H-atom product channels in the photodissociation of CH3Cl, CH3Br, and CH3I at 121.6 nm

H-atom product channels in the photodissociation of jet-cooled methyl halides (CH3X, X = Cl, Br, and I) at 121.6 nm are studied by using the high-n Rydberg-atom time-of-flight technique. Bimodal product center-of-mass (c.m.) translational energy distributions (with a fast and a slow component) and isotropic angular distributions have been observed. In going from CH3Cl to CH3Br to CH3I, the translational energy of the fast component in the bimodal profiles decreases slightly, while that of the slow component, as well as its relative intensity, increases. The fast component, with a significant energy release, corresponds to direct production of H + CH2X((X) over tilde B-2(1)). The slow component, as suggested by the correlation of its translational energy distribution with the parent molecule CH3X, might be mainly due to H + H-2 + CX(X(2)Pi) and/or X(P-2(3/2))/X*(P-2(1/2)) + H + CH2((XB1)-B-3) channels via the concerted three-body and/or two-step sequential dissociation. Although CH3I is excited above its first two ionization thresholds, its neutral fragmentation could compete with the photoionization processes.