Collision-Induced Singlet→Triplet Intersystem Crossing of Methylene and Methylene-d2

Abstract

The photolysis of ketene or ketene‐d₂ in the presence of propane, oxygen, and inert gas was studied at 300°K at each of the four wavelengths 2600, 3130, 3340, and 3500 Å. Inert gases used were He, Ar, Xe, N₂, and CF₄. The inert gases cause the transition of methylene from the first singlet state to the triplet ground state to occur. The observed kinetics of the intersystem crossing process, being first order in methylene and first order in inert gas, are second order. The second‐order rate constants, relative to the rate constant for reaction of singlet methylene with propane, are independent of wavelength, and increase in the order He4≤Xe. The rate‐constant ratios for methylene‐d₂ show the same trend with inert gas but are 2–3 times larger. The results are consistent with a bimolecular intersystem‐crossing mechanism where the role of the collision partner is to cause the transition to occur by sufficiently perturbing the singlet methylene stationary states.