopy, the interactions between Cs atoms, Cs+ ions and electrons that lead to the formation of metastable long-range molecules. The types of molecules observed in these experiments and the mechanisms leading to the aggregation of atoms in weakly bound molecules are very different from those encountered in warmer samples. In particular, we present results on molecules with binding energies of less than 0.05 J/mol and discuss their properties in the context of a new category of molecular states arising from slow-electron–atom scattering and their relation to atomic and molecular Rydberg states. One of the astonishing aspects of these types of molecules is that they can still be treated in good approximation in the realm of the Born-Oppenheimer approximation despite a huge electronic-state density. Non-Born-Oppenheimer effects are revealed by the decay of the molecules into neutral and charged fragments.

@article{sasmannshausen_long-range_2016,
	title = {Long-range {Rydberg} molecules, {Rydberg} macrodimers and {Rydberg} aggregates in an ultracold {Cs} gas},
	volume = {225},
	issn = {1951-6355, 1951-6401},
	url = {http://link.springer.com/article/10.1140/epjst/e2016-60124-9},
	doi = {10.1140/epjst/e2016-60124-9},
	abstract = {We present an overview of our recent investigations of long-range interactions in an ultracold Cs Rydberg gas. These interactions are studied by high-resolution photoassociation spectroscopy, using excitation close to one-photon transitions into np3/2 Rydberg states with pulsed and continuous-wave ultraviolet laser radiation, and lead to the formation of long-range Cs2 molecules. We observe two types of molecular resonances. The first type originates from the correlated excitation of two atoms into Rydberg-atom-pair states interacting at long range via multipole-multipole interactions. The second type results from the interaction of one atom excited to a Rydberg state with one atom in the electronic ground state. Which type of resonances is observed in the experiments depends on the laser intensity and frequency and on the pulse sequences used to prepare the Rydberg states. We obtain insights into both types of molecular resonances by modelling the interaction potentials, using a multipole expansion of the long-range interaction for the first type of resonances and a Fermi-contact pseudo-potential for the second type of resonances. We analyse the relation of these long-range molecular resonances to molecular Rydberg states and ion-pair states, and discuss their decay channels into atomic and molecular ions. In experiments carried out with a two-colour two-photon excitation scheme, we observe a large enhancement of Rydberg-excitation probability, which we interpret as a saturable autocatalytic antiblockade phenomenon.},
	language = {en},
	number = {15-16},
	urldate = {2016-12-23},
	journal = {The European Physical Journal Special Topics},
	author = {Saßmannshausen, Heiner and Deiglmayr, Johannes and Merkt, Frédéric},
	month = dec,
	year = {2016},
	keywords = {Review, Ultracold Rydberg gases},
	pages = {2891--2918},
}

We present an overview of our recent investigations of long-range interactions in an ultracold Cs Rydberg gas. These interactions are studied by high-resolution photoassociation spectroscopy, using excitation close to one-photon transitions into np3/2 Rydberg states with pulsed and continuous-wave ultraviolet laser radiation, and lead to the formation of long-range Cs2 molecules. We observe two types of molecular resonances. The first type originates from the correlated excitation of two atoms into Rydberg-atom-pair states interacting at long range via multipole-multipole interactions. The second type results from the interaction of one atom excited to a Rydberg state with one atom in the electronic ground state. Which type of resonances is observed in the experiments depends on the laser intensity and frequency and on the pulse sequences used to prepare the Rydberg states. We obtain insights into both types of molecular resonances by modelling the interaction potentials, using a multipole expansion of the long-range interaction for the first type of resonances and a Fermi-contact pseudo-potential for the second type of resonances. We analyse the relation of these long-range molecular resonances to molecular Rydberg states and ion-pair states, and discuss their decay channels into atomic and molecular ions. In experiments carried out with a two-colour two-photon excitation scheme, we observe a large enhancement of Rydberg-excitation probability, which we interpret as a saturable autocatalytic antiblockade phenomenon.

@article{thiele_imaging_2015,
	title = {Imaging electric fields in the vicinity of cryogenic surfaces using {Rydberg} atoms},
	volume = {92},
	url = {http://link.aps.org/doi/10.1103/PhysRevA.92.063425},
	doi = {10.1103/PhysRevA.92.063425},
	abstract = {The ability to characterize static and time-dependent electric fields in situ is an important prerequisite for quantum-optics experiments with atoms close to surfaces. Especially in experiments which aim at coupling Rydberg atoms to the near field of superconducting circuits, the identification and subsequent elimination of sources of stray fields are crucial. We present a technique that allows the determination of stray-electric-field distributions [Fstrx(r⃗ ),Fstry(r⃗ ),Fstrz(r⃗ )] at distances of less than 2 mm from (cryogenic) surfaces using coherent Rydberg-Stark spectroscopy in a pulsed supersonic beam of metastable 1s12s11S0 helium atoms. We demonstrate the capabilities of this technique by characterizing the electric stray field emanating from a structured superconducting surface. Exploiting coherent population transfer with microwave radiation from a coplanar waveguide, the same technique allows the characterization of the microwave-field distribution above the surface.},
	number = {6},
	urldate = {2016-02-06},
	journal = {Physical Review A},
	author = {Thiele, T. and Deiglmayr, J. and Stammeier, M. and Agner, J.-A. and Schmutz, H. and Merkt, F. and Wallraff, A.},
	year = {2015},
	keywords = {Cavity QED},
	pages = {063425},
}

The ability to characterize static and time-dependent electric fields in situ is an important prerequisite for quantum-optics experiments with atoms close to surfaces. Especially in experiments which aim at coupling Rydberg atoms to the near field of superconducting circuits, the identification and subsequent elimination of sources of stray fields are crucial. We present a technique that allows the determination of stray-electric-field distributions [Fstrx(r⃗ ),Fstry(r⃗ ),Fstrz(r⃗ )] at distances of less than 2 mm from (cryogenic) surfaces using coherent Rydberg-Stark spectroscopy in a pulsed supersonic beam of metastable 1s12s11S0 helium atoms. We demonstrate the capabilities of this technique by characterizing the electric stray field emanating from a structured superconducting surface. Exploiting coherent population transfer with microwave radiation from a coplanar waveguide, the same technique allows the characterization of the microwave-field distribution above the surface.

@article{sasmannshausen_experimental_2015,
	title = {Experimental {Characterization} of {Singlet} {Scattering} {Channels} in {Long}-{Range} {Rydberg} {Molecules}},
	volume = {114},
	url = {http://link.aps.org/doi/10.1103/PhysRevLett.114.133201},
	doi = {10.1103/PhysRevLett.114.133201},
	abstract = {We observe the formation of long-range Cs2 Rydberg molecules consisting of a Rydberg and a ground-state atom by photoassociation spectroscopy in an ultracold Cs gas near 6s1/2(F=3,4)→np3/2 resonances (n=26–34). The spectra reveal two types of molecular states recently predicted by D. A. Anderson, S. A. Miller, and G. Raithel [Phys. Rev. A 90, 062518 (2014)]: states bound purely by triplet s-wave scattering with binding energies ranging from 400 MHz at n=26 to 80 MHz at n=34, and states bound by mixed singlet-triplet s-wave scattering with smaller and F-dependent binding energies. The experimental observations are accounted for by an effective Hamiltonian including s-wave scattering pseudopotentials, the hyperfine interaction of the ground-state atom, and the spin-orbit interaction of the Rydberg atom. The analysis enables the characterization of the role of singlet scattering in the formation of long-range Rydberg molecules and the determination of an effective singlet s-wave scattering length for low-energy-electron–Cs collisions.},
	number = {13},
	urldate = {2015-04-03},
	journal = {Physical Review Letters},
	author = {Saßmannshausen, Heiner and Merkt, Frédéric and Deiglmayr, Johannes},
	year = {2015},
	keywords = {Ultracold Rydberg gases},
	pages = {133201},
}

