A complete look at the multi-channel dissociation of propenal photoexcited at 193 nm: branching ratios and distributions of kinetic energy

Abstract

We observed fifteen photofragments upon photolysis of propenal (acrolein, CH₂CHCHO) at 193 nm using photofragment translational spectroscopy and selective vacuum-ultraviolet (VUV) photoionization. All the photoproducts arise from nine primary and two secondary dissociation pathways. We measured distributions of kinetic energy of products and determined branching ratios of dissociation channels. Dissociation to CH₂CHCO + H and CH₂CH + HCO are two major primary channels with equivalent branching ratios of 33%. The CH₂CHCO fragment spontaneously decomposes to CH₂CH + CO. A proportion of primary products CH₂CH from the fission of bond C–C of propenal further decompose to CHCH + H but secondary dissociation HCO → H + CO is negligibly small. Binary dissociation to CH₂CH₂ (or CH₃CH) + CO and concerted three-body dissociation to C₂H₂ + CO + H₂ have equivalent branching ratios of 14%–15%. The other channels have individual branching ratios of ∼1%. The production of HCCO + CH₃ indicates the formation of intermediate methyl ketene (CH₃CHCO) and the production of CH₂CCH + OH and CH₂CC + H₂O indicate the formation of intermediate hydroxyl propadiene (CH₂CCHOH) from isomerization of propenal. Distributions of kinetic energy release and dissociation mechanisms are discussed. This work provides a complete look and profound insight into the multi-channel dissociation mechanisms of propenal. The combination of a molecular beam apparatus and synchrotron VUV ionization allowed us to untangle the complex mechanisms of nine primary and two secondary dissociation channels.