Desorption from thin films of amorphous HCl hydrate

Amorphous HCl hydrate films of known stoichiometry were prepared using a molecular beam and their desorption behaviour examined. HCl adsorbed onto the surface of ice at low temperature (85<T<125 K) incorporates into the ice as it is heated above 125 K. HCl concentrates in the film as water desorbs above ∼150 K, only desorbing with the last of the water. Partial saturation of thin ice films (<20 bilayers) with HCl at temperatures between 125 and 140 K results in HCl and H2O desorbing together above 150 K. HCl desorption from amorphous HCl trihydrate, produced by exposing an ice film to HCl at 135 K, starts at 140 K and is characterised by an activation barrier of 67±3 kJ mol−1 which does not change significantly during desorption of the trihydrate film. Thicker films show a reduction in the desorption rate as the hydrate film crystallises during desorption, two separate HCl desorption peaks developing. Below 130 K additional HCl can be adsorbed onto the surface of the HCl trihydrate, desorbing with an activation barrier of 31 kJ mol−1 around 130 K.