Imaging Purely Nuclear Quantum Dynamics in Molecules by Combined X-ray and Electron Diffraction.

Monitoring the motions of atoms and molecules in the course of chemical processes is a central goal of femtochemistry. Optical spectroscopic signals are usually sensitive to electronic properties such as dipoles, polarizabilities, and electronic charge densities rather than to nuclear motions. In this theoretical study, we propose a novel measurement that solely and directly monitors the evolving nuclear wave packet and can thereby unambiguously image photochemical events in real time. We demonstrate how nuclear charge densities can be singled out by subtracting the ultrafast gas-phase X-ray and electron diffraction signals in the photodissociation of thiophenol as it passes through two conical intersections. This signal can reveal the shape and trajectory of the nuclear wave packets as well as the electronic coherences in the vicinity of conical intersections.