Determination of chiral asymmetries in the valence photoionization of camphor enantiomers by photoelectron imaging using tunable circularly polarized light

Abstract

An electron imaging technique has been used to study the full angular distribution of valence photoelectrons produced from enantiomerically pure molecular beams of camphor when these are photoionized with circularly polarized light. In addition to the familiar beta parameter, this provides a new chiral term, taking the form of an additional cosine function in the angular distribution which consequently displays a forward-backward electron ejection asymmetry. Several ionization channels have been studied using synchrotron radiation in the 8.85-26 eV photon energy range. With alternating left and right circularly polarized radiations the photoelectron circular dichroism (PECD) in the angular distribution can be measured and shows some strong dynamical variations with the photon energy, depending in sign and intensity on the ionized orbital. For all orbitals the measured PECD has a quite perfect antisymmetry when switching between R and S enantiomers, as expected from theory. In the HOMO(-1) channel the PECD chiral asymmetry curves show a double maxima reaching nearly 10% close to threshold, and peaking again at approximately 20% some 11 eV above threshold. This is attributed to a resonance that is also visible in the beta parameter curve. Newly optimized CMS-Xalpha photoionization dynamics calculations are also presented. They are in reasonably good agreement with the experimental data, including in the very challenging threshold regions. These calculations show that PECD in such randomly oriented samples can be understood in the electric dipole approximation and that, unlike the case pertaining in core-shell ionization-where a highly localized achiral initial orbital means that the dichroism arises purely as a final state scattering effect-in valence shell ionization there is a significant additional influence contributed by the initial orbital density.

Determination of accurate electron chiral asymmetries in fenchone and camphor in the VUV range: sensitivity to isomerism and enantiomeric purity

“…Conversely, to obtain the PECD of the deeper--lying orbitals, we have obtained the electron images correlated In addition, the double imaging coincidence scheme permits removing common background impurities such as water, N2, O2, which would increase the achiral content in the total signal used for normalization and therefore decrease the observed PECD. Overall, the procedure dramatically increases the signal--to--noise ratio with respect to previous non--coincident data 36,40 recorded for camphor and fenchone, as evidenced with the low error bars and excellent mirror behavior. Indeed, in Figure 1c the PECD baseline, BL, barely deviates from the ideal zero value over the whole kinetic energy range, with a weighted mean value of 0.0003 ± 0.0005.…”

“…In the higher resolution PES recorded by Rennie et al 51 with a hemispherical analyzer the electronic band structure is slightly more marked and seen to correlate with the PECD changes. The OVGF calculations published by Rennie et al also correspond to the observed oscillations, which highlights the enhanced sensitivity of PECD to electronic structure when compared to usual observables such as ionization cross--sections -even in the case of congested electronic bands as already observed in camphor 36 and glycidol 13 .…”

“…The images are then summed according to the light helicity to provide a single pair composed of the left (LCP) and right (RCP) circularly polarized measurements. The PES is then obtained by Abel inversion of the LCP+RCP image with a kinetic energy resolution of 3% for the fastest electrons, while the b0b1 term, where b0 is the total cross--section of ionization of a given electronic band as given by the integral of the PES, is obtained from the LCP--RCP difference, leading to the final PECD, 2b1, through normalization by b0 (see 36 for a complete description of the procedure). The statistical error bars are given as the standard error on the principle that each image pixel acts as an independent counter that follows a Poisson distribution, with the associated error properly propagated through all subsequent operations.…”

“…Thus, by filtering the electron images according to the coincident parent ion mass, the electrons ejected from the HOMO can be better isolated. This becomes 36 extremely useful at high photon energies where the moderate resolution of the VMI leads to overlap between the HOMO and the inner orbital bands. Figure 2 illustrates the result of applying the filtering for the parent ions at hν=12.3 eV, and can be directly compared to Fig.…”

“…This is one of the motivations for the present study, which provides for the first time complete and very accurate measurements of b1 parameters for the two outermost orbitals of camphor and fenchone up to hν 16 eV. Indeed, in the past some of us 34,36,40 and other authors 35 published some data for these molecules but, because of technical SR light source limitations at the time, there were uncovered regions of the spectrum -notably in the crucial 10 to 15 eV photon energy range for which theoretical models 36,41 predict large oscillations of b1. The benchmarking of theory versus experiment, in this specific range, with very accurate data obtained with a state--of--the--art double imaging electron/ion coincidence (i 2 PEPICO) spectrometer set--up and quasi--perfect CPL is the second motivation of the present study, especially in a context where multi--photon/high field PECD modeling has just been initiated on these showcase molecules 42,43 .…”