Water adsorption on (111) surfaces ofBaF2andCaF2

Water adsorption on polished (111) surfaces of ${\mathrm{BaF}}_2$ and ${\mathrm{CaF}}_2$ was studied at room temperature under ultrahigh vacuum conditions by optical second-harmonic generation. Above ${10}^4$ langmuirs, the total second-harmonic yield decreases with increasing exposure. The azimuthal anisotropy is influenced differently on each substrate. Below ${10}^4$ langmuirs the 3m symmetry of the crystal surface is preserved in both cases, but more pronounced for ${\mathrm{CaF}}_2$. At high doses the symmetry remains for ${\mathrm{BaF}}_2$, but vanishes for ${\mathrm{CaF}}_2$. The results point to pseudomorphous epitaxial growth of water on ${\mathrm{BaF}}_2$ but not on ${\mathrm{CaF}}_2$.