Real-space imaging of valence-electron bonding dynamics with ultrafast hard X-ray scattering.

Chemical reactivity is governed by the time-dependent redistribution of valence electron density, yet ultrafast diffractive imaging probes are typically dominated by scattering from the atoms and/or core electrons and therefore primarily report nuclear motion. Building on recent hard X-ray time-resolved scattering measurements of photoexcited deuterated ammonia, we extend ultrafast X-ray scattering off the evolving valence electron density into a real-space framework by retrieving the time-dependent electron pair density (PD). Using scattering difference signals, we reconstruct transient real-space ΔPDs that directly recover: (i) the prompt redistribution of valence electron density following excitation into a diffuse Rydberg orbital, (ii) the subsequent localization of the electron density on the fragments following N-D bond fission, and (iii) distinct electronic-structure fingerprints of ballistic dissociation channels. These ΔPDs provide an intuitive, chemically interpretable measure of bond breaking and rearrangement by tracking the emergence, displacement, and loss of electron density between atomic centers while retaining femtosecond time resolution. Comparison with ab initio simulations guides the interpretation of the recovered ΔPDs and supports the inferred pathway assignments.