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 mu s 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 similar to 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 similar to 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.