Water Absorption on Perfect CaF2(111) Studied with He Scattering: Experimental Evidence for Ordering of Nanoclusters

Elastic and inelastic He-atom scattering were used to nondestructively investigate the water adsorption on UHV cleaved Ca${\mathrm{F}}_2$(111) surfaces. Below 145 K a $p(4\mathrm{}\times 4)$ adsorbate structure was observed. The corrugation function deduced from the diffraction intensities directly shows the formation of a two-dimensional periodic structure from water nanoclusters. Using the observed lateral extension and employing the Bernal-Fowler-Pauling rules, we arrive at six ${\mathrm{H}}_2$O molecules per cluster. This surprising result is confirmed by the observation of a very low energetic Einstein mode at ∼6 meV.