General theory of photoexcitation induced photoelectron circular dichroism

The photoionization of chiral molecules prepared in a coherent superposition of excited states can give access to the underlying chiral coherent dynamics in a procedure known as photoexcitation induced photoelectron circular dichroism (PXECD). As in photoelectron circular dichroism (PECD), chirality manifests as asymmetric photoelectron emission in the forward/backward direction (relative to the laser propagation direction). However, in PXECD, the asymmetric photoemission is additionally contingent on coherence. This exclusive dependence on coherence can also be seen in a different part of the photoelectron angular distribution (PAD), where it is not contingent on the chirality of the molecule, thus allowing extension of PXECD's sensitivity to tracking coherence to non-chiral molecules. Here we present a general theory of PXECD based on angular momentum algebra and derive explicit expressions for all pertinent asymmetry parameters which arise for the arbitrary polarization of pump (which prepares the superposition of excited states) and ionizing probe pulses. The theory is developed in a way that clearly and simply separates chiral and non-chiral contributions to the PAD and also demonstrates how PXECD and PECD-type contributions, which may be distinguished by whether the pump or ionizing probe pulse enables chiral response, are mixed when arbitrary polarization is used.