Vibrationally induced inversion of photoelectron forward-backward asymmetry in chiral molecule photoionization by circularly polarized light

Abstract

Electron–nuclei coupling accompanying excitation and relaxation processes is a fascinating phenomenon in molecular dynamics. A striking and unexpected example of such coupling is presented here in the context of photoelectron circular dichroism measurements on randomly oriented, chiral methyloxirane molecules, unaffected by any continuum resonance. Here, we report that the forward-backward asymmetry in the electron angular distribution, with respect to the photon axis, which is associated with photoelectron circular dichroism can surprisingly reverse direction according to the ion vibrational mode excited. This vibrational dependence represents a clear breakdown of the usual Franck–Condon assumption, ascribed to the enhanced sensitivity of photoelectron circular dichroism (compared with other observables like cross-sections or the conventional anisotropy parameter-β) to the scattering phase off the chiral molecular potential, inducing a dependence on the nuclear geometry sampled in the photoionization process. Important consequences for the interpretation of such dichroism measurements within analytical contexts are discussed.

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

“…Future developments are likely to introduce electron correlations into the N--1 electron potential model, and the current experimental data could serve as a benchmark for judging performance of such developments. Equally, there is accumulating evidence of the need to take fuller account of vibrational motions in refining out understanding of PECD 21 , something which is absent from the current capabilities of fixed--geometry computations.…”

“…However, unlike the usual cross section σ or the asymmetry parameter β, this new observable has a pure quantum origin in the scattering of the outgoing electron by an intrinsically chiral potential, being also fully dependent on the sine of the relative phase between adjacent outgoing partial wave continuum functions 5 . This phase dependence makes b1 a very sensitive probe of the whole molecular potential, much more so than σ or β, with a demonstrated dependence on conformers 11--15 , chemical substitution 16,17 , dimerization 18 and clustering 19,20 , and even vibrational--dynamics 21,22 .…”

Identifying and Understanding Strong Vibronic Interaction Effects Observed in the Asymmetry of Chiral Molecule Photoelectron Angular Distributions

“…[11,15] There remains, in any case, little evidence to indicate notable vibrational influence on molecular PADs in regions removed from continuum resonances, other than a near threshold, K-edge study of fixed-in-space CO. [16,17] Set against this background, a suggestion of a vibrational dependency in the non-resonant outer valence electron PAD of methyloxirane [18] was interesting and stimulated a further PECD investigation. [19] This latter study provided the unexpected and surprising result that excitation of a specific vibrational mode could provoke a complete reversal in the PAD forward-backward emission asymmetry (switching from -6% to +6%) near to threshold. Furthermore, this reversal appeared to be part of a wider pattern of similarly unexpected, large magnitude variations in the chiral PAD parameters associated with other vibrational modes that could be observed at a higher photon energy.…”

Determining the polarization state of an extreme ultraviolet free-electron laser beam using atomic circular dichroism

“…Accordingly, CD in photoemission from solids has been widely used for the investigation of local magnetization and other properties of solids and surfaces (for example, refs 10,11). In atomic and molecular physics, circularly polarized photon beams have been applied for example in studies of electron spin-related phenomena and its polarization control 12,13 as well as of symmetry breaking in chiral molecules 14,15 . With FERMI many experimental techniques, already well established for applications of CD using synchrotron radiation, high order harmonics or optical lasers [16][17][18][19][20][21] , can now be extended to studies at FEL sources.…”