The mechanism of ketene photolysis

Ketene has been photolysed both alone and in the presence of inert gases at 3660 and 3130 A/O, at various temperatures. The mechanism for the photolysis at shorter wavelengths, around 2700 A/O, is fairly straightforward because each ketene molecule that absorbs a quantum decomposes and the carbon monoxide quantum yield, equal to two, is independent of temperature and pressure. At 3130 A/O and 3660 A/O, however, deactivation processes play an important part in the photolysis and the mechanisms are more complicated at these wavelengths. At 3660 A/O collisional deactivation has been found to be one-step process, but at 3130 A/O this deactivation process becomes two-step, in certain circumstances. Mechanisms are proposed for ketene photolysis at both these wavelengths. A simpler mechanism used by previous workers is shown to be invalid by its failure to reconcile the experimentally determined rate constants for decomposition, assuming the mechanism, with those calculated from unimolecular reaction theory. The new mechanisms proposed include intersystem crossing whereby excited singlet state molecules cross over to the triplet manifold. Decomposition of both excited singlet state and triplet state ketene molecules occurs, resulting in the production of both singlet and triplet methylene. These two methylenes react differently with hydrocarbons. Recent determinations, with ketene – olefin mixtures, of the proportions of singlet and triplet methylene formed are reviewed.