Primary and Secondary Dissociation Pathways in the UV Photochemistry of α-Dicarbonyls.

Photolysis of the α-dicarbonyls biacetyl (BiAc, CHCOCOCH) and acetylpropionyl (AcPr, CHCOCOCH) following UV excitation to the S state at 280 nm was studied using velocity-map ion imaging. Single-photon VUV ionization at 118 nm was used to detect alkyl and acyl radical photoproducts. Photolysis of BiAc at 280 nm yields the expected Norrish Type I photofragments CH and CHCO in a 1.0:1.3 ratio. The CHCO speed distribution is bimodal; the fast component is assigned to formation of a CHCO fragment pair on the T surface while the slow component most likely results from prompt secondary dissociation of energized CHCOCO radicals initially produced in conjunction with CH, tentatively assigned to dissociation on T. AcPr photolysis at 280 nm produces CH, CHCO and additionally CH and CHCO radicals, with a total alkyl to acyl ratio of 1.0:0.7. Both types of acyl radicals have bimodal speed distributions, which are momentum-matched only for the fast tails. By analogy with BiAc, the fast component is attributed to formation of the CHCO + CHCO pair on the T surface. The slower components are attributed to secondary dissociation of the corresponding energized RCOCO radicals formed in conjunction with the detected alkyl radicals. The results highlight the role that characterization of the detailed partitioning of the available energy can play in identifying mechanisms and quantifying branching between competitive pathways.