On the problem of water adsorption on alkali halide cleavage planes, investigated by secondary ion mass spectroscopy

Abstract

In the epitaxial growth of thin metallic films, atomic beam scattering and other work on (001) cleavage planes of alkali halides, the problem of water adsorption (physically or chemically) and its consequences often arose. In this paper (001) planes of LiF, NaF and NaCl were investigated in a UHV apparatus by secondary ion mass spectroscopy (SIMS) under a variety of conditions: air and vacuum cleaved, with and without heat annealing, with and without exposure to water at various crystal temperatures. The main results are: cleaved crystal surfaces are free of water and hydroxide layers even in the submonolayer region under UHV conditions. Water vapour at a partial pressure of 10 −9 Torr will only adsorb on cooled cleavage planes: for LiF below 200 K, for NaF below 250 K and for NaCl below 260 K, the adsorption energy for the fluorides being in the 200 meV region. Thicker layers of adsorbed water on LiF (001) will grow according to the Weber-Volmer mechanism. The formation of hydroxide layers on the fluorides is possible only under extremely forced conditions. Direct evidence is given that the “active sites” for the adsorption of H 2O consist in surplus Li metal atoms on the surface. Heat treatment in UHV at temperatures up to the melting point for several hours removes these active sites and yields crystal surfaces which do not adsorb water even if cooled. Cleavage planes of the water soluble NaCl showed a somewhat different behaviour.