Imaging valence electron structural rearrangement in ammonia using hard x-ray scattering

We report here the observation of the hard x-ray scattering signature of valence electron rearrangement in photoexcited ammonia. While ultrafast hard x-ray scattering has been a powerful tool for imaging structural rearrangement in molecules, it has historically been used primarily to track the motion of the atomic centers. The dominant contribution to the structural information encoded in the total scattering signal usually originates from the tightly bound core electrons around each atom, allowing one to neglect valence electron structure and utilize the independent atom model approximation. We performed an ultrafast hard x-ray scattering experiment to investigate the extent to which valence electron rearrangement contributes to the scattering signal. Gas-phase deuterated ammonia, ND₃, was photoexcited with a 200 nm pump pulse to a 3s Rydberg state and probed with a 10 keV x-ray pulse with sufficient time resolution to observe scattering changes due to the initial photoexcitation, umbrella unbending motion, and subsequent deuterium dissociation dynamics. The use of ultrafast hard x-ray scattering to image the structural rearrangement of single valence electrons constitutes an important advance in the experimental techniques used to study ultrafast photochemistry.