The interaction of CsCl with films of solid water

Abstract

The interaction of CsCl molecules with films of solid water (three layers thick, typically), deposited on a tungsten crystal at 130 K, was studied. Metastable impact electron spectroscopy (MIES) and UPS(HeI) were applied to study the emission from Cl3p and Cs5p and the highest occupied states 1b 1, 3a 1 and 1b 2 of molecular water. Below a critical stoichiometry of about CsCl · nH 2O with n=6 the UPS spectra are quite similar to those from chlorides solvated in liquid water in as much as the relative positions and intensities of the water and salt features are concerned; very little emission from the ionization of Cl3p and Cs5p is observed with MIES. We conclude that the CsCl molecules become solvated in the water film. As long as n>6, the water spectrum remains characteristic for condensed water; at n<6 it develops gas phase-like features. Moreover, the Cs and Cl ions located in the near-surface region are not entirely shielded anymore by their solvation shells and are accessed by MIES. When heating films with n>6, water molecules not involved directly into the hydration of the salt molecules desorb around 140 K. Around 160 K all water has disappeared from the surface. Above this temperature only Cl and Cs (no OH-fragments) are detected on the surface up to about 400 K.