Abstract:

Carbonyls are an important class of volatile organic compounds present in the atmosphere, and photolysis is the major sink. While monocarbonyls have been studied extensively, the photochemistry of multifunctional carbonyl species is less well studied. Identification of the products and the photodissociation mechanisms is important to understand the role they may have in the atmosphere. Velocity-map ion imaging (VMI) coupled with single-photon VUV ionization has been used to investigate the UV photochemistry of several multifunctional carbonyls. The technique allows the measurement of photofragment speed distributions, which provides information critical to identifying mechanisms and quantifying the branching between competitive dissociation pathways. First, the photolysis of the β-dicarbonyl acetylacetone, which exists primarily as the enolone tautomer, was investigated following excitation to the S₂ state. Two Norrish Type I pathways leading to radical products were identified. The products CH₃ + C(O)CH=C(OH)CH₃ were attributed to dissociation of the enolone form, most likely on the T₁ surface. In contrast, the alternative pathway, which produces CH₃CO + CH2C(O)CH₃, most likely occurs on S₀ following phototautomerization to the diketone. The photolysis of the α-dicarbonyls biacetyl and acetylpropionyl following excitation to S₂ was also investigated. Fast acyl radical (RCO) products were produced with bimodal speed distributions that could be assigned to primary products formed on T₁ and secondary products resulting from dissociation of energized alkylglyoxyl (RCOCO) radicals. The latter were formed in conjunction with alkyl radicals, through a mechanism that is tentatively assigned to occur on the T₂ surface.