Abstract

Second harmonic generation (SHG) with p-p polarization has been used for studying the symmetry of the crystal-water interface during water adsorption on polished surfaces of alkali halide and alkaline-earth halide crystals in UHV. The adsorption dynamics can be explained by Langmuir kinetics, by assuming several different adsorption sites. The azimuthal anisotropy of the SHG yield, reflecting 3m symmetry for (111) surfaces, shows different adsorption dependencies for BaF₂ and CaF₂. We conclude that water grows epitaxially on BaF₂(111). In contrast, for CaE₂ the adsorbate symmetry differs from the one of the substrate, with increasing coverages. Unexpectedly, LiF(100) shows a pronounced m symmetry caused by water adsorption, which is not the case for NaF(100). The m symmetry on LiF is stable and indicates a preferred direction of the water dipoles in the adsorbate layer.

© 1992 Optical Society of America