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

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.