A high-level ab initio study of the photodissociation of the HCCl radical

The photodissociation dynamics of the HCCl radical is investigated to explore its potential impact on the atmospheric chemistry. To this purpose, two-dimensional potential-energy surfaces along the C-Cl and C-H dissociating bond distances are calculated for the ground and several excited electronic states, by means of high-level multireference configuration interaction ab initio calculations. The electronic potential surfaces computed are in the energy range of the first and second absorption bands. The states involved in the first band are bound, so dissociation of HCCl upon excitation to this band is not possible. In contrast, the higher excited states involved in the second absorption band are repulsive or nearly repulsive along the C-Cl dissociation coordinate, while they are bound along the C-H coordinate. This implies that the photodissociation pathway of HCCl producing CH and Cl fragments upon excitation to the second absorption band, is expected to be the largely dominant one. The calculated HCCl absorption spectrum shows an appreciable absorption intensity in the wavelength region 190 − 280 nm , where solar actinic fluxes are significant at stratospheric altitudes in which ozone is abundant. Since Cl atoms are well known contributors to ozone depletion, the present results predict that HCCl photodissociation in the second absorption band may have implications for atmospheric ozone destruction.