We observe the formation of long-range Cs2 Rydberg molecules consisting of a Rydberg and a ground-state atom by photoassociation spectroscopy in an ultracold Cs gas near 6s1/2(F=3,4)→np3/2 resonances (n=26–34). The spectra reveal two types of molecular states recently predicted by D. A. Anderson, S. A. Miller, and G. Raithel [Phys. Rev. A 90, 062518 (2014)]: states bound purely by triplet s-wave scattering with binding energies ranging from 400 MHz at n=26 to 80 MHz at n=34, and states bound by mixed singlet-triplet s-wave scattering with smaller and F-dependent binding energies. The experimental observations are accounted for by an effective Hamiltonian including s-wave scattering pseudopotentials, the hyperfine interaction of the ground-state atom, and the spin-orbit interaction of the Rydberg atom. The analysis enables the characterization of the role of singlet scattering in the formation of long-range Rydberg molecules and the determination of an effective singlet s-wave scattering length for low-energy-electron–Cs collisions.

@article{sasmannshausen_pulsed_2015,
	title = {Pulsed excitation of {Rydberg}-atom-pair states in an ultracold {Cs} gas},
	volume = {92},
	url = {http://link.aps.org/doi/10.1103/PhysRevA.92.032505},
	doi = {10.1103/PhysRevA.92.032505},
	abstract = {Pulsed laser excitation of a dense ultracold Cs vapor has been used to study the pairwise interactions between Cs atoms excited to np3/2 Rydberg states of principal quantum numbers in the range n=22–36. Molecular resonances were observed that correspond to excitation of Rydberg-atom-pair states correlated not only to the np3/2+np3/2 dissociation asymptotes, but also to ns1/2+(n+1)s1/2, ns1/2+n′fj, and (n−4)fj+(n−3)fj(j=5/2,7/2) dissociation asymptotes. These pair resonances are interpreted as arising from dipole-dipole and higher-order long-range-interaction terms between the Rydberg atoms on the basis of (i) their spectral positions, (ii) their response to static and pulsed electric fields, and (iii) millimeter-wave spectra between pair states correlated to different pair-dissociation asymptotes. The Rydberg-atom-pair states were found to spontaneously decay by Penning ionization and the dynamics of the ionization process were investigated during the first 15 μs following initial photoexcitation. To interpret the experimental observations, a potential model was derived that is based on the numerical determination of the eigenvalues and eigenfunctions of the long-range interaction Hamiltonian. With this potential model, which does not include adjustable parameters, all experimental observations could be accounted for, and the results demonstrate that long-range-interaction models provide a global and accurate description of interactions in ultracold Rydberg gases and that they correctly account for, and enable the analysis of, phenomena as diverse as the formation of Rydberg macrodimers, Penning ionization in dense Rydberg gases, and Rydberg-excitation-blockade effects.},
	number = {3},
	urldate = {2015-09-11},
	journal = {Physical Review A},
	author = {Saßmannshausen, Heiner and Merkt, Frédéric and Deiglmayr, Johannes},
	month = sep,
	year = {2015},
	keywords = {Ultracold Rydberg gases},
	pages = {032505},
}

Pulsed laser excitation of a dense ultracold Cs vapor has been used to study the pairwise interactions between Cs atoms excited to np3/2 Rydberg states of principal quantum numbers in the range n=22–36. Molecular resonances were observed that correspond to excitation of Rydberg-atom-pair states correlated not only to the np3/2+np3/2 dissociation asymptotes, but also to ns1/2+(n+1)s1/2, ns1/2+n′fj, and (n−4)fj+(n−3)fj(j=5/2,7/2) dissociation asymptotes. These pair resonances are interpreted as arising from dipole-dipole and higher-order long-range-interaction terms between the Rydberg atoms on the basis of (i) their spectral positions, (ii) their response to static and pulsed electric fields, and (iii) millimeter-wave spectra between pair states correlated to different pair-dissociation asymptotes. The Rydberg-atom-pair states were found to spontaneously decay by Penning ionization and the dynamics of the ionization process were investigated during the first 15 μs following initial photoexcitation. To interpret the experimental observations, a potential model was derived that is based on the numerical determination of the eigenvalues and eigenfunctions of the long-range interaction Hamiltonian. With this potential model, which does not include adjustable parameters, all experimental observations could be accounted for, and the results demonstrate that long-range-interaction models provide a global and accurate description of interactions in ultracold Rydberg gases and that they correctly account for, and enable the analysis of, phenomena as diverse as the formation of Rydberg macrodimers, Penning ionization in dense Rydberg gases, and Rydberg-excitation-blockade effects.

@article{thiele_manipulating_2014,
	title = {Manipulating {Rydberg} atoms close to surfaces at cryogenic temperatures},
	volume = {90},
	url = {http://link.aps.org/doi/10.1103/PhysRevA.90.013414},
	doi = {10.1103/PhysRevA.90.013414},
	abstract = {Helium atoms in Rydberg states have been manipulated coherently with microwave radiation pulses near a gold surface and near a superconducting NbTiN surface at a temperature of 3K. The experiments were carried out with a skimmed supersonic beam of metastable (1s)1(2s)11S0 helium atoms excited with laser radiation to np Rydberg levels with principal quantum number n between 30 and 40. The separation between the cold surface and the center of the collimated beam is adjustable down to 250μm. Short-lived np Rydberg states were coherently transferred to the long-lived ns state to avoid radiative decay of the Rydberg atoms between the photoexcitation region and the region above the cold surfaces. Further coherent manipulation of the ns Rydberg states with pulsed microwave radiation above the surfaces enabled measurements of stray electric fields and allowed us to study the decoherence of the atomic ensemble. Adsorption of residual gas onto the surfaces and the resulting slow buildup of stray fields was minimized by controlling the temperature of the surface and monitoring the partial pressures of H2O, N2, O2, and CO2 in the experimental chamber during the cool-down procedure. Compensation of the stray electric fields to levels below 100mV/cm was achieved over a region of 6mm along the beam-propagation direction which, for the 1770-m/s beam velocity, implies the possibility to preserve the coherence of the atomic sample for several microseconds above the cold surfaces.},
	number = {1},
	urldate = {2014-10-15},
	journal = {Physical Review A},
	author = {Thiele, T. and Filipp, S. and Agner, J. A. and Schmutz, H. and Deiglmayr, J. and Stammeier, M. and Allmendinger, P. and Merkt, F. and Wallraff, A.},
	month = jul,
	year = {2014},
	keywords = {Cavity QED},
	pages = {013414},
}

Helium atoms in Rydberg states have been manipulated coherently with microwave radiation pulses near a gold surface and near a superconducting NbTiN surface at a temperature of 3K. The experiments were carried out with a skimmed supersonic beam of metastable (1s)1(2s)11S0 helium atoms excited with laser radiation to np Rydberg levels with principal quantum number n between 30 and 40. The separation between the cold surface and the center of the collimated beam is adjustable down to 250μm. Short-lived np Rydberg states were coherently transferred to the long-lived ns state to avoid radiative decay of the Rydberg atoms between the photoexcitation region and the region above the cold surfaces. Further coherent manipulation of the ns Rydberg states with pulsed microwave radiation above the surfaces enabled measurements of stray electric fields and allowed us to study the decoherence of the atomic ensemble. Adsorption of residual gas onto the surfaces and the resulting slow buildup of stray fields was minimized by controlling the temperature of the surface and monitoring the partial pressures of H2O, N2, O2, and CO2 in the experimental chamber during the cool-down procedure. Compensation of the stray electric fields to levels below 100mV/cm was achieved over a region of 6mm along the beam-propagation direction which, for the 1770-m/s beam velocity, implies the possibility to preserve the coherence of the atomic sample for several microseconds above the cold surfaces.

