Photodissociation dynamics of the chloromethanes at the Lyman-alpha wavelength (121.6 nm)

The gas-phase dissociation dynamics of CH3Cl, CH2Cl2, and CHCl3 after photoexcitation at the Lyman-alpha wavelength (121.6 nm) were studied under collision-free conditions at room temperature. Narrow-band tunable Lyman-alpha laser radiation (lambda(L alpha) approximate to 121.6 MI) was generated by resonant third-order sum-difference frequency conversion of pulsed-dye-laser radiation and used both to photodissociate the parent molecules and to detect the nascent H atom products via (2p(2)P <-- 1s(2)S) laser induced fluorescence. Absolute H atom quantum yields Phi(H)(CH3Cl)=(0.53+/-0.05), Phi(H)(CH2Cl2)=(0.28+/-0.03), and Phi(H)(CHCl3)=(0.23+/-0.03) were determined employing a photolytic calibration method where the Lyman-alpha photolysis of H2O was used as a reference source of well-defined H atom concentrations. H atom Doppler profiles were measured for all chlorinated methanes. In the case of CH3Cl the line shapes of the profiles indicate a pronounced bimodal translational energy distribution suggesting the presence of two H atom formation mechanisms leading to a markedly different H atom translational energy release. The observed ''slow'' component of the H atom translational energy distribution corresponds to an average kinetic energy of (55+/-5) kJ/mol, while the ''fast'' component leads to an average kinetic energy of (320+/-17) kJ/mol. The relative branching ratio between the ''fast'' and the ''slow'' H atom channel was determined to be (0.71+/-0.15). For CH2Cl2 and CHCl3 no bimodal translational energy distributions were observed. Here the translational energy distributions could each be well described by a single Maxwell-Boltzmann distribution, corresponding to an average translational energy of (81+/-9) kJ/mol and (75+/-4) kJ/mol, respectively. (C) 1997 American Institute of Physics.