A photodissociation study of CH2BrCl in the A-band using the time-sliced ion velocity imaging method

摘要

Employing a high-resolution (velocity resolution Delta nuu < 1.5%) time-sliced ion velocity imaging apparatus, we have examined the photodissociation of CH2BrCl in the photon energy range of 448.6-618.5 kJ/mol (193.3-266.6 nm). Precise translational and angular distributions for the dominant Br(P-2(3/2)) and Br(P-2(1/2)) channels have been determined from the ion images observed for Br(P-2(3/2)) and Br(P-2(1/2)). In confirmation with the previous studies, the kinetic-energy distributions for the Br(P-2(1/2)) channel are found to fit well with one Gaussian function, whereas the kinetic- energy distributions for the Br(P-2(3/2)) channel exhibit bimodal structures and can be decomposed into a slow and a fast Gaussian component. The observed kinetic-energy distributions are consistent with the conclusion that the formation of the Br(P-2(3/2)) and Br(P-2(1/2)) channels takes place on a repulsive potential-energy surface, resulting in a significant fraction (0.40-0.47) of available energy to appear as translational energy for the photofragments. On the basis of the detailed kinetic-energy distributions and anisotropy parameters obtained in the present study, together with the specific features and relative absorption cross sections of the excited 2A('), 1A('), 3A('), 4A('), and 2A(') states estimated in previous studies, we have rationalized the dissociation pathways of CH2BrCl in the A-band, leading to the formation of the Br(P-2(3/2)) and Br(P-2(1/2)) channels. The analysis of the ion images observed at 235 nm for Cl(P-2(3/2,1/2)) provides strong evidence that the formation of Cl mainly arises from the secondary photodissociation process CH2Cl+h nu -> CH2+Cl.