Resolving Competing Conical Intersection Pathways: Time-Resolved X-ray Absorption Spectroscopy of trans-1,3-Butadiene

TL;DR

Time-resolved X-ray absorption spectroscopy (TRXAS) can distinguish between competing conical intersection pathways in photo-excited molecules, exemplified by trans-1,3-butadiene, revealing different electronic structures and dynamics.

Contribution

This study demonstrates the capability of TRXAS to resolve multiple conical intersection pathways and their associated electronic structures in excited-state molecular dynamics.

Findings

TRXAS can differentiate pathways involving different conical intersections.

Distinct electronic structures produce unique XAS signals.

Chemical substitution effects on dynamics can be probed with XAS.

Abstract

Time-resolved X-ray absorption spectroscopy is emerging as a uniquely powerful tool to probe coupled electronic-nuclear dynamics in photo-excited molecules. Theoretical studies to date have established that time-resolved X-ray absorption spectroscopy is an atom-specific probe of excited-state wave packet passage through a seam of conical intersections (CIs). However, in many molecular systems, there are competing dynamical pathways involving CIs of different electronic and nuclear character. Discerning these pathways remains an important challenge. Here, we demonstrate that time-resolved X-ray absorption spectroscopy (TRXAS) has the potential to resolve competing channels in excited-state non-adiabatic dynamics. Using the example of 1,3-butadiene, we show how TRXAS discerns the different electronic structures associated with passage through multiple conical intersections. Trans 1,3-butadiene exhibits a branching between polarized and radicaloid pathways associated with ethylenic "twisted-pyramidalized" and excited-state cis-trans isomerization dynamics, respectively. The differing electronic structures along these pathways give rise to different XAS signals, indicating the possibility of resolving them. Furthermore, this indicates that XAS, and other core-level spectroscopic techniques, offer the appealing prospect of directly probing the effects of selective chemical substitution and its ability to affect chemical control over excited-state molecular dynamics.