Adsorption and Desorption of HCl on Ice

Abstract

Pulsed molecular beam and mass spectrometric techniques are used to study the adsorption of hydrogen chloride on thin ice films at temperatures from 100 to 170 K. The adsorption and desorption of HCl from an ice surface is relevant to the polar stratosphere where it is thought that chlorine atoms are liberated from reservoir species such as HCl by heterogeneous reactions occurring on the surface of polar stratospheric clouds. We have measured the sticking coefficient for HCl at an incident translational energy of 0.09 eV on thin film ice surfaces using a modified version of the reflectivity technique of King and Wells. By modeling the HCl partial pressure versus time waveforms for surface temperatures of 100−125 K, we obtain a sticking coefficient of 0.91 ± 0.06. The model incorporates first-order HCl desorption and a loss term also first order in HCl. Fitted kinetic parameters are Edes = 28 kJ/mol, νdes = 2 × 1014 s-1 for desorption and Eloss = 21 kJ/mol, νloss = 4 × 1011 s-1 for the loss. The loss may be associated with the onset of water diffusion on the ice surface and subsequent ionization or hydration of the HCl. The measured waveforms are inconsistent with diffusion of HCl into the bulk. The apparent reflectivity decreases substantially in the temperature range of 126 to 140 K. This decrease cannot be attributed to an increase in sticking coefficient, a phase change in the ice, or the formation of the hexahydrate state of HCl.