Imaging valence electron rearrangement in a chemical reaction using hard X-ray scattering

TL;DR

This study demonstrates the use of ultrafast hard X-ray scattering to directly image valence electron rearrangement in a photoexcited molecule, revealing electronic and structural dynamics with high sensitivity.

Contribution

It introduces a novel application of time-resolved hard X-ray scattering to observe valence electron dynamics in molecules, overcoming typical core-electron dominance in scattering signals.

Findings

Detected valence electron rearrangement in ammonia after photoexcitation.

Observed structural changes due to deuterium dissociation.

Confirmed sensitivity of scattering signals to valence electron dynamics.

Abstract

We have observed the signatures of valence electron rearrangement in photoexcited ammonia using ultrafast hard X-ray scattering. Time-resolved X-ray scattering is a powerful tool for imaging structural dynamics in molecules because of the strong scattering from the core electrons localized near each nucleus. Such core-electron contributions generally dominate the differential scattering signal, masking any signatures of rearrangement in the chemically important valence electrons. Ammonia represents an exception to the typically high core-to-valence electron ratio. We measured 9.8 keV X-ray scattering from gas-phase deuterated ammonia following photoexcitation via a 200 nm pump pulse to the 3s Rydberg state. We observed changes in the recorded scattering patterns due to the initial photoexcitation and subsequent deuterium dissociation. Ab initio calculations confirm that the observed signal is sensitive to the rearrangement of the single photoexcited valence electron as well as the interplay between adiabatic and nonadiabatic dissociation channels. The use of ultrafast hard X-ray scattering to image the structural rearrangement of single valence electrons constitutes an important advance in tracking valence electronic structure in photoexcited atoms and molecules.