Chlorine atom formation dynamics in the dissociation of CH3CF2Cl(HCFC-142b) after UV laser photoexcitation

Abstract

The dynamics of chlorine atom formation after UV photoexcitation of CH3CF2Cl(HCFC-142b) in the gas phase was studied by a pulsed laser photolysis/laser-induced fluorescence (LIF) “pump-and-probe” technique at room temperature. The parent molecule was excited at the ArF excimer laser wavelength (193.3 nm) and nascent ground state Cl(2P3/2) and spin-orbit excited Cl*(2P1/2) photofragments were detected under collision-free conditions via laser induced fluorescence in the vacuum ultraviolet spectral region. Narrow-band probe laser radiation, tunable over the wavelength range 133.5–136.4 nm, was generated via resonant third-order sum-difference frequency conversion of dye laser radiation in Krypton. Using HCl photolysis at 193.3 nm as a source of well-defined Cl(2P3/2) and Cl*(2P1/2) concentrations, values for the total Cl atom quantum yield (ΦCl+Cl*=0.90±0.17) and the [Cl*]/[Cl] branching ratio 0.39±0.11 were determined by means of a photolytic calibration method. From the measured Cl and Cl* atom Doppler profiles the average relative translational energy of the fragments could be determined to be 125±25 kJ/mol. The corresponding value fT=0.48±0.10 of the fraction of total available energy channeled into product translational energy was found to be (within experimental uncertainty) in agreement with the result fT=0.39 of a dynamical simulation assuming a repulsive model for single C–Cl bond cleavage. Both the measured total Cl atom quantum yield and the energy disposal indicates that direct C–Cl bond cleavage is a primary fragmentation mechanism for CH3CF2Cl after photoexcitation at 193.3 nm.