Dynamics of H Atom Formation in the Photodissociation of Chloromethanes at 193.3 nm

Using the laser photolysis/vacuum−ultraviolet laser-induced fluorescence (LP/VUV−LIF) “pump-and-probe” technique the dynamics of H atom formation after photoexcitation of chloromethanes at 193.3 nm were studied in the gas phase at room temperature under collision-free conditions. For all chloromethanes, H atoms were detected by ( 2 p 2 P ← 1 s 2 S)-LIF using tunable narrow-band Lyman-α laser radiation (λ Lα ≈ 121.6 nm) generated by resonant third-order sum−difference frequency conversion of pulsed-dye-laser radiation. However, only in the cases of CH 3 Cl and CH 2 Cl 2 were the H atoms found to originate from a primary photodissociation step. Absolute quantum yields for the formation of primary H atoms were measured by means of a calibration method to be φ H (CH 3 Cl) = (1.2 ± 0.6) × 10 -2 and φ H (CH 2 Cl 2 ) = (0.2 ± 0.1) × 10 -2 . From H atom Doppler profiles measured under single-collision conditions, the average translational energy released to the H + CH 2 Cl and H + CHCl 2 products in the center-of-mass system was determined to be: E T (H−CH 2 Cl) = (86.6 ± 14.2) kJ/mol and E T (H−CHCl 2 ) = (84.3 ± 8.9) kJ/mol. On the basis of available thermochemical data, the corresponding fraction of the available energy released as product translational energy was determined to be f T (H−CH 2 Cl) = (0.44 ± 0.07) and f T (H−CHCl 2 ) = (0.41 ± 0.04). In the CHCl 3 photodissociation, primary H atom formation was not observed. The H atoms detectable after laser irradiation of CHCl 3 at 193.3 nm were found to originate from secondary photodissociation of the CHCl 2 radical.