Atomically Resolved Trajectories of a Bimolecular Photochemical Reaction

Abstract Bimolecular processes are ubiquitous in chemistry but incredibly difficult to study with the spatio-temporal resolution required to record molecular movies. Mechanistic studies on ultrafast timescales have been carried out predominantly in the liquid state, either by spectroscopic1,2 or structural3 methods, but this approach is inherently limited to the picosecond time domain due to the wide distribution of orientations and positions of the reactant molecules, causing timing uncertainty between collisional encounters that cannot be disentangled4. The prospect of atomically resolving bimolecular collision trajectories would seem out of reach as it demands not just the utmost in terms of temporal and spatial resolution, but also strong signal to detect small changes and well-defined initial states to ensure minimal signal overlap in space or time that can blur out collisional details. Here, we exploit solid-state spatial alignment to track a photoinduced bimolecular disproportionation reaction involving two pairs of adjacent triiodide molecular anions (I3-) using femtosecond electron diffraction5,6. We succeeded in reconstructing atomic coordinates from the diffraction data, uncovering the real-space transfer of iodine liberated by photodissociation of I3- and captured by its nearest neighbour, yielding a metastable tetraiodide species (I4−) within one picosecond.