@article{allmendinger_surface-electrode_2014,
	title = {Surface-electrode decelerator and deflector for {Rydberg} atoms and molecules},
	volume = {90},
	url = {http://link.aps.org/doi/10.1103/PhysRevA.90.043403},
	doi = {10.1103/PhysRevA.90.043403},
	abstract = {A surface-electrode decelerator and deflector for Rydberg atoms and molecules has been developed with the goal of performing collisional experiments. Translationally cold H2 molecules in a supersonic beam were excited to Rydberg-Stark states of principal quantum number n=31, loaded into electric traps moving at a predetermined speed above the surface of a bent printed circuit board, decelerated, and deflected from the original direction of the supersonic beam by an angle of 10∘. The phase-space characteristics of the deflected beam were characterized by measuring the time-of-flight distribution and images of the Rydberg molecules and comparing them to the results of numerical particle-trajectory simulations. More than 1000 H2 molecules were deflected per experimental cycle at a repetition rate of 25 Hz. The phase-space characteristics of the deflector make it attractive to study ion-molecule reactions at low collision energies.},
	number = {4},
	urldate = {2014-10-23},
	journal = {Physical Review A},
	author = {Allmendinger, P. and Deiglmayr, J. and Agner, J. A. and Schmutz, H. and Merkt, F.},
	year = {2014},
	keywords = {Cold chemistry, Cold molecules},
	pages = {043403},
}

A surface-electrode decelerator and deflector for Rydberg atoms and molecules has been developed with the goal of performing collisional experiments. Translationally cold H2 molecules in a supersonic beam were excited to Rydberg-Stark states of principal quantum number n=31, loaded into electric traps moving at a predetermined speed above the surface of a bent printed circuit board, decelerated, and deflected from the original direction of the supersonic beam by an angle of 10∘. The phase-space characteristics of the deflected beam were characterized by measuring the time-of-flight distribution and images of the Rydberg molecules and comparing them to the results of numerical particle-trajectory simulations. More than 1000 H2 molecules were deflected per experimental cycle at a repetition rate of 25 Hz. The phase-space characteristics of the deflector make it attractive to study ion-molecule reactions at low collision energies.

@article{deiglmayr_observation_2014,
	title = {Observation of {Dipole}-{Quadrupole} {Interaction} in an {Ultracold} {Gas} of {Rydberg} {Atoms}},
	volume = {113},
	url = {http://link.aps.org/doi/10.1103/PhysRevLett.113.193001},
	doi = {10.1103/PhysRevLett.113.193001},
	abstract = {We observe the direct excitation of pairs of Cs atoms from the ground state to molecular states correlating asymptotically to nsn′f asymptotes. The molecular resonances are interpreted as originating from the dipole-quadrupole interaction between the nsn′f pair states and close-by npnp asymptotes (22≤n≤32). This interpretation is supported by Stark spectroscopy of the pair states and a detailed modeling of the interaction potentials. The dipole-quadrupole interaction mixes electronic states of opposite parity and, thus, requires a coupling between electronic and nuclear motion to conserve the total parity of the system. This non-Born-Oppenheimer coupling is facilitated by the near-degeneracy of even- and odd-L partial waves in the atom-atom scattering which have opposite parity.},
	number = {19},
	urldate = {2014-11-05},
	journal = {Physical Review Letters},
	author = {Deiglmayr, Johannes and Saßmannshausen, Heiner and Pillet, Pierre and Merkt, Frédéric},
	month = nov,
	year = {2014},
	keywords = {Ultracold Rydberg gases},
	pages = {193001},
}

We observe the direct excitation of pairs of Cs atoms from the ground state to molecular states correlating asymptotically to nsn′f asymptotes. The molecular resonances are interpreted as originating from the dipole-quadrupole interaction between the nsn′f pair states and close-by npnp asymptotes (22≤n≤32). This interpretation is supported by Stark spectroscopy of the pair states and a detailed modeling of the interaction potentials. The dipole-quadrupole interaction mixes electronic states of opposite parity and, thus, requires a coupling between electronic and nuclear motion to conserve the total parity of the system. This non-Born-Oppenheimer coupling is facilitated by the near-degeneracy of even- and odd-L partial waves in the atom-atom scattering which have opposite parity.

@article{sasmannshausen_high-resolution_2013,
	title = {High-resolution spectroscopy of {Rydberg} states in an ultracold cesium gas},
	volume = {87},
	url = {http://link.aps.org/doi/10.1103/PhysRevA.87.032519},
	doi = {10.1103/PhysRevA.87.032519},
	abstract = {Transitions between high Rydberg states of 133Cs atoms have been studied by high-resolution millimeter-wave spectroscopy of an ultracold sample. The spectroscopic measurements were performed after releasing the atoms from a magneto-optical trap. Switching off all trapping fields and compensating the stray electric and magnetic fields to below 1 mV/cm and 2 mG, respectively, prior to the spectroscopic measurement enabled the recording of millimeter-wave spectra of Rydberg states with principal quantum number beyond n=100 under conditions where the inhomogeneous broadening by stray fields is minimal and no dephasing of the Rydberg-atom sample can be detected over measurement times up to 60 μs. The Fourier-transform-limited line widths of better than 20 kHz enabled the observation of the hyperfine structure of nS and nP Rydberg states of Cs beyond n=90. The analysis of the line shapes of transitions to Rydberg states with n≈230 indicated that field inhomogeneities across the atomic sample represent the dominant cause of spectral broadening at high n values. The analysis also revealed that the initial polarization of the atomic sample (F=4, MF=4) is preserved for several tens of microseconds, the depolarization being caused by slow precession along the magnetic stray fields.},
	number = {3},
	urldate = {2014-06-02},
	journal = {Physical Review A},
	author = {Saßmannshausen, H. and Merkt, F. and Deiglmayr, J.},
	year = {2013},
	keywords = {Ultracold Rydberg gases},
	pages = {032519},
}

Transitions between high Rydberg states of 133Cs atoms have been studied by high-resolution millimeter-wave spectroscopy of an ultracold sample. The spectroscopic measurements were performed after releasing the atoms from a magneto-optical trap. Switching off all trapping fields and compensating the stray electric and magnetic fields to below 1 mV/cm and 2 mG, respectively, prior to the spectroscopic measurement enabled the recording of millimeter-wave spectra of Rydberg states with principal quantum number beyond n=100 under conditions where the inhomogeneous broadening by stray fields is minimal and no dephasing of the Rydberg-atom sample can be detected over measurement times up to 60 μs. The Fourier-transform-limited line widths of better than 20 kHz enabled the observation of the hyperfine structure of nS and nP Rydberg states of Cs beyond n=90. The analysis of the line shapes of transitions to Rydberg states with n≈230 indicated that field inhomogeneities across the atomic sample represent the dominant cause of spectral broadening at high n values. The analysis also revealed that the initial polarization of the atomic sample (F=4, MF=4) is preserved for several tens of microseconds, the depolarization being caused by slow precession along the magnetic stray fields.

