Photochemistry of HgBr₂ in methanol investigated using time-resolved X-ray liquidography

We investigate the photoinduced dissociation of HgBr₂ in methanol and the ensuring structural dynamics of the photo-products over a time span from 100 ps to 1 µs after photolysis at 267 nm by using time-resolved X-ray liquidography (TRXL). By making use of the atomic-level structural sensitivity of X-ray scattering and the superb 100 ps time resolution of X-ray pulses from a 3rd-generation synchrotron, the structural dynamics of a chemical reaction in solution can be directly monitored. The measured time-dependent X-ray solution scattering signals, analyzed using global-fitting based on DFT calculations and MD simulations, show that photoexcited HgBr₂ dissociates via both two-body (HgBr + Br) and three-body (Hg + Br + Br) dissociation pathways with a ∼2: 1 branching ratio. Following dissociation, the photoproducts recombine via three reactions involving Br species: (1) Hg + Br, (2) HgBr + Br, and (3) Br + Br. The associated rate constants and branching ratios are determined from the global-fitting analysis. Also, we examine the energy dissipation from reacting solute molecules and relaxation of excited molecules to solvent bath accompanying the temperature rise of 0.54 K. Compared to a previous TRXL study of the photodissociation of HgI₂, the results of this work suggest that the photodissociation pathway of HgBr₂ is different from that of HgI₂, which dissociates predominantly via two-body dissociation, at least to within the currently available time resolution of ∼100 ps. In addition, the error analysis of the fit parameters used in the global-fitting are discussed in detail with a comparison of various error estimation algorithms.