Ultrafast X-ray spectroscopy of conical intersections

TL;DR

This paper demonstrates how ultrafast X-ray spectroscopy can directly observe the complex electronic and nuclear dynamics of molecules near conical intersections, revealing charge localization effects during excited state processes.

Contribution

It introduces a novel application of time-resolved X-ray absorption and photoelectron spectroscopy to study conical intersections in molecules, supported by high-level quantum dynamics simulations.

Findings

TRXAS is highly sensitive to molecular dynamics near conical intersections

X-ray spectroscopies reveal charge localization effects during excited state evolution

Ultrafast X-ray techniques can directly observe non-adiabatic molecular processes

Abstract

Ongoing developments in ultrafast X-ray sources offer powerful new means of probing the com- plex non-adiabatically coupled structural and electronic dynamics of photoexcited molecules. These non-Born-Oppenheimer effects are governed by general electronic degeneracies termed conical in- tersections which play a key role, analogous to that of a transition state, in the electronic-nuclear dynamics of excited molecules. Using high level ab initio quantum dynamics simulations, we studied time-resolved X-ray absorption and photoelectron spectroscopy (TRXAS and TRXPS, respectively) of the prototypical unsaturated organic chromophore, ethylene, following excitation to its S2 state. The TRXAS in particular is highly sensitive to all aspects of the ensuing dynamics. These X-ray spectroscopies provide a clear signature of the wavepacket dynamics near conical intersections, related to charge localization effects driven by the nuclear dynamics. Given the ubiquity of charge localization in excited state dynamics, we believe that ultrafast X-ray spectroscopies offer a unique and powerful route to the direct observation of dynamics around conical intersections.