UV-induced dissociation dynamics of bromoform probed by Ultrafast Electron Diffraction

The photochemistry of halogen-containing molecules has significant implications for polar ozone depletion, to which bromoform makes a major contribution. In previous work, using UV pump – XUV probe femtosecond transient absorption spectroscopy, it was shown that the major UV-induced dissociation channel of bromoform is the production of atomic bromine via a symmetric fragmentation pathway.¹ Molecular dynamics simulations predicted that a minor channel proceeds through an intermediate iso-CHBr₃ configuration. An unresolved photochemical curiosity is that the quantum yield for the C-Br fission following excitation at 266 nm is only 0.76±0.03 and the remaining relaxation channels are unknown. Here, the UV photochemistry of bromoform is investigated using mega-electron-volt ultrafast electron diffraction (MeV-UED). UED is a direct probe of the changes in nuclear degrees of freedom and complementary to the transient absorption study. Although, the excitation ratio is only ~0.3%, we can follow the C-Br dissociation in 50 fs time steps. Applying a test structure-based fit to the experimental data in k-space shows contributions of other products besides CHBr₂ + Br. Individual features are associated with distinct timescales. The findings will be discussed in terms of the possible associated photofragmentation pathways.

¹B. W. Toulson et al., Struct. Dyn. 6,054304 (2019)