Bifurcation of UV-induced reaction pathways in bromoform probed with ultrafast electron diffraction

The UV photochemistry of bromoform (CHBr₃) is an important aspect of atmospheric ozone depletion. However, understanding the chemical reaction pathways and relative yields, especially the role of isomerization versus direct C-Br bond scission, has been challenging. Spectroscopic product yield measurements have been hindered by possible contributions from multiphoton processes, and ultrafast spectroscopy experiments have not provided a consistent quantification of relative reaction pathways. Here, gas phase ultrafast electron diffraction is used to directly study 267 nm induced structural dynamics in bromoform with femtosecond temporal resolution. The experiment is performed with an excitation ratio of only 0.3% and the observed dynamics can be confidently assigned to single-photon processes. By analyzing the time-dependent excited state diffraction images in both real space and momentum space, we show that direct C-Br bond breaking within <200 fs competes with formation of iso-CHBr₃ on the same timescale. Approximately 60% of molecules undergo isomerization within the first few hundred femtoseconds after UV excitation. The long-speculated isomer structure is experimentally determined for the first time and compared to theoretical predictions. The observed branching ratio is in reasonable agreement with theoretical estimates, but absolute rates and lifetimes differ significantly from theoretical models.