Primary Photochemical Processes in Glyoxal at 4358 Å

Abstract

Quantum yields of CO and of H₂ have been measured in the gas‐phase photolysis with 4358‐Å radiation at temperatures between 37° and 207°C. φ_CO is generally about 0.6; φH₂ is about 0.01. The data suggest that about 70% of the excited molecules cross to the triplet state by a first‐order process, and the remaining excited singlet molecules return to the ground state with fluorescence. The principal reaction of triplet molecules is a dissociation into CO and HCHO, while a small fraction of triplet molecules are destroyed by collision with ground‐state glyoxal molecules. About 3% of the triplet molecules undergo a first‐order intersystem crossing to the ground state, forming molecules with about 55 kcal of vibrational energy. These either dissociate into H₂ and CO or relax to vibrational equilibrium by collision with heat‐bath molecules. At pressures near 100 Torr, where higher concentrations of triplet molecules are formed because of increased absorption, reactions of free radicals produced in collisions between triplet molecules also contribute significantly to φH₂. The data suggest that temperature variation has little effect on the reactions destroying the triplet state, but that the rate of vibrational relaxation in the ground state increases markedly at higher temperatures.