A Valence Photoelectron Imaging Investigation of Chiral Asymmetry in the Photoionization of Fenchone and Camphor

Abstract

Photoelectron circular dichroism (PECD) is investigated in the valence ionization of selected fenchone enantiomers using a photoelectron imaging technique and circularly polarized synchrotron radiation. Theoretical modelling of the results using electron scattering calculations demonstrates that the observed chiral asymmetry in the photoelectron angular distributions depends strongly upon the final state scattering, and upon the quality of the molecular potential used for these calculations. However, very pronounced dependence on the orbital from which ionization occurs is also observed. Comparison with analogous results previously obtained for camphor reveals striking differences in the PECD, even when the ionizing orbitals are themselves left substantially unaffected by the changes in methyl groups' substitution site. PECD measurements readily differentiate these molecules despite their very similar photoelectron spectra, demonstrating PECD to be a structurally sensitive probe.

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.…”

“…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 .…”

“…6) is more complex since due to our limited electron energy resolution and to the congestion of the PES, what we labeled as "HOMO--1" is in fact a mixture of several closely lying electronic orbitals such as the one labeled 39, 40 and 41 in Ref. 40 . In consequence the b1 value observed in the A band (10.05-10.61 eV binding energy range) represents a blend of these individual ionizations.…”

“…S2. In this respect, Powis et al 40 already commented on the different PECD values measured on camphor and fenchone, albeit in an energy range restricted to electron KE > 4 eV for the HOMO orbital.…”

Ion spectroscopy: Where did it come from; where is it now; and where is it going?

“…A unique feature of the French synchrotron, Soleil, is its crossed undulators that permit the generation of circularly polarized light, which can oscillate between left and right in about 20 s. However, soon this will be upgraded to a switching time of 0.2 s. As shown by Powis and Nahon at the conference, this polarization switching ability has been used to collect UV and VUV circular dichroism spectra of optically active molecules [20,21]. These experiments are carried out at fixed UV or VUV photon energies while collecting angularly resolved photoelectron spectra on an electron imaging detector.…”