Mapping the Complete Reaction Path of a Complex Photochemical Reaction

TL;DR

This study uses femtosecond extreme ultraviolet photoelectron spectroscopy to map the entire reaction pathway of dissociating CS$_2$ molecules, revealing the dominance of triplet products and the role of internal conversion and intersystem crossing.

Contribution

It provides a comprehensive experimental and theoretical analysis of the full reaction pathway, including potential energy curves and dynamics of singlet and triplet states.

Findings

Triplet state products dominate the final yield.

Singlet dissociation occurs but is a minor pathway.

Internal conversion stabilizes singlet populations and facilitates intersystem crossing.

Abstract

We probe the dynamics of dissociating CS$_2$ molecules across the entire reaction pathway upon excitation. Photoelectron spectroscopy measurements using laboratory-generated femtosecond extreme ultraviolet pulses monitor the competing dissociation, internal conversion, and intersystem crossing dynamics. Dissociation occurs either in the initially excited singlet manifold or, via intersystem crossing, in the triplet manifold. Both product channels are monitored and show that, despite being more rapid, the singlet dissociation is the minor product and that triplet state products dominate the final yield. We explain this by a consideration of accurate potential energy curves for both the singlet and triplet states. We propose that rapid internal conversion stabilizes the singlet population dynamically, allowing for singlet-triplet relaxation via intersystem crossing and the efficient formation of spin-forbidden dissociation products on longer timescales. The study demonstrates the importance of measuring the full reaction pathway for defining accurate reaction mechanisms.