Dynamic and static secondary ion mass spectrometry studies of the solvation of HCl by ice

Abstract

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 surfaces of the films was studied using static secondary ion mass spectrometry (SIMS). The temperature of the films ranged from 90 to a maximum of 150 K, the temperature at which water begins to desorb rapidly. Under all conditions studied, HCl dissociated rapidly and completely to give 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-uniform, with the concentration of ionic species tending to decrease towards the interior of the film. Measurable changes in concentration occurred 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 layered structure was evidently quite stable, persisting over the several hours it took to complete the experiment. The situation was quite different at 150 K, the temperature at which the water molecules become very mobile and begin to desorb rapidly. At that temperature, no ionic concentration gradient was measured. The dynamic conditions of this experiment more closely mimicked those found in the stratosphere, suggesting that the migration of ions between the surface and the interior of the film may play a role in the atmospheric chemistry of HCl.