Energy partitioning in the photodissociation of C₃H₄O near 200 nm

The photodissociation of methylketene and acrolein near 200 nm has been investigated using a CO laser probing and conventional gas analysis methods. The dissociation of methylketene in a Vycor tube (λ ≃226 nm) produces CO with a Boltzmann vibrational temperature of about 2300 K and with an average vibrational energy of 2.0 ± 0.2 kcal/mol. These results are consistent with the mechanism proposed by Kistiakowsky and co-workers in which the ethylidene diradical is formed in the first step of the reaction CH₃CHCO + hv → CH₃CH^† (→ C₂H₄^†) + CO and subsequently isomerizes to ethylene. The CO was found to have a vibrational energy distribution similar to that found previously in the O(³P) + CH₃C₂H reaction which is believed to occur via a methylketene intermediate. The dissociation of both methylketene and acrolein below 220 nm produced CO with similar, but nonstatistical vibrational energy distributions. A mechanism involving H-atom migration in both systems is postulated in which the direct production of C₂H₄ + CO takes place in addition to the major, but less exothermic CH₃CH + CO channel.