Photodissociation of ClONO2 at 193 and 248 nm Original Research Article

The photodissociation of chlorine nitrate (ClONO2) at 193 and 248 nm was investigated by photofragment translational energy spectroscopy. Only two primary dissociation channels, Cl + NO3 and ClO + NO2, were observed. The branching ratios between Cl and CIO photoproducts were found to be ΦClO : ΦClO = 0.66:0.34 at 193 nm and 0.54:0.46 at 248 nm, with uncertainties of ±0.08 in the relative yields. The yield of CIO products is a lower bound, because a fraction of primary ClO fragments underwent secondary photodissociation. No compelling evidence was found for photodissociation via a ClONO + O channel at either wavelength. Center-of-mass total translational energy and angular distributions were obtained for each observed primary channel. In the analysis of the 248 nm data, the secondary photodissociation of CIO was modeled in order to permit deconvolution of all contributions to the signal at m/z= 35 (Cl+). All channels had positive anisotropy parameters, indicating that dissociation was prompt (occurring in less than a rotational period) and that parallel transitions were excited. The similarity of the ClO + NO2 translational energy distributions (< ET - 95 kJ/mol, ⩽ 25% of available energy) for both photodissociation wavelengths suggests that dissociation occurred on the same final potential energy surface. The high translational energy of the Cl and NO3 products from photolysis at either wavelength implies distortion in the transition state to a geometry with a larger ClON bond angle than that in the ground state. Significant fractions of the primary NO3 photoproducts (0.17 at 248 nm, 0.85 at 193 nm) were unstable and dissociated, most likely by spontaneous secondary dissociation. The secondary dissociation of ClO and NO3 could account for previous reports of atomic oxygen products from chlorine nitrate photolysis.