No abstract

A liquid microjet was employed to examine the gas/liquid interface of aqueous sodium halide (Na + X -, X ) Cl, Br, I) salt solutions. Laser excitation at 193 nm produced and removed cations of the form H + (H 2 O) n and Na + (H 2 O) m from liquid jet surfaces containing either NaCl, NaBr or NaI. The protonated water cluster yield varied inversely with increasing salt concentration, while the solvated sodium ion cluster yield varied by anion type. The distribution of H + (H 2 O) n at low salt concentration is identical to that observed from lowenergy electron irradiated amorphous ice, and the production of these clusters can be accounted for using a localized ionization/Coulomb expulsion model. Production of Na + (H 2 O) m is not quantitatively accounted for by this model but requires ionization of solvation shell waters and a contact ion/Coulomb expulsion mechanism. The reduced yields of Na + (H 2 O) m from high concentration (10 -2 and 10 -1 M) NaBr and NaI solutions indicate a propensity for Brand Iat the solution surfaces and interfaces. This is supported by the observation of multiphoton induced production and desorption of Br + and I + from the 10 -2 and 10 -1 M solution surfaces.

The interaction and autoionization of HCl on low-temperature (80-140 K) water ice surfaces has been studied using low-energy (5-250 eV) electron-stimulated desorption (ESD) and temperature programmed desorption (TPD). There is a reduction of H(+) and H(2)(+) and a concomitant increase in H(+)(H(2)O)(n=1-7) ESD yields due to the presence of submonolayer quantities of HCl. These changes are consistent with HCl induced reduction of dangling bonds required for H(+) and H(2)(+) ESD and increased hole localization necessary for H(+)(H(2)O)(n=1-7) ESD. For low coverages, this can involve nonactivated autoionization of HCl, even at temperatures as low as 80 K; well below those typical of polar stratospheric cloud particles. The uptake and autoionization of HCl is supported by TPD studies which show that for HCl doses ≤0.5 ± 0.2 ML (ML = monolayer) at 110 K, desorption of HCl begins at 115 K and peaks at 180 K. The former is associated with adsorption of a small amount of molecular HCl and is strongly dependent on the annealing history of the ice. The latter peak at 180 K is commensurate with desorption of HCl via recombinative desorption of solvated separated ion pairs. The activation energy for second-order desorption of HCl initially in the ionized state is 43 ± 2 kJ/mol. This is close to the zero-order activation energy for ice desorption.