Angular Distributions of 1sσ Photoelectrons from Fixed-in-Space N2. Shigemasa, E, Adachi, J, Oura, M, & Yagishita, A Physical Review Letters, 74(3):359–362, January, 1995. tex.ids= Shigemasa1995
Angular Distributions of 1sσ Photoelectrons from Fixed-in-Space N2 [link]Paper  doi  abstract   bibtex   
The angular distributions of 1so-photoelectrons from N2 molecules held fixed in space have been measured around the or* shape resonance for the first time, The angular distributions have been very rich in structure, which are completely different from usual photoelectron angular distributions from randomly oriented molecules, as predicted by Dill. The orbital angular momentum properties of the 1scr photoelectrons around the o. * shape resonance have been made clear from the angular distribution patterns. PACS numbers: 33.80.Eh, 33.90.+h Molecular K-shell spectra are known to depart remark-ably from corresponding atomic spectra [1]. Specifically, above the K edge there is a broad band of enhanced pho-toabsorption in place of the smooth monotonic decrease one might expect. The novel feature, so called shape res-onance, results from the interaction between the photo-electron escaping from the K shell and the anisotropic molecular field [2]. Direct study of the orbital angular momentum properties of the photoelectrons in the shape resonance is not pos-sible by conventional gas-phase photoelectron angular distribution studies, owing to the random orientations of the molecules. If, however, the molecules have a definite orientation, the angular distributions of photoelectrons ejected by the electric-dipole interaction has the general form [3] 2~max g ~KM' KM(0 tt') @=0 M where the angles (0, tb) are measured from the molecule g axis, I " is the maximum orbital angular momentum component of the outgoing photoelectron, and Y~M are the spherical harmonics. Thus, such photoelectron angular distributions of fixed-in-space molecules can offer a direct probe of the orbital angular momentum composition of the molecular photoelectrons. In the limited case, the angular distribution of photoelectrons from oriented H2 [4], CO [5], and N2 and CO [6] molecules were predicted by three groups. The present experiment has been undertaken to study the orbital angular momentum properties of the photo-electrons in a prototype molecular shape resonance — the o. * shape resonance of nitrogen molecules. In this Let-ter, we present the first experimental results on the an-gular distributions of lsd-photoelectrons ejected from Nz molecules held fixed in space. The photoionization pro-cesses of fixed-in-space molecules in a gas phase can be realized by detecting photoelectrons in coincidence with fragment ions as reported by Golovin et al. [7,8]. Since the dissociation time of the molecular ions produced by a subsequent Auger decay of the K-shell vacancy is much shorter than the molecular rotation period, the emission direction of the fragment ion is considered to be equiva-lent to the molecular orientation at a moment of absorp-tion of a photon. The experiments have been carried out on beam line BL-28, supplying the synchrotron radiation emitted from an undulator [9] inserted in the 2.5 GeV positron stor-age ring at the Photon Factory. The undulator radia-tion, monochromatized with a 10 m grazing incidence monochromator [10], was focused onto an effusive beam of the sample gas at the center of the ionization region of the experimental chamber. The entrance and exit slit openings of the monochromator were both set to 50 p, m. The expected energy resolution was about 0.6 eV at h p = 400 eV. The diameter of the photon beam spot at the sam-ple position was less than 0.2 mm. The first harmonic of the undulator radiation was used for all measurements de-scribed here. It had been confirmed in our previous work that the degree of linear polarization within the first har-monic peak of the undulator radiation is more than 95% [11]. Two identical ion detectors (channeltrons) with re-tarding grids were installed in the chamber at 0 and 90 relative to the polarization vector of the incident radiation in the plane perpendicular to the radiation path. A re-tarding potential of +3 V was applied to the grid of each detector to select energetic fragment ions only. Each de-tector has a collection half-angle of 10.5 . A parallel plate electrostatic analyzer with a position sensitive detector (a microchannel plate and a resistive anode), which can be rotated around the photon beam axis, was used to energy analyze photoelectrons [11]. The effective acceptance an-gle of the analyzer was estimated to be ~5 . To obtain coincidence signals between photoelectrons and fragment ions, event signals from the electron detector were fed into two time-to-amplitude cogverters to start them at the same time, and signals from one ion detector were used to stop one of the converters and signals from the other detector to stop the other. As demonstrated in our previous work [11 — 19], the en-ergetic fragment-ion angular distributions after the K-shell excitations of diatomic molecules induced by linearly polarized radiation have enabled us to decompose the conventional photoabsorption spectra into their molecular 0031-9007/95/74(3)/359(4)$06. 00 1995 The American Physical Society 359
@article{Shigemasa1995,
	title = {Angular {Distributions} of 1sσ {Photoelectrons} from {Fixed}-in-{Space} {N2}},
	volume = {74},
	issn = {0031-9007},
