The surfaces of beryllium and their interaction with hydrogen

Beryllium is an s-p bonded metal, so it is tractable by detailed first-principles calculations yet it is far from being a simple metal. The bulk is nearly a semiconductor while two of the three surfaces investigated here are high electron-density metals, due to free-electron like surface states. In many ways, beryllium is an ideal system to use in investigating the structural properties at surfaces, given the unusual bonding mechanism and the unique electronic structure, so very different at the surface compared to the bulk. Our low-energy electron diffraction experiments show that Be(0001) exhibits a large thermal expansion of the first interlayer spacing even though high-resolution electron energy loss spectroscopy shows no enhancement in the surface anharmonicity. The toplayer spacing expands from +3.1% at 110 K, +4.3% at 300 K, to +6.7% at 700 K resulting in a thermal expansion coefficient at the surface of 70 $\times$ 10$\sp{-6}$K$\sp{-1}$, six times the bulk value, without any sign of anharmonicity in the interlayer potential. We find the (1120) surface to be the first simple metal to reconstruct in a missing row structure, so far only seen on three transition and noble metal surfaces. Until now, it was believed that localized d bands were responsible for dramatic surface restructuring. Our experiments on Be(1010) indicate that from the two possible terminations of the bulk, the one with the shorter first interlayer spacing is realized. We have found that hydrogen adsorption on Be(0001) induces massive reconstruction of the surface by forming a honeycomb structure of Be vacancies each decorated with three H adatoms, bonded in tilted bridge sites. This marks the first successful determination of H adsorption site on a reconstructed surface. We can present a complete phase diagram combining our results from low-energy electron diffraction, electron energy loss spectroscopy and high resolution X-ray core level spectroscopy. The H induced vacancy reconstruction of the Be surface grows in islands at the lowest coverage and temperatures of 100 K