Second Harmonic Generation Studies of Ozone Depletion Reactions on Ice Surfaces under Stratospheric Conditions

Hypochlorous acid, HOCl, an important species in the proposed heterogeneous mechanism for stratospheric ozone depletion, has been observed directly at submonolayer amounts on a single crystalline basal ice surface at 155-188 K, using the nonlinear optical method second harmonic generation. The ice is held in equilibrium with its vapor pressure. Second harmonic generation signals from 290 to 310 nm spectroscopically characterize the species and enable us to follow isothermal desorption kinetics in situ. HOCl desorbs as a single species with a ΔG*_des = 48 ± 4 kJ/mol, close to the cohesive energy of ice itself. The lifetime of HOCl on the clean ice surface at 185 K is estimated to be 4 s and the equilibrium surface coverage at 10^-11 Torr HOCl to be around 4 × 10¹¹ molecules/cm², corresponding to about 0.1% of a monolayer. However, these same measurements performed on ice predosed with varying amounts of HNO₃ show that the HOCl lifetime is lengthened by coadsorbed HNO₃, depending on the HNO₃ surface density.