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 (2p(2)P <-- 1s(2)S)-LIF using tunable narrow-band Lyman-alpha laser radiation (lambda(L alpha) approximate to 121.6 nm) generated by resonant third-order sum-difference frequency conversion of pulsed-dye-laser radiation. However, only in the cases of CH3Cl and CH2Cl2 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 phi(H)(CH3Cl) = (1.2 +/- 0.6) x 10(-2) and phi(H)(CH2Cl2) = (0.2 +/- 0.1) x 10(-2). From H atom Doppler profiles measured under single-collision conditions, the average translational energy released to the H + CH2Cl and H + CHCl2 products in the center-of-mass system was determined to be: E-T(H-CH2Cl) = (86.6 +/- 14.2) kJ/mol and E-T(H-CHCl2) = (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-CH2Cl) = (0.44 +/- 0.07) and f(T)(H-CHCl2) = (0.41 +/- 0.04). In the CHCl3 photodissociation, primary H atom formation was not observed. The H atoms detectable after laser irradiation of CHCl3 at 193.3 nm were found to originate from secondary photodissociation of Phe CHCl2 radical.