The role of excited states in the gas-phase photolysis of acetaldehyde

The cis-trans isomerization of butene-2 has been used to measure the triplet state yields in the photolysis of acetaldehyde at various wavelengths between 313 and 254 nm over the temperature range 35 to 140 °C. The results, together with those derived from chemical product formation, are consistent with data from luminescence studies. Dissociation into molecular products occurs rapidly, probably by predissociation, from a non-quenchable excited state formed by absorption. The main free radical decomposition occurs from the triplet state and this, in the absence of additives, such as butene-2, is responsible for the chain decomposition. The intersystem crossing and non-quenchable processes are independent of temperature. Isopentyl radicals formed from methyl addition to butene-2 can also propagate a chain reaction for acetaldehyde decomposition. At high temperatures and low pressures, dissociation of vibrationally excited isopentyl radicals can contribute to the measured isomerization yield. This is shown by the effect of addition of inert gas. Evidence is put forward that geometrical isomerization of the olefin involves a triplet aldehyde-olefin complex that can be decomposed by collision with ground state aldehyde molecules without cis-trans rearrangement of the olefin. This conclusion is consistent with other work in the gas and liquid phases.