The methanol photodecomposition quantum yield increases with the addition of different foreign gases (alkanes, rare gases, C 0 2 , SF6). Whereas the methanol absorption spectrum presents a continuum in the 1850 8, region, the added gas effect implies that collisions are involved in reaching the dissociative state. This apparent contradiction between photochemical and spectroscopic data leads to the postulate of the existence of a collision-induced predissociation. The efficiency of the different gases in promoting the predissociation correlates with their polarizability. In absence of spin-orbit coupling, the collision-induced predissociationc an be explained on the basis of a van der Wads interaction. With Kr and Xe as added gases, spin-orbit coupling is an additional dissociative channel.

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The quantum yields for methanol photodecomposition have been found to be pressure dependent. Apart from methanol itself, other gases (alkane, N2, SF6, CO2, Ar, Kr, Xe) are effective in promoting the transition to dissociative states. A collision induced predissociation is postulated and the relative efficiencies of the different gases analyzed using a model based on simple degenerate perturbation theory.