DYNAMIC AND STATIC SECONDARY-ION MASS-SPECTROMETRY STUDIES OF THE SOLVATION OF HCL BY ICE

Water-ice films designed to simulate type II polar stratospheric cloud (PSC) particles have been exposed to various levels of HCl vapour in an ultra-high vacuum chamber. The interaction between HCl and the surf aces of the films was studied using static secondary ion mass spectrom etry (SIMS). The temperature of the films ranged from 90 to a maximum of 150 K, the temperature at which water begins to desorb rapidly. Und er all conditions studied, HCl dissociated rapidly and completely to g ive solvated ions. There was no evidence for the adsorption of intact molecular HCl. The extent to which these ions penetrated into the bulk of the ice films was studied by using a high-energy beam of Ar+ ions to etch through the film, then probing the newly exposed surface using static SIMS. At 90 and 135 K, the distribution of ions was non-unifor m, with the concentration of ionic species tending to decrease towards the interior of the film. Measurable changes in concentration occurre d rapidly, within the first five to ten layers. From this we inferred the existence of a relatively thin, ion-rich 'skin', condensed on top of a film of either pure ice or of some stable HCl hydrate. This layer ed structure was evidently quite stable, persisting over the several h ours it took to complete the experiment. The situation was quite diffe rent at 150 K, the temperature at which the water molecules become ver y mobile and begin to desorb rapidly. Ar that temperature, no ionic co ncentration gradient was measured. The dynamic conditions of this expe riment more closely mimicked those found in the stratosphere, suggesti ng that the migration of ions between the surface and the interior of the him may play a role in the atmospheric chemistry of HCl.