@article{deiglmayr_reactive_2012,
	title = {Reactive collisions of trapped anions with ultracold atoms},
	volume = {86},
	issn = {1050-2947},
	doi = {10.1103/PhysRevA.86.043438},
	abstract = {We present a scheme to embed molecular anions in a gas of ultracold   rubidium atoms as a route towards the preparation of cold molecular ions   by collisional cooling with ultracold atoms. Associative detachment as   an important loss process in collisions between OH- molecules and   rubidium atoms is studied. The density distribution of trapped negative   ions in the multipole radio frequency trap is measured by   photodetachment tomography, which allows us to derive absolute rate   coefficients for the process. We define a regime where translational and   internal cooling of molecular ions embedded into the ultracold atomic   cloud can be achieved.},
	number = {4},
	journal = {Physical Review A},
	author = {Deiglmayr, J. and Goeritz, A. and Best, T. and Weidemueller, M. and Wester, R.},
	month = oct,
	year = {2012},
	note = {WOS:000310434800012},
	keywords = {Cold chemistry, Cold molecules},
}

We present a scheme to embed molecular anions in a gas of ultracold rubidium atoms as a route towards the preparation of cold molecular ions by collisional cooling with ultracold atoms. Associative detachment as an important loss process in collisions between OH- molecules and rubidium atoms is studied. The density distribution of trapped negative ions in the multipole radio frequency trap is measured by photodetachment tomography, which allows us to derive absolute rate coefficients for the process. We define a regime where translational and internal cooling of molecular ions embedded into the ultracold atomic cloud can be achieved.

@article{ulmanis_ultracold_2012,
	title = {Ultracold {Molecules} {Formed} by {Photoassociation}: {Heteronuclear} {Dimers}, {Inelastic} {Collisions}, and {Interactions} with {Ultrashort} {Laser} {Pulses}},
	volume = {112},
	issn = {0009-2665},
	shorttitle = {Ultracold {Molecules} {Formed} by {Photoassociation}},
	url = {http://dx.doi.org/10.1021/cr300215h},
	doi = {10.1021/cr300215h},
	number = {9},
	urldate = {2016-08-26},
	journal = {Chemical Reviews},
	author = {Ulmanis, Juris and Deiglmayr, Johannes and Repp, Marc and Wester, Roland and Weidemüller, Matthias},
	month = sep,
	year = {2012},
	keywords = {Cold chemistry, Cold molecules, Review},
	pages = {4890--4927},
}

@article{deiglmayr_population_2011,
	title = {Population redistribution in optically trapped polar molecules},
	volume = {65},
	issn = {1434-6060},
	doi = {10.1140/epjd/e2011-20072-9},
	abstract = {We investigate the rovibrational population redistribution of polar   molecules in the electronic ground state induced by spontaneous emission   and blackbody radiation. As a model system we use optically trapped LiCs   molecules formed by photoassociation in an ultracold two-species gas.   The population dynamics of vibrational and rotational states is modeled   using an ab initio electric dipole moment function and experimental   potential energy curves. Comparison with the evolution of the vaEuro(3)   = 3 electronic ground state yields good qualitative agreement. The   analysis provides important input to assess applications of ultracold   LiCs molecules in quantum simulation and ultracold chemistry.},
	number = {1-2},
	journal = {European Physical Journal D},
	author = {Deiglmayr, J. and Repp, M. and Dulieu, O. and Wester, R. and Weidemueller, M.},
	month = nov,
	year = {2011},
	note = {WOS:000297137000013},
	keywords = {Cold chemistry, Cold molecules},
	pages = {99--104},
}

We investigate the rovibrational population redistribution of polar molecules in the electronic ground state induced by spontaneous emission and blackbody radiation. As a model system we use optically trapped LiCs molecules formed by photoassociation in an ultracold two-species gas. The population dynamics of vibrational and rotational states is modeled using an ab initio electric dipole moment function and experimental potential energy curves. Comparison with the evolution of the vaEuro(3) = 3 electronic ground state yields good qualitative agreement. The analysis provides important input to assess applications of ultracold LiCs molecules in quantum simulation and ultracold chemistry.

@article{deiglmayr_inelastic_2011,
	title = {Inelastic collisions of ultracold polar {LiCs} molecules with caesium   atoms in an optical dipole trap},
	volume = {13},
	issn = {1463-9076},
	doi = {10.1039/c1cp21396b},
	abstract = {We investigate collisions of ultracold polar LiCs molecules and   ultracold caesium atoms. LiCs molecules are formed in an optical dipole   trap by photoassociation of caesium and lithium atoms via the B(1)Pi   excited state followed by spontaneous emission to the X(1)Sigma(+)   ground state and the lowest triplet state a(3)Sigma(+). The molecules   are then stored together with caesium atoms in the same optical trap.   Rate coefficients for the loss of molecules induced by collisions with   surrounding Cs atoms are measured for molecular ensembles produced via   different photoassociation resonances. The results are analyzed in terms   of the unitarity limit for the inelastic rates and predictions from the   universal model of Idziaszek and Julienne (Phys. Rev. Lett., 2010, 104,   113202).},
	number = {42},
	journal = {Physical Chemistry Chemical Physics},
	author = {Deiglmayr, Johannes and Repp, Marc and Wester, Roland and Dulieu, Olivier and Weidemueller, Matthias},
	year = {2011},
	note = {WOS:000296027500047},
	keywords = {Cold chemistry, Cold molecules},
	pages = {19101--19105},
}

We investigate collisions of ultracold polar LiCs molecules and ultracold caesium atoms. LiCs molecules are formed in an optical dipole trap by photoassociation of caesium and lithium atoms via the B(1)Pi excited state followed by spontaneous emission to the X(1)Sigma(+) ground state and the lowest triplet state a(3)Sigma(+). The molecules are then stored together with caesium atoms in the same optical trap. Rate coefficients for the loss of molecules induced by collisions with surrounding Cs atoms are measured for molecular ensembles produced via different photoassociation resonances. The results are analyzed in terms of the unitarity limit for the inelastic rates and predictions from the universal model of Idziaszek and Julienne (Phys. Rev. Lett., 2010, 104, 113202).

@article{deiglmayr_permanent_2010,
	title = {Permanent dipole moment of {LiCs} in the ground state},
	volume = {82},
	issn = {1050-2947},
	doi = {10.1103/PhysRevA.82.032503},
	abstract = {Ultracold gases of dipolar molecules have been proposed as candidates   for the exploration of quantum phases in dipolar gases [1], the   development of quantum computation techniques [2], or precision   measurements of fundamental constants [3]. LiCs is predicted to have the   largest permanent electric dipole moment (EDM) of all alkali-metal   dimers [4] and is thus very advantageous for such schemes. Recently we   have reported the formation of LiCs molecules in deeply bound levels of   the X(1)Sigma(+) ground state [5], including the rovibrational ground   state [6], after a single step of photoassociation (PA). Here we report   on an experimental determination of the permanent EDM of deeply bound   LiCs molecules.},
	number = {3},
	journal = {Physical Review A},
	author = {Deiglmayr, J. and Grochola, A. and Repp, M. and Dulieu, O. and Wester, R. and Weidemueller, M.},
	month = sep,
	year = {2010},
	note = {WOS:000281719700006},
	keywords = {Cold molecules},
}

Ultracold gases of dipolar molecules have been proposed as candidates for the exploration of quantum phases in dipolar gases [1], the development of quantum computation techniques [2], or precision measurements of fundamental constants [3]. LiCs is predicted to have the largest permanent electric dipole moment (EDM) of all alkali-metal dimers [4] and is thus very advantageous for such schemes. Recently we have reported the formation of LiCs molecules in deeply bound levels of the X(1)Sigma(+) ground state [5], including the rovibrational ground state [6], after a single step of photoassociation (PA). Here we report on an experimental determination of the permanent EDM of deeply bound LiCs molecules.

