Absorption cross sections of HCl and DCl at 135-232 nanometers: Implications for photodissociation on Venus

Cross sections for photoabsorption of HCl and DCl are determined in the spe ctral region of 135-232 nm using radiation from a synchrotron light source. At wavelengths near the onset of absorption (lambda > 200 nm), cross secti ons of HCl are approximately 5-10 times larger than those of DCl. These dat a are used to calculate rates of photodissociation of HCl and DCl in the Ve nusian atmosphere. For the entire wavelength region measured, the rate of p hotodissociation of DCl is only 16% that of HCl. The difference in rates of photodissociation contributes to the exceptionally large [D]/[H] ratio of the Venusian atmosphere.