Photoelectron asymmetry

Figure 6. The measured phase lag of the photoelectron asymmetry parameter for the 6,v2.S i /2 and the continua of Ba. (Reproduced with permission from Ref. 73, Copyright 2007...

Figure 10. K-shell photoelectron asymmetry parameters in N2s present results with a relaxed ion core shifted down in photon energy only by the same amount as the shift in the corresponding cross section in Figure 8 , synchrotron radiation data of Reference 31.

Figure 8.22 Photoion vibrational branching fractions (left side) and photoelectron asymmetry parameters (right side) for CO+ X2S+ = 0 — 2 produced by photoionization of CO X1 E+ v = 0 (Hardis, et al., 1988). The ft parameters are obtained from the photoelectron spectra measured simultaneously at two observation angles. The heavy lines are the experimental values and the light lines are the MQDT results from Leyh and Ra eev (1988), convoluted to the experimental energy resolution. The peak at 17.02 eV corresponds to the 3p7r i>+ = 0 Rydberg state converging to the CO+ B2E+ state.

Photoion vibrational branching fractions and photoelectron asymmetry parameters for CO+ X2E+. 

In addition to primary features from copper in Eig. 2.7 are small photoelectron peaks at 955 and 1204 eV kinetic energies, arising from the oxygen and carbon Is levels, respectively, because of the presence of some contamination on the surface. Secondary features are X-ray satellite and ghost lines, surface and bulk plasmon energy loss features, shake-up lines, multiplet splitting, shake-off lines, and asymmetries because of asymmetric core levels [2.6]. 

This forward-backward asymmetry of the photoelectron distribution, expected when a randomly oriented sample of molecular enantiomers is ionized by circularly polarized light, is central to our discussion. The photoelectron angular... 

Figure 2. Photoelectron chiral asymmetry factor, y, obtained as a function of electron kinetic energy at hv = 21.2 eV for the (R)- and (S)- enantiomers of glycidol. Also included is a moderate resolution photoelectron spectrum recorded under identical conditions. Data from Refs. [37, 38].

Figure 15. Circular dichroism of the C=0 C li peak (BE = 292.7 eV) in fenchone at three different photon energies, indicated, (a) Photoelectron spectrum of the carbonyl peak of the (1S,4R) enantiomer, recorded with right (solid line) and left (broken line) circularly polarized radiation at the magic angle, 54.7° to the beam direction, (b) The circular dichroism signal for fenchone for (1R,4A)-fenchone (x) and the (lS,41 )-fenchone (+) plotted as the raw difference / p — /rep of the 54.7° spectra, for example, as in the row above, (c) The asymmetry factor, F, obtained by normalizing the raw difference. In the lower rows, error bars are included, but are often comparable to size of plotting symbol (l/ ,4S)-fenchone (x), (lS,4R)-fenchone (+). Data are taken from Ref. [38],...

The experimental work described in this chapter clearly demonstrates that chiral asymmetries in the forward-backward distribution of photoelectrons emitted from randomly oriented enantiomers when ionized with circularly polarized light can be spectacularly large (to borrow and apply a superlative from previous accounts of an unprecedented chiral asymmetry)—on the order of 20%. The theory discussed here, as implemented in two computational methods, is fully capable of predicting this and being applied to develop an understanding of a phenomenon that at times displays some counterintuitive properties. Doing so is very much an ongoing quest. 

The latter equation assumes a 100% linearly polarized ionizing radiation, a is the fine structure constant, Nni is the number of electrons in a nl subshell, Dni->ei l is a radial dipole photoionization amplitude, fini is the dipole photoelectron angular asymmetry parameter, and A i2 is the electric dipole-quadrupole interference term arising due to the correction term ikr in the above expression for Mab,... 

Figure 6 Nondipole asymmetry parameter xis(e) as a function of the photoelectron energy s for the Ne Is photoionization from e Cgo calculated [36] in two different approximations 1, the 5-potential model. 2, 3 and 4, the A-potential model with the following values for Rgq, f/gQ and A (all in au) ...

As an illustration, calculated data [36] for the Ne Is, 2s and 2p photoelectron angular asymmetry parameters from Ne C60, both in the 5- and A-potential models, are depicted in Figures 6-8. Note, the dipole parameter /3ns = 2,... 

The discovery of confinement resonances in the photoelectron angular distribution parameters from encaged atoms may shed light [36] on the origin of anomalously high values of the nondipole asymmetry parameters observed in diatomic molecules [62]. Following [36], consider photoionization of an inner subshell of the atom A in a diatomic molecule AB in the gas phase, i.e., with random orientation of the molecular axis relative to the polarization vector of the radiation. The atom B remains neutral in this process and is arbitrarily located on the sphere with its center at the nucleus of the atom A with radius equal to the interatomic distance in this molecule. To the lowest order, the effect of the atom B on the photoionization parameters can be approximated by the introduction of a spherically symmetric potential that represents the atom B smeared over... 

Figure 11 Nondipole photoelectron angular asymmetry parameter y A ( >) for Is photoionization of free Ne, Ne from neutral Ne Cgo as well as Ne from the fullerene anion Ne q0 (z < 0) [28], as indicated.

Figure 12 Ne Is nondipole photoelectron angular asymmetry parameter y A(a>) for the Ne C605 anion along with the dipole D and quadrupole Q amplitudes and cos A (A = 2 — ti) [28].

Calculated results for the Ar nondipole Is photoelectron angular-asymmetry parameter yis(< ) for free Ar, Ar C60, Ar C60 (-240 and Ar C6o C240 C540 are shown in Figure 15 (the nondipole parameter Sns vanishes, by definition, and the dipole parameter fins = 2, i.e., is constant, nonrelativistically for the photoionization of ns-states). 

Figure 15 Calculated results [32] for a nondipole photoelectron angular-asymmetry parameter Ki aM for the Is photoionization of Ard>C6o, Ar Cgo C24o and Ar Cgo C24o C54o, as indicated, as well as of free Ar (dashed line).

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