Atomic Trajectories of a Bimolecular Reaction Visualized by Ultrafast Electron Diffraction

Bimolecular reactions are ubiquitous in chemistry, but it is exceptionally difficult to resolve the motion of atoms for such processes on the ultrafast time scales that the breaking and creation of chemical bonds occur. Detecting small changes in atom positions requires high temporal and spatial resolution and reliable signal-to-noise characteristics. Here, we have exploited solid-state alignment to track a photoinduced bimolecular disproportionation reaction that transforms two pairs of adjacent triiodide anions (I₃^-) into the metastable tetraiodide molecule and the diiodide fragment. We use ultrafast electron diffraction and dynamical structure refinement, where the atomic displacements are directly reconstructed from the diffraction data with plausible restraints on the atomic trajectories starting from the known ground-state structure. The excited-state structures are supported by theoretical calculations in the solid state and the reaction courses show clear site dependence on the ultrafast time scale. Our results provide a close-up view of the atomic motions involved in this classic reaction in real time, from bond elongation to breaking at the parent ion, to collision and subsequent bond formation of the liberated I atom with the neighboring I₃^-, all within a picosecond.