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

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.