Fragmentation dynamics of the ethyl bromide and ethyl iodide cations: a velocity-map imaging study

Abstract

The photodissociation dynamics of ethyl bromide and ethyl iodide cations (C₂H₅Br⁺ and C₂H₅I⁺) 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 ab initio calculations of cuts through the potential energy surfaces (along the R_C–Br/I stretch coordinate) for the ground and first few excited states of the respective cations. Analysis of the velocity-map images indicates that photoexcited C₂H₅Br⁺ cations undergo prompt C–Br bond fission to form predominantly C₂H₅⁺ + Br* products with a near-limiting ‘parallel’ recoil velocity distribution. The observed C₂H₃⁺ + H₂ + Br product channel is thought to arise via unimolecular decay of highly internally excited C₂H₅⁺ products formed following radiationless transfer from the initial excited state populated by photon absorption. Broadly similar behaviour is observed in the case of C₂H₅I⁺, along with an additional energetically accessible C–I bond fission channel to form C₂H₅ + I⁺ products. HX (X = Br, I) elimination from the highly internally excited C₂H₅X⁺ cation is deemed the most probable route to forming the C₂H₄⁺ fragment ions observed from both cations. Finally, both ethyl halide cations also show evidence of a minor C–C bond fission process to form CH₂X⁺ + CH₃ products.