The photodissociation dynamics of ethyl bromide and ethyl iodide cations (CHBr and CHI) have been studied. Ethyl halide cations were formed through vacuum ultraviolet (VUV) photoionization of the respective neutral parent molecules at 118.2 nm, and were photolysed at a number of ultraviolet (UV) photolysis wavelengths, including 355 nm and wavelengths in the range from 236 to 266 nm. Time-of-flight mass spectra and velocity-map images have been acquired for all fragment ions and for ground (Br) and spin–orbit excited (Br*) bromine atom products, allowing multiple fragmentation pathways to be investigated. The experimental studies are complemented by spin–orbit resolved calculations of cuts through the potential energy surfaces (along the stretch coordinate) for the ground and first few excited states of the respective cations. Analysis of the velocity-map images indicates that photoexcited CHBr cations undergo prompt C–Br bond fission to form predominantly CH + Br* products with a near-limiting ‘parallel’ recoil velocity distribution. The observed CH + H + Br product channel is thought to arise unimolecular decay of highly internally excited CH products formed following radiationless transfer from the initial excited state populated by photon absorption. Broadly similar behaviour is observed in the case of CHI, along with an additional energetically accessible C–I bond fission channel to form CH + I products. HX (X = Br, I) elimination from the highly internally excited CHX cation is deemed the most probable route to forming the CH fragment ions observed from both cations. Finally, both ethyl halide cations also show evidence of a minor C–C bond fission process to form CHX + CH products.
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