@article{aymar_systematic_2009,
	title = {Systematic trends in electronic properties of alkali hydrides},
	volume = {87},
	issn = {0008-4204},
	doi = {10.1139/P09-005},
	abstract = {Obtaining ultracold samples of dipolar molecules is a current challenge,   which requires an accurate knowledge of their electronic properties to   guide the ongoing experiments. Alkali hydride molecules have permanent   dipole moments significantly larger than those of mixed alkali species,   and, as pointed out by Taylor-Juarros et al. (Eur. Phys. J. D, 31, 213   (2004)) and by Juarros et al. (Phys. Rev. A, 73, 041403 (2006)), are   thus good candidates for cold molecule formation. In this paper, using a   standard quantum chemistry approach based on pseudopotentials for atomic   core representation, large Gaussian basis sets, and effective core   polarization potential, we systematically investigate the electronic   properties of the alkali hydrides LiH to CsH, to discuss general trends   of their behavior. We computed (for the first time for NaH, KH, RbH, and   CsH) the variation of their static polarizability with the internuclear   distance. Moreover, in addition to potential curves, we determine   accurate values of permanent and transition dipole moments for ground   and excited states depending on the internuclear distance. The   electronic properties of all alkali hydrides are compared with one   another, in the light of the numerous other data available in the   literature. Finally, the influence of the quality of the representation   of the hydrogen electronic affinity in the approach on the results is   discussed.},
	number = {5},
	journal = {Canadian Journal of Physics},
	author = {Aymar, Mireille and Deiglmayr, Johannes and Dulieu, Olivier},
	month = may,
	year = {2009},
	note = {WOS:000269834000012},
	keywords = {Quantum chemistry},
	pages = {543--556},
}

Obtaining ultracold samples of dipolar molecules is a current challenge, which requires an accurate knowledge of their electronic properties to guide the ongoing experiments. Alkali hydride molecules have permanent dipole moments significantly larger than those of mixed alkali species, and, as pointed out by Taylor-Juarros et al. (Eur. Phys. J. D, 31, 213 (2004)) and by Juarros et al. (Phys. Rev. A, 73, 041403 (2006)), are thus good candidates for cold molecule formation. In this paper, using a standard quantum chemistry approach based on pseudopotentials for atomic core representation, large Gaussian basis sets, and effective core polarization potential, we systematically investigate the electronic properties of the alkali hydrides LiH to CsH, to discuss general trends of their behavior. We computed (for the first time for NaH, KH, RbH, and CsH) the variation of their static polarizability with the internuclear distance. Moreover, in addition to potential curves, we determine accurate values of permanent and transition dipole moments for ground and excited states depending on the internuclear distance. The electronic properties of all alkali hydrides are compared with one another, in the light of the numerous other data available in the literature. Finally, the influence of the quality of the representation of the hydrogen electronic affinity in the approach on the results is discussed.

@article{deiglmayr_influence_2009,
	title = {Influence of a {Feshbach} resonance on the photoassociation of {LiCs}},
	volume = {11},
	issn = {1367-2630},
	doi = {10.1088/1367-2630/11/5/055034},
	abstract = {We analyze the formation of ultracold (7)Li(133)Cs molecules in the   rovibrational ground state through photoassociation into the B(1)Pi   state, which has recently been reported (Deiglmayr et al 2008 Phys. Rev.   Lett. 101 133004). Absolute rate constants for photoassociation at large   detunings from the atomic asymptote are determined and are found to be   surprisingly large. The photoassociation process is modeled using a full   coupled-channel calculation for the continuum state, taking all relevant   hyperfine states into account. The enhancement of the photoassociation   rate is found to be caused by an 'echo' of the triplet component in the   singlet component of the scattering wave function at the inner turning   point of the lowest triplet a(3)Sigma(+) potential. This perturbation   can be ascribed to the existence of a broad Feshbach resonance at low   scattering energies. Our results elucidate the important role of   couplings in the scattering wave function for the formation of deeply   bound ground state molecules via photoassociation.},
	journal = {New Journal of Physics},
	author = {Deiglmayr, J. and Pellegrini, P. and Grochola, A. and Repp, M. and Cote, R. and Dulieu, O. and Wester, R. and Weidemueller, M.},
	month = may,
	year = {2009},
	note = {WOS:000266234500025},
	keywords = {Cold chemistry, Cold molecules},
}

We analyze the formation of ultracold (7)Li(133)Cs molecules in the rovibrational ground state through photoassociation into the B(1)Pi state, which has recently been reported (Deiglmayr et al 2008 Phys. Rev. Lett. 101 133004). Absolute rate constants for photoassociation at large detunings from the atomic asymptote are determined and are found to be surprisingly large. The photoassociation process is modeled using a full coupled-channel calculation for the continuum state, taking all relevant hyperfine states into account. The enhancement of the photoassociation rate is found to be caused by an 'echo' of the triplet component in the singlet component of the scattering wave function at the inner turning point of the lowest triplet a(3)Sigma(+) potential. This perturbation can be ascribed to the existence of a broad Feshbach resonance at low scattering energies. Our results elucidate the important role of couplings in the scattering wave function for the formation of deeply bound ground state molecules via photoassociation.

@article{deiglmayr_formation_2009,
	title = {Formation of ultracold dipolar molecules in the lowest vibrational   levels by photoassociation},
	volume = {142},
	issn = {1364-5498},
	doi = {10.1039/b818391k},
	abstract = {We recently reported the formation of ultracold LiCs molecules in the   rovibrational ground state X(1)Sigma(+), v '' = 0, J '' = 0 (J.   Deiglmayr et al., Phys. Rev. Lett., 2008, 101, 133004). Here we discuss   details of the experimental setup and present a thorough analysis of the   photoassociation step including the photoassociation line shape. We   predict the distribution of produced ground state molecules using   accurate potential energy curves combined with an ab initio dipole   transition moment and compare this prediction with experimental   ionization spectra. Additionally we improve the value of the   dissociation energy for the X(1)Sigma(+) state by high resolution   spectroscopy of the vibrational ground state.},
	journal = {Faraday Discussions},
	author = {Deiglmayr, J. and Repp, M. and Grochola, A. and Moertlbauer, K. and Glueck, C. and Dulieu, O. and Lange, J. and Wester, R. and Weidemueller, M.},
	year = {2009},
	note = {WOS:000268974900021},
	keywords = {Cold molecules},
	pages = {335--349},
}

We recently reported the formation of ultracold LiCs molecules in the rovibrational ground state X(1)Sigma(+), v '' = 0, J '' = 0 (J. Deiglmayr et al., Phys. Rev. Lett., 2008, 101, 133004). Here we discuss details of the experimental setup and present a thorough analysis of the photoassociation step including the photoassociation line shape. We predict the distribution of produced ground state molecules using accurate potential energy curves combined with an ab initio dipole transition moment and compare this prediction with experimental ionization spectra. Additionally we improve the value of the dissociation energy for the X(1)Sigma(+) state by high resolution spectroscopy of the vibrational ground state.

