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

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Abstract
The gas-phase dissociation dynamics of CH3Cl, CH2Cl2, and CHCl3 after photoexcitation at the Lyman-α wavelength (121.6 nm) were studied under collision-free conditions at room temperature. Narrow-band tunable Lyman-α laser radiation (λLα≈121.6 nm) 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 (2p2P←1s2S) laser induced fluorescence. Absolute H atom quantum yields ΦH(CH3Cl)=(0.53±0.05), ΦH(CH2Cl2)=(0.28±0.03), and ΦH(CHCl3)=(0.23±0.03) were determined employing a photolytic calibration method where the Lyman-α 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.