Lyman-α photodissociation of CH3CFCl2 (HCFC-141b): Quantum yield and translational energy of hydrogen atoms

Abstract

The collision-free, room temperature gas-phase photodissociation dynamics of CH3CFCl2 (HCFC-141b) was studied using Lyman-α laser radiation (121.6 nm) by the laser photolysis/laser-induced fluorescence ‘pump/probe’ technique. Lyman-α radiation was used both to photodissociate the parent molecule and to detect the nascent H atom products via (2p 2P → 1s 2S) laser-induced fluorescence. Absolute H atom quantum yield, ϕH = (0.39 ± 0.09) was determined by calibration method in which CH4 photolysis at 121.6 nm was used as a reference source of well-defined H atom concentrations. The line shapes of the measured H atom Doppler profiles indicate a Gaussian velocity distribution suggesting the presence of indirect H atom formation pathways in the Lyman-α photodissociation of CH3CFCl2. The average kinetic energy of H atoms calculated from Doppler profiles was found to be E T(lab) = (50 ± 3) kJ/mol. The nearly statistical translational energy together with the observed Maxwell-Boltzmann velocity distribution indicates that for CH3CFCl2 the H atom forming dissociation process comes closer to the statistical limit.