@article{deiglmayr_ultracold_2009,
	title = {Ultracold polar molecules in the rovibrational ground state},
	abstract = {Recently, there has been important progress in the formation of   ultracold polar molecules in the rovibrational ground state, thus   opening intriguing perspectives for the investigation of strongly   correlated quantum systems under the influence of long-range dipolar   forces. After an brief introduction into the field of ultracold   molecules, we will review our recent experiments on the formation of   ultracold LiCs molecules in the absolute ground state X 1Sigma +, v=0,   J=0 via a single photo-association step starting from laser-cooled   atoms.},
	journal = {Proceedings of the XIX International Conference on Laser Spectroscopy},
	author = {Deiglmayr, J. and Repp, M. and Grochola, A. and Dulieu, O. and Wester, R. and Weidemuumlller, M.},
	editor = {Katoria, H. and Yoneda, H. and Nakagawa, K. and Shimizu, F.},
	year = {2009},
	note = {INSPEC:11950919},
	keywords = {Cold molecules},
	pages = {235--46},
}

Recently, there has been important progress in the formation of ultracold polar molecules in the rovibrational ground state, thus opening intriguing perspectives for the investigation of strongly correlated quantum systems under the influence of long-range dipolar forces. After an brief introduction into the field of ultracold molecules, we will review our recent experiments on the formation of ultracold LiCs molecules in the absolute ground state X 1Sigma +, v=0, J=0 via a single photo-association step starting from laser-cooled atoms.

@article{grochola_photoassociation_2009,
	title = {Photoassociation spectroscopy of the {B} (1){Pi} state of {LiCs}},
	volume = {131},
	issn = {0021-9606},
	doi = {10.1063/1.3180820},
	abstract = {We present an accurate potential energy curve of the B (1)Pi state in   the LiCs molecule for which vibrational levels between v(')=0 and   v(')=35 (bound by 11.4 GHz) were measured by photoassociation   spectroscopy in an ultracold ensemble of (7)Li and (133)Cs atoms. By the   combination of conventional spectroscopic data of the B-X system and the   new photoassociation measurements a very precise value of the   dissociation energy of the ground state X (1)Sigma(+) of LiCs was   determined to be D(0)=5783.495(5) cm(-1).},
	number = {5},
	journal = {Journal of Chemical Physics},
	author = {Grochola, A. and Pashov, A. and Deiglmayr, J. and Repp, M. and Tiemann, E. and Wester, R. and Weidemueller, M.},
	month = aug,
	year = {2009},
	note = {WOS:000268809800015},
	keywords = {Cold molecules, Precision Measurements},
}

We present an accurate potential energy curve of the B (1)Pi state in the LiCs molecule for which vibrational levels between v(')=0 and v(')=35 (bound by 11.4 GHz) were measured by photoassociation spectroscopy in an ultracold ensemble of (7)Li and (133)Cs atoms. By the combination of conventional spectroscopic data of the B-X system and the new photoassociation measurements a very precise value of the dissociation energy of the ground state X (1)Sigma(+) of LiCs was determined to be D(0)=5783.495(5) cm(-1).

@article{guerout_core_2009,
	title = {Core {Repulsion} {Effects} in {Alkali} {Trimers}},
	volume = {109},
	issn = {0020-7608},
	doi = {10.1002/qua.22304},
	abstract = {The present article is related to a talk presented during the Symposium   on Coherent Control and Ultracold Chemistry held during the Sixth   Congress of the International Society for Theoretical Chemical Physics   (ISTCP-VI, July 2008). The talk was entitled "Electronic structure   properties of alkali dimers and trimers. Prospects for alignment of   ultracold molecules." Here we report on the electrostatic repulsion   forces of the ionic cores at short separation, involved when the   potential energy surfaces of alkali trimers are calculated with a   quantum chemistry approach based on effective large-core potentials for   ionic core description. We demonstrate that such forces in the triatomic   molecule can be obtained as the sum of three pairwise terms. We   illustrate our results on the lowest electronic states of Cs(3), which   are computed for the first time within a full configuration interaction   based on a large Gaussian basis set. As a preliminary section, we also   propose a brief introduction about the importance of alkali trimer   systems in the context of cold and ultracold molecules. (C) 2009 Wiley   Periodicals, Inc. Int J Quantum Chem 109:3387-3398, 2009},
	number = {14},
	journal = {International Journal of Quantum Chemistry},
	author = {Guerout, R. and Soldan, P. and Aymar, M. and Deiglmayr, J. and Dulieu, O.},
	month = nov,
	year = {2009},
	note = {WOS:000270884200020},
	keywords = {Quantum chemistry},
	pages = {3387--3398},
}

The present article is related to a talk presented during the Symposium on Coherent Control and Ultracold Chemistry held during the Sixth Congress of the International Society for Theoretical Chemical Physics (ISTCP-VI, July 2008). The talk was entitled "Electronic structure properties of alkali dimers and trimers. Prospects for alignment of ultracold molecules." Here we report on the electrostatic repulsion forces of the ionic cores at short separation, involved when the potential energy surfaces of alkali trimers are calculated with a quantum chemistry approach based on effective large-core potentials for ionic core description. We demonstrate that such forces in the triatomic molecule can be obtained as the sum of three pairwise terms. We illustrate our results on the lowest electronic states of Cs(3), which are computed for the first time within a full configuration interaction based on a large Gaussian basis set. As a preliminary section, we also propose a brief introduction about the importance of alkali trimer systems in the context of cold and ultracold molecules. (C) 2009 Wiley Periodicals, Inc. Int J Quantum Chem 109:3387-3398, 2009

@article{reetz-lamour_rabi_2008,
	title = {Rabi oscillations between ground and {Rydberg} states and van der {Waals} blockade in a mesoscopic frozen {Rydberg} gas},
	volume = {10},
	issn = {1367-2630},
	url = {http://stacks.iop.org/1367-2630/10/i=4/a=045026},
	doi = {10.1088/1367-2630/10/4/045026},
	abstract = {We present a detailed analysis of our recent observation of synchronous Rabi oscillations between the electronic ground state and Rydberg states in a mesoscopic ensemble containing roughly 100 ultracold atoms (Reetz-Lamour et al submitted, Preprint 0711.4321). The mesoscopic cloud is selected out of a sample of laser-cooled Rb atoms by optical pumping. The atoms are coupled to a Rydberg state with principal quantum number around 30 by a two-photon scheme employing flat-top laser beams. The influence of residual spatial intensity fluctuations as well as sources of decoherence such as redistribution to other states, radiative lifetime and laser bandwidth are analysed. The results open up new possibilities for the investigation of coherent many-body phenomena in dipolar Rydberg gases. As an example we demonstrate the van der Waals blockade, a variant of the dipole blockade, for a mesoscopic atom sample.},
	language = {en},
	number = {4},
	urldate = {2016-08-26},
	journal = {New Journal of Physics},
	author = {Reetz-Lamour, M. and Deiglmayr, J. and Amthor, T. and Weidemüller, M.},
	year = {2008},
	pages = {045026},
}

We present a detailed analysis of our recent observation of synchronous Rabi oscillations between the electronic ground state and Rydberg states in a mesoscopic ensemble containing roughly 100 ultracold atoms (Reetz-Lamour et al submitted, Preprint 0711.4321). The mesoscopic cloud is selected out of a sample of laser-cooled Rb atoms by optical pumping. The atoms are coupled to a Rydberg state with principal quantum number around 30 by a two-photon scheme employing flat-top laser beams. The influence of residual spatial intensity fluctuations as well as sources of decoherence such as redistribution to other states, radiative lifetime and laser bandwidth are analysed. The results open up new possibilities for the investigation of coherent many-body phenomena in dipolar Rydberg gases. As an example we demonstrate the van der Waals blockade, a variant of the dipole blockade, for a mesoscopic atom sample.

