UV Photodissociation Dynamics of Bromoform Studied by Ultrafast Inner-Shell Transient Absorption Spectroscopy

Small bromoalkanes have been the focus of multiple studies owing to the importance of bromine chemistry in the atmosphere as bromine is ~60 times more destructive towards stratospheric ozone than chlorine. Here we study the photochemistry of the low-lying electronically excited states of bromoform (CHBr₃). To understand the physical mechanism of UV-induced C-Br bond breaking, fewest switches surface hopping (FSSH) calculations are performed to simulate the dynamics of CHBr₃ after photon excitation. Using intermediate geometries and electronic structures from the FSSH simulation, XUV absorption spectra are calculated with the linear-response time-dependent DFT method for comparison with femtosecond time-resolved inner-shell absorption measurements. The combined theoretical-experimental study indicates that C-Br bond scission proceeds on a timescale of ~30-40 fs, followed by continued electronic interaction between the departing Br atom and the remaining CHBr₂ fragment on an ~80-90 fs timescale. The study demonstrates how photochemical processes may be probed through a combination of ultrafast XUV/X-ray transient absorption spectroscopy, excited state molecular dynamics simulations, and core-level near-edge XUV/X-ray absorption calculations.