Atomic Trajectories of a Bimolecular Reaction Visualized by Ultrafast Electron Diffraction

29 April 2025, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Bimolecular reactions are ubiquitous in chemistry, but it is exceptionally difficult to resolve the motion of atoms for such processes on the ultrafast timescales 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 into the metastable tetraiodide molecule and the diiodide fragment. We use ultrafast electron diffraction and dynamic 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 timescale. Our results provide a close-up view of the atomic motions involved in the 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 neighbouring triiodide, all within a picosecond.

Supplementary materials

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Supporting information
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The file include extended information on experimental testing of sample conditions, dynamic structure refinement procedure to recover transient atomic structures, and quantum chemical calculations of the excited-state structures in the solid state.
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Supplementary movie
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The movie include frame-by-frame comparison between differential diffraction data, simulated results, and reconstructed transient atomic coordinates.
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