A time-resolved X-ray solution scattering study of bromine molecules in CCl4is presented as an example of how to track atomic motions in a simple chemical reaction. The structures of the photoproducts are tracked during the recombination process, geminate and non-geminate, from 100 ps to 10 µs after dissociation. The relaxation of hot Br2*molecules heats the solvent. At early times, from 0.1 to 10 ns, an adiabatic temperature rise is observed, which leads to a pressure gradient that forces the sample to expand. The expansion starts after about 10 ns with the laser beam sizes used here. When thermal artefacts are removed by suitable scaling of the transient solvent response, the excited-state solute structures can be obtained with high fidelity. The analysis shows that 30% of Br2*molecules recombine directly along theXpotential, 60% are trapped in theA/A′ state with a lifetime of 5.5 ns, and 10% recombine non-geminatelyviadiffusive motion in about 25 ns. The Br—Br distance distribution in theA/A′ state peaks at 3.0 Å.
Determination of Excited State Molecular Structures from Time-Resolved Gas-Phase X-Ray Scattering
