X-ray Coulomb explosion imaging reveals role of molecular structure in internal conversion

TL;DR

This study uses X-ray Coulomb explosion imaging to directly observe the molecular structural changes, specifically deplanarization, during nucleobase photorelaxation, linking geometric rearrangements to electronic dynamics.

Contribution

It introduces a novel application of Coulomb explosion imaging with X-ray free-electron lasers to directly measure molecular geometry changes in real-time during electronic relaxation.

Findings

Deplanarization of the molecule was observed during relaxation.

Protons serve as effective messengers of molecular geometry.

The method links electronic and nuclear dynamics in complex molecules.

Abstract

Molecular photoabsorption results in an electronic excitation/ionization which couples to the rearrangement of the nuclei. The resulting intertwined change of nuclear and electronic degrees of freedom determines the conversion of photoenergy into other molecular energy forms. Nucleobases are excellent candidates for studying such dynamics, and great effort has been taken in the past to observe the electronic changes induced by the initial excitation in a time-resolved manner using ultrafast electron spectroscopy. The linked geometrical changes during nucleobase photorelaxation have so far not been observed directly in time-resolved experiments. Here, we present a study on a thionucleobase, where we extract comprehensive information on the molecular rearrangement using Coulomb explosion imaging. Our measurement links the extracted deplanarization of the molecular geometry to the previously studied temporal evolution of the electronic properties of the system. In particular, the protons of the exploded molecule are well-suited messengers carrying rich information on the molecule's geometry at distinct times after the initial electronic excitation. The combination of ultrashort laser pulses to trigger molecular dynamics, intense X-ray free-electron laser pulses for the explosion of the molecule, and multi-particle coincidence detection opens new avenues for time-resolved studies of complex molecules in the gas phase.