@article{reetz-lamour_rabi_2008-1,
	title = {Rabi oscillations and excitation trapping in the coherent excitation of   a mesoscopic frozen {Rydberg} gas},
	volume = {100},
	issn = {0031-9007},
	doi = {10.1103/PhysRevLett.100.253001},
	abstract = {We demonstrate the coherent excitation of a mesoscopic ensemble of about   100 ultracold atoms to Rydberg states by driving Rabi oscillations from   the atomic ground state. We employ a dedicated beam shaping and optical   pumping scheme to compensate for the small transition matrix element. We   study the excitation in a weakly interacting regime and in the regime of   strong interactions. When increasing the interaction strength by pair   state resonances, we observe an increased excitation rate through   coupling to high angular momentum states. This effect is in contrast to   the proposed and previously observed interaction-induced suppression of   excitation, the so-called dipole blockade.},
	number = {25},
	journal = {Physical Review Letters},
	author = {Reetz-Lamour, M. and Amthor, T. and Deiglmayr, J. and Weidemueller, M.},
	month = jun,
	year = {2008},
	note = {WOS:000257230500020},
}

We demonstrate the coherent excitation of a mesoscopic ensemble of about 100 ultracold atoms to Rydberg states by driving Rabi oscillations from the atomic ground state. We employ a dedicated beam shaping and optical pumping scheme to compensate for the small transition matrix element. We study the excitation in a weakly interacting regime and in the regime of strong interactions. When increasing the interaction strength by pair state resonances, we observe an increased excitation rate through coupling to high angular momentum states. This effect is in contrast to the proposed and previously observed interaction-induced suppression of excitation, the so-called dipole blockade.

@article{deiglmayr_formation_2008,
	title = {Formation of ultracold polar molecules in the rovibrational ground state},
	volume = {101},
	issn = {0031-9007},
	doi = {10.1103/PhysRevLett.101.133004},
	abstract = {Ultracold LiCs molecules in the absolute ground state X(1) Sigma(+), nu   '' = 0, J '' = 0 are formed via a single photoassociation step starting   from laser-cooled atoms. The selective production of nu '' = 0, J '' = 2   molecules with a 50-fold higher rate is also demonstrated. The   rotational and vibrational state of the ground state molecules is   determined in a setup combining depletion spectroscopy with   resonant-enhanced multiphoton ionization time-of-flight spectroscopy.   Using the determined production rate of up to 5 x 10(3) molecules/s, we   describe a simple scheme which can provide large samples of externally   and internally cold dipolar molecules.},
	number = {13},
	journal = {Physical Review Letters},
	author = {Deiglmayr, J. and Grochola, A. and Repp, M. and Moertlbauer, K. and Glueck, C. and Lange, J. and Dulieu, O. and Wester, R. and Weidemueller, M.},
	month = sep,
	year = {2008},
	note = {WOS:000259680600024},
	keywords = {Cold molecules},
}

Ultracold LiCs molecules in the absolute ground state X(1) Sigma(+), nu '' = 0, J '' = 0 are formed via a single photoassociation step starting from laser-cooled atoms. The selective production of nu '' = 0, J '' = 2 molecules with a 50-fold higher rate is also demonstrated. The rotational and vibrational state of the ground state molecules is determined in a setup combining depletion spectroscopy with resonant-enhanced multiphoton ionization time-of-flight spectroscopy. Using the determined production rate of up to 5 x 10(3) molecules/s, we describe a simple scheme which can provide large samples of externally and internally cold dipolar molecules.

@article{deiglmayr_calculations_2008,
	title = {Calculations of static dipole polarizabilities of alkali dimers: {Prospects} for alignment of ultracold molecules},
	volume = {129},
	issn = {0021-9606, 1089-7690},
	shorttitle = {Calculations of static dipole polarizabilities of alkali dimers},
	url = {http://scitation.aip.org/content/aip/journal/jcp/129/6/10.1063/1.2960624},
	doi = {10.1063/1.2960624},
	abstract = {The rapid development of experimental techniques to produce ultracold alkali molecules opens the ways to manipulate them and to control their dynamics using external electric fields. A prerequisite quantity for such studies is the knowledge of their static dipole polarizability. In this paper, we computed the variations with internuclear distance and with vibrational index of the static dipole polarizability components of all homonuclear alkali dimers including Fr 2 , and of all heteronuclear alkali dimers involving Li to Cs, in their electronic ground state and in their lowest triplet state. We use the same quantum chemistry approach as in our work on dipole moments [Aymar and Dulieu, J. Chem. Phys.122, 204302 (2005)], based on pseudopotentials for atomic core representation, Gaussian basis sets, and effective potentials for core polarization. Polarizabilities are extracted from electronic energies using the finite-field method. For the heaviest species Rb 2 , Cs 2 , and Fr 2 and for all heteronuclear alkali dimers, such results are presented for the first time. The accuracy of our results on atomic and molecular static dipole polarizabilities is discussed by comparing our values with the few available experimental data and elaborate calculations. We found that for all alkali pairs, the parallel and perpendicular components of the ground state polarizabilities at the equilibrium distance R e scale as ( R e ) 3 , which can be related to a simple electrostatic model of an ellipsoidal charge distribution. Prospects for possible alignment and orientation effects with these molecules in forthcoming experiments are discussed.},
	number = {6},
	urldate = {2016-08-26},
	journal = {The Journal of Chemical Physics},
	author = {Deiglmayr, Johannes and Aymar, Mireille and Wester, Roland and Weidemüller, Matthias and Dulieu, Olivier},
	month = aug,
	year = {2008},
	keywords = {Quantum chemistry},
	pages = {064309},
}

The rapid development of experimental techniques to produce ultracold alkali molecules opens the ways to manipulate them and to control their dynamics using external electric fields. A prerequisite quantity for such studies is the knowledge of their static dipole polarizability. In this paper, we computed the variations with internuclear distance and with vibrational index of the static dipole polarizability components of all homonuclear alkali dimers including Fr 2 , and of all heteronuclear alkali dimers involving Li to Cs, in their electronic ground state and in their lowest triplet state. We use the same quantum chemistry approach as in our work on dipole moments [Aymar and Dulieu, J. Chem. Phys.122, 204302 (2005)], based on pseudopotentials for atomic core representation, Gaussian basis sets, and effective potentials for core polarization. Polarizabilities are extracted from electronic energies using the finite-field method. For the heaviest species Rb 2 , Cs 2 , and Fr 2 and for all heteronuclear alkali dimers, such results are presented for the first time. The accuracy of our results on atomic and molecular static dipole polarizabilities is discussed by comparing our values with the few available experimental data and elaborate calculations. We found that for all alkali pairs, the parallel and perpendicular components of the ground state polarizabilities at the equilibrium distance R e scale as ( R e ) 3 , which can be related to a simple electrostatic model of an ellipsoidal charge distribution. Prospects for possible alignment and orientation effects with these molecules in forthcoming experiments are discussed.

