Shake-down spectroscopy as state- and site-specific probe of ultrafast chemical dynamics

TL;DR

This paper introduces a novel X-ray spectroscopy method using free-electron lasers to simultaneously probe valence and core electronic states, revealing detailed ultrafast molecular dynamics during photochemical reactions.

Contribution

The study demonstrates that shake-down satellite states in X-ray photoelectron spectra can serve as sensitive probes of electronic and geometric changes in molecules during ultrafast dynamics, providing new insights into pre-dissociation processes.

Findings

Shake-down satellites are highly sensitive to electronic and geometric changes.

The method allows unambiguous assignment of singlet and triplet state contributions.

Spin-selectivity of shake-downs enables tracking of excited state populations.

Abstract

Tracking the multifarious ultrafast electronic and structural changes occurring in a molecule during a photochemical transformation is a challenging endeavor that benefits from recent experimental and computational progress in time-resolved techniques. Measurements of valence electronic states, which provide a global picture of the bonding structure of the molecule, and core electronic states, which provide insight into the local environment, traditionally require different approaches and are often studied separately. Here, we demonstrate that X-ray pulses from a seeded free-electron laser (FEL) enable the measurement of high-resolution, time-resolved X-ray photoelectron spectra (XPS) that capture weak satellite states resulting from shake-down processes in a valence-excited molecule. This approach effectively combines the advantages of both valence- and core-state investigations. We applied this method to investigate photoexcited CS$_2$ molecules, where the role of internal conversion (IC) and intersystem crossing (ISC) in determining the pre-dissociation dynamics is controversial. We present XPS spectra from photoexcited CS$_2$, obtained at the FERMI FEL. High-resolution measurements, compared to the corresponding spectra obtained from accurate multireference quantum chemical calculations, reveal that shake-down satellite channels are highly sensitive to both valence electronic and geometric changes. Previous studies of the pre-dissociation dynamics have led to uncertain assignments of the branching between singlet and triplet excited states. We derive a propensity rule that demonstrates the spin-selectivity of the shake-downs. This selectivity allows us to unequivocally assign contributions from the bright and dark singlet excited states, with populations tracked along the pre-dissociation dynamic pathway.