	url = {https://link.aps.org/doi/10.1103/PhysRevLett.74.359},
	doi = {10.1103/PhysRevLett.74.359},
	abstract = {The angular distributions of 1so-photoelectrons from N2 molecules held fixed in space have been measured around the or* shape resonance for the first time, The angular distributions have been very rich in structure, which are completely different from usual photoelectron angular distributions from randomly oriented molecules, as predicted by Dill. The orbital angular momentum properties of the 1scr photoelectrons around the o. * shape resonance have been made clear from the angular distribution patterns. PACS numbers: 33.80.Eh, 33.90.+h Molecular K-shell spectra are known to depart remark-ably from corresponding atomic spectra [1]. Specifically, above the K edge there is a broad band of enhanced pho-toabsorption in place of the smooth monotonic decrease one might expect. The novel feature, so called shape res-onance, results from the interaction between the photo-electron escaping from the K shell and the anisotropic molecular field [2]. Direct study of the orbital angular momentum properties of the photoelectrons in the shape resonance is not pos-sible by conventional gas-phase photoelectron angular distribution studies, owing to the random orientations of the molecules. If, however, the molecules have a definite orientation, the angular distributions of photoelectrons ejected by the electric-dipole interaction has the general form [3] 2{\textasciitilde}max g {\textasciitilde}KM' KM(0 tt') @=0 M where the angles (0, tb) are measured from the molecule g axis, I " is the maximum orbital angular momentum component of the outgoing photoelectron, and Y{\textasciitilde}M are the spherical harmonics. Thus, such photoelectron angular distributions of fixed-in-space molecules can offer a direct probe of the orbital angular momentum composition of the molecular photoelectrons. In the limited case, the angular distribution of photoelectrons from oriented H2 [4], CO [5], and N2 and CO [6] molecules were predicted by three groups. The present experiment has been undertaken to study the orbital angular momentum properties of the photo-electrons in a prototype molecular shape resonance — the o. * shape resonance of nitrogen molecules. In this Let-ter, we present the first experimental results on the an-gular distributions of lsd-photoelectrons ejected from Nz molecules held fixed in space. The photoionization pro-cesses of fixed-in-space molecules in a gas phase can be realized by detecting photoelectrons in coincidence with fragment ions as reported by Golovin et al. [7,8]. Since the dissociation time of the molecular ions produced by a subsequent Auger decay of the K-shell vacancy is much shorter than the molecular rotation period, the emission direction of the fragment ion is considered to be equiva-lent to the molecular orientation at a moment of absorp-tion of a photon. The experiments have been carried out on beam line BL-28, supplying the synchrotron radiation emitted from an undulator [9] inserted in the 2.5 GeV positron stor-age ring at the Photon Factory. The undulator radia-tion, monochromatized with a 10 m grazing incidence monochromator [10], was focused onto an effusive beam of the sample gas at the center of the ionization region of the experimental chamber. The entrance and exit slit openings of the monochromator were both set to 50 p, m. The expected energy resolution was about 0.6 eV at h p = 400 eV. The diameter of the photon beam spot at the sam-ple position was less than 0.2 mm. The first harmonic of the undulator radiation was used for all measurements de-scribed here. It had been confirmed in our previous work that the degree of linear polarization within the first har-monic peak of the undulator radiation is more than 95\% [11]. Two identical ion detectors (channeltrons) with re-tarding grids were installed in the chamber at 0 and 90 relative to the polarization vector of the incident radiation in the plane perpendicular to the radiation path. A re-tarding potential of +3 V was applied to the grid of each detector to select energetic fragment ions only. Each de-tector has a collection half-angle of 10.5 . A parallel plate electrostatic analyzer with a position sensitive detector (a microchannel plate and a resistive anode), which can be rotated around the photon beam axis, was used to energy analyze photoelectrons [11]. The effective acceptance an-gle of the analyzer was estimated to be {\textasciitilde}5 . To obtain coincidence signals between photoelectrons and fragment ions, event signals from the electron detector were fed into two time-to-amplitude cogverters to start them at the same time, and signals from one ion detector were used to stop one of the converters and signals from the other detector to stop the other. As demonstrated in our previous work [11 — 19], the en-ergetic fragment-ion angular distributions after the K-shell excitations of diatomic molecules induced by linearly polarized radiation have enabled us to decompose the conventional photoabsorption spectra into their molecular 0031-9007/95/74(3)/359(4)\$06. 00 1995 The American Physical Society 359},
	number = {3},
	urldate = {2016-11-23},
	journal = {Physical Review Letters},
	author = {Shigemasa, E and Adachi, J and Oura, M and Yagishita, A},
	month = jan,
	year = {1995},
	note = {tex.ids= Shigemasa1995},
	pages = {359--362},
}
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