Absolute chlorine and hydrogen atom quantum yield measurements in the 193.3 nm photodissociation of CH3CFCl2 (HCFC-141b)

Abstract

The dynamics of chlorine and hydrogen atom formation in the 193.3 nm gas-phase laser photolysis of room-temperature 1,1-dichloro-1-fluoroethane, CH3CFCl2 (HCFC-141b), were studied by means of the pulsed-laser-photolysis and laser-induced fluorescence (LIF) “pump-and-probe” technique. Nascent ground-state Cl(2P3/2) and spin–orbit excited Cl*(2P1/2) as well as H(2S) atom photofragments were detected under collision-free conditions by pulsed Doppler-resolved laser-induced fluorescence measurements employing narrow-band vacuum ultraviolet probe laser radiation, generated via resonant third-order sum-difference frequency conversion of dye laser radiation in krypton. Using HCl photolysis as a reference source of well-defined Cl(2P3/2), Cl*(2P1/2), and H atom concentrations, values for the chlorine-atom spin–orbit branching ratio [Cl*]/[Cl]=0.36±0.08, the total chlorine atom quantum yield (ΦCl+Cl*=1.01±0.14), and the H atom quantum yield (ΦH=0.04±0.01) were determined by means of a photolytic calibration method. From the measured Cl and Cl* atom Doppler profiles the mean relative translational energy of the chlorine fragments could be determined to be ET(Cl)=157±12 kJ/mol and ET(Cl*)=165±12 kJ/mol. The corresponding average values 0.56 and 0.62 of the fraction of total available energy channeled into CH3CFCl+Cl/Cl* product translational energy were found to lie between the limiting values 0.36 and 0.85 predicted by a soft impulsive and a rigid rotor model of the CH3CFCl2→CH3CFCl+Cl/Cl* dissociation processes, respectively. The measured total chlorine atom quantum yield along with the rather small H atom quantum yield as well as the observed energy disposal indicates that direct C–Cl bond cleavage is the most important primary fragmentation mechanism for CH3CFCl2 after photoexcitation in the first absorption band.