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

The dynamics of chlorine atom formation after UV photoexcitation of CH3CF2C l(HCFC-142b) in the gas phase was studied by a pulsed laser photolysis/lase r-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(P-2(3/2)) and spin-orbit excited Cl*(P-2( 1/2)) photofragments were detected under collision-free conditions via lase r induced fluorescence in the vacuum ultraviolet spectral region. Narrow-ba nd 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 s ource of well-defined Cl(P-2(3/2)) and Cl-*(P-2(1/2)) concentrations, value s for the total Cl atom quantum yield (Phi(Cl+Cl*)=0.90+/-0.17) and the [Cl *]/[Cl] branching ratio 0.39+/-0.11 were determined by means of a photolyti c calibration method. From the measured Cl and Cl* atom Doppler profiles th e average relative translational energy of the fragments could be determine d to be 125+/-25 kJ/mol. The corresponding value f(T)=0.48+/-0.10 of the fr action of total available energy channeled into product translational energ y was found to be (within experimental uncertainty) in agreement with the r esult f(T)=0.39 of a dynamical simulation assuming a repulsive model for si ngle 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 f ragmentation mechanism for CH3CF2Cl after photoexcitation at 193.3 nm. (C) 1999 American Institute of Physics. [S0021-9606(99)00624-8].