Theoretical description of circular dichroism in photoelectron angular distributions of randomly oriented chiral molecules after multi-photon photoionization

TL;DR

This paper develops a theoretical framework for understanding circular dichroism in photoelectron angular distributions of chiral molecules after multi-photon ionization, aligning well with experimental observations.

Contribution

It introduces a novel theoretical model combining perturbation theory and ab initio calculations to describe circular dichroism in multi-photon photoionization of chiral molecules.

Findings

Model reproduces symmetry behavior under handedness and helicity exchange

Semi-quantitative agreement with experimental data for fenchone and camphor

D wave character of excited state is crucial for results

Abstract

Photoelectron circular dichroism refers to the forward/backward asymmetry in the photoelectron angular distribution with respect to the propagation axis of circularly polarized light. It has recently been demonstrated in femtosecond multi-photon photoionization experiments with randomly oriented camphor and fenchone molecules [C. Lux et al., Angew. Chem. Int. Ed. 51, 5001 (2012);C. S. Lehmann et al., J. Chem. Phys. 139, 234307 (2013)]. A theoretical framework describing this process as (2+1) resonantly enhanced multi-photon ionization is constructed, which consists of two-photon photoselection from randomly oriented molecules and successive one-photon ionisation of the photoselected molecules. It combines perturbation theory for the light-matter interaction with ab initio calculations for the two-photon absorption and a single-center expansion of the photoelectron wavefunction in terms of hydrogenic continuum functions. It is verified that the model correctly reproduces the basic symmetry behavior expected under exchange of handedness and light helicity. When applied it to fenchone and camphor, semi-quantitative agreement with the experimental data is found, for which a sufficient d wave character of the electronically excited intermediate state is crucial.