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

Photolysis of the α-dicarbonyls biacetyl (BiAc, CH₃COCOCH₃) and acetylpropionyl (AcPr, CH₃COCOC₂H₅) following UV excitation to the S₂ 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 CH₃ and CH₃CO in a 1.0:1.3 ratio. The CH₃CO speed distribution is bimodal; the fast component is assigned to formation of a CH₃CO fragment pair on the T₁ surface while the slow component most likely results from prompt secondary dissociation of energized CH₃COCO radicals initially produced in conjunction with CH₃, tentatively assigned to dissociation on T₂. AcPr photolysis at 280 nm produces CH₃, CH₃CO and additionally C₂H₅ and C₂H₅CO 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 CH₃CO + C₂H₅CO pair on the T₁ 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.