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

The UV photochemistry of bromoform is investigated by using mega-electron-volt ultrafast electron diffraction (MeV-UED) to study excited-state dissociation dynamics on femtosecond to picosecond timescales. In previous work, using UV pump – XUV probe transient absorption spectroscopy, it was shown that two timescales dominate the photodissociation dynamics of bromoform, each related to a characteristic timescale for either Br or C/H atomic motion.¹ Molecular dynamics simulations identified that two pathways to dissociation may exist: (1) C-Br fission along an approximately C_S symmetrical reaction pathway and (2) a minor pathway that forms a transient species containing a carbon-halogen-halogen bond, iso-CHBr₃, on a timescale of several hundred femtoseconds. During the first 40 fs of the dominant pathway distortion from C_3V to a quasi-planar CHBr₂ causes significant changes to the excited state valence structure. The displacement of the heavy Br atoms requires up to 300 fs to form separate Br + CHBr₂ products. UED enables a direct probe of the changes in the nuclear degrees of freedom and is complementary to the transient absorption study. Here, we present first results from a recent MeV-UED experiment at SLAC National Accelerator Laboratory, which further elucidate the 266 nm induced photodissociation of bromoform. ¹B. W. Toulson et al., Struct. Dyn. 6,054304 (2019)