@article{kraft_high-resolution_2007,
	title = {A high-resolution time-of-flight mass spectrometer for the detection of   ultracold molecules},
	volume = {89},
	issn = {0946-2171},
	doi = {10.1007/s00340-007-2855-8},
	abstract = {We have realized a high-resolution time-of-flight mass spectrometer   combined with a magneto-optical trap. The spectrometer enables excellent   optical access to the trapped atomic cloud using specifically devised   acceleration and deflection electrodes. The ions are extracted along a   laser beam axis and deflected onto an off-axis detector. The setup is   applied to detect atoms and molecules photoassociated from ultracold   atoms. The detection is based on resonance-enhanced multi-photon   ionization. Mass resolution up to m/Delta m(rms)=1000 at the mass of   (133)Cs is achieved. The performance of this spectrometer is   demonstrated in the detection of photoassociated ultracold (7)Li(133)Cs   molecules near a large signal of (133)Cs ions.},
	number = {4},
	journal = {Applied Physics B-Lasers and Optics},
	author = {Kraft, S. D. and Mikosch, J. and Staanum, P. and Deiglmayr, J. and Lange, J. and Fioretti, A. and Wester, R. and Weidemueller, M.},
	month = dec,
	year = {2007},
	note = {WOS:000251425100005},
	keywords = {Cold molecules},
	pages = {453--457},
}

We have realized a high-resolution time-of-flight mass spectrometer combined with a magneto-optical trap. The spectrometer enables excellent optical access to the trapped atomic cloud using specifically devised acceleration and deflection electrodes. The ions are extracted along a laser beam axis and deflected onto an off-axis detector. The setup is applied to detect atoms and molecules photoassociated from ultracold atoms. The detection is based on resonance-enhanced multi-photon ionization. Mass resolution up to m/Delta m(rms)=1000 at the mass of (133)Cs is achieved. The performance of this spectrometer is demonstrated in the detection of photoassociated ultracold (7)Li(133)Cs molecules near a large signal of (133)Cs ions.

@article{westermann_dynamics_2006,
	title = {Dynamics of resonant energy transfer in a cold {Rydberg} gas},
	volume = {40},
	issn = {1434-6060, 1434-6079},
	url = {http://link.springer.com/article/10.1140/epjd/e2006-00130-3},
	doi = {10.1140/epjd/e2006-00130-3},
	abstract = {We investigate excitation transfer and migration processes in a cold gas of rubidium Rydberg atoms. Density-dependent measurements of the resonant population exchange for atoms initially excited into the 32P3/2({\textbar}mJ{\textbar}=3/2) state are compared with a Monte Carlo model for coherent energy transfer. The model is based on simulations of small atom subensembles involving up to ten atoms interacting via coherent pair processes. The role of interatomic mechanical forces due to the resonant dipole-dipole interaction is investigated. Good agreement is found between the experimental data and the predictions of the model, from which we infer that atomic motion has negligible influence on the energy transfer up to Rydberg densities of 108 cm-3, that the system has to be described in terms of many-body dynamics, and that the energy transfer preserves coherence on microsecond timescales.},
	language = {en},
	number = {1},
	urldate = {2014-03-25},
	journal = {The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics},
	author = {Westermann, S. and Amthor, T. and Oliveira, A. L. de and Deiglmayr, J. and Reetz-Lamour, M. and Weidemüller, M.},
	month = oct,
	year = {2006},
	pages = {37--43},
}

We investigate excitation transfer and migration processes in a cold gas of rubidium Rydberg atoms. Density-dependent measurements of the resonant population exchange for atoms initially excited into the 32P3/2(\textbarmJ\textbar=3/2) state are compared with a Monte Carlo model for coherent energy transfer. The model is based on simulations of small atom subensembles involving up to ten atoms interacting via coherent pair processes. The role of interatomic mechanical forces due to the resonant dipole-dipole interaction is investigated. Good agreement is found between the experimental data and the predictions of the model, from which we infer that atomic motion has negligible influence on the energy transfer up to Rydberg densities of 108 cm-3, that the system has to be described in terms of many-body dynamics, and that the energy transfer preserves coherence on microsecond timescales.

@article{reetz-lamour_prospects_2006,
	title = {Prospects of ultracold {Rydberg} gases for quantum information processing},
	volume = {54},
	issn = {0015-8208},
	doi = {10.1002/prop.200610318},
	abstract = {We present our experiments on the applicability of ultracold Rydberg   gases for quantum information processing. As quantum computing relies on   entanglement mediated through interparticle interactions, we have   experimentally implemented three approaches to induce ultralong-range   interactions between Rydberg atoms: van-der-Waals interactions,   interactions of permanent dipoles induced by an electric field, and   resonant dipole-dipole interactions (Forster resonances). Advantages and   limitations of the approaches are discussed.},
	number = {8-10},
	journal = {Fortschritte Der Physik-Progress of Physics},
	author = {Reetz-Lamour, Markus and Amthor, Thomas and Deighnayr, Johannes and Westermann, Sebastian and Singer, Kilian and de Oliveira, Andre Luiz and Marcassa, Luis Gustavo and Weidemueller, Matthias},
	month = oct,
	year = {2006},
	note = {WOS:000240242000011},
	keywords = {Review},
	pages = {776--787},
}

We present our experiments on the applicability of ultracold Rydberg gases for quantum information processing. As quantum computing relies on entanglement mediated through interparticle interactions, we have experimentally implemented three approaches to induce ultralong-range interactions between Rydberg atoms: van-der-Waals interactions, interactions of permanent dipoles induced by an electric field, and resonant dipole-dipole interactions (Forster resonances). Advantages and limitations of the approaches are discussed.

@article{deiglmayr_coherent_2006,
	title = {Coherent excitation of {Rydberg} atoms in an ultracold gas},
	volume = {264},
	issn = {0030-4018},
	doi = {10.1016/j.optcom.2006.02.058},
	abstract = {We demonstrate two schemes for the coherent excitation of Rydberg atoms   in an ultracold gas of rubidium atoms employing the three-level ladder   system 5S(1/2)-5P(3/2)-nl(j). In the first approach rapid adiabatic   passage with pulsed laser fields yields Rydberg excitation probabilities   of 90\% in the center of the laser focus. In a second experiment   two-photon Rydberg excitation with continuous-wave fields is applied   which results in Rabi oscillations between the ground and Rydberg state.   The experiments represent a prerequisite for the control of interactions   in ultracold Rydberg gases and the application of ultracold Rydberg   gases for quantum information processing. (c) 2006 Elsevier BV All   rights reserved.},
	number = {2},
	journal = {Optics Communications},
	author = {Deiglmayr, J. and Reetz-Lamour, M. and Amthor, T. and Westermann, S. and de Oliveira, A. L. and Weidemueller, M.},
	month = aug,
	year = {2006},
	pages = {293--298},
}

We demonstrate two schemes for the coherent excitation of Rydberg atoms in an ultracold gas of rubidium atoms employing the three-level ladder system 5S(1/2)-5P(3/2)-nl(j). In the first approach rapid adiabatic passage with pulsed laser fields yields Rydberg excitation probabilities of 90% in the center of the laser focus. In a second experiment two-photon Rydberg excitation with continuous-wave fields is applied which results in Rabi oscillations between the ground and Rydberg state. The experiments represent a prerequisite for the control of interactions in ultracold Rydberg gases and the application of ultracold Rydberg gases for quantum information processing. (c) 2006 Elsevier BV All rights reserved.

@incollection{weidemuller_interactions_2005,
	title = {Interactions in an ultracold gas of rydberg atoms},
	isbn = {978-981-256-659-1},
	url = {https://www.worldscientific.com/doi/abs/10.1142/9789812701473_0027},
	urldate = {2018-06-22},
	booktitle = {Laser {Spectroscopy}},
	publisher = {WORLD SCIENTIFIC},
	author = {Weidemüller, M. and Reetz-Lamour, M. and Amthor, T. and Deiglmayr, J. and Singer, K. and Marcassa, L. G.},
	month = dec,
	year = {2005},
	doi = {10.1142/9789812701473_0027},
	keywords = {Review},
	pages = {264--274},
}