UV-induced Dissociation Dynamics of Bromoform Probed by Ultrafast Electron Diffraction

The UV photochemistry of bromoform significantly contributes to stratospheric ozone depletion. It is a major source of bromine radicals, which have an ozone destruction potential 100 times higher than that of chlorine atoms. After 267 nm excitation, multiple reaction pathways are energetically accessible. The dominant single-photon dissociation channel is C-Br bond fission, but questions remain regarding the transient intermediates participating in the process as well as potential alternative reaction channels. For example, HBr and Br₂ elimination have been reported, too, but largely attributed to multiphoton excitation. A recent femtosecond XUV transient absorption study found no evidence for a Br₂ species and, supported by theoretical calculations, it was concluded that the dominant reaction mechanism is direct dissociation of atomic Br.¹ However, an alternative roaming-like mechanism was previously reported in which a transitory iso-CHBr₃ is formed.² Here the UV-induced photodissociation of bromoform is investigated by ultrafast electron diffraction (UED). With an excitation ratio of only 0.3%, multiphoton processes are vanishingly small, and all results can be confidently assigned to single-photon channels. Dissociation trajectories based on structures retrieved from UED patterns spaced by as little as 50 fs time delays will be presented. Parallels and distinguishing features between experimental results and theoretically predicted trajectories will be discussed.

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

²A. S. Mereshchenko et al., Nat. Chem. 7, 562–568 (2015)