Phase diagram of hydrogen on Be(0001) from reconstruction-induced surface core-level shifts

The surface core-level shifts (SCLS) on the (0001) surface of beryllium were used to study the phase diagram of the hydrogen-induced reconstructions on this surface. Two different top-layer reconstruction phases are formed upon atomic hydrogen adsorption below and above room temperature, respectively. We are able to assign a characteristic core-level spectrum to an individual reconstruction phase and trace its presence at various hydrogen coverages and sample temperatures. This technique makes it possible to survey the lowcoverage regions that are lacking long-range ordering. Adsorption of atomic hydrogen in a temperature range of 100 to 270 K induces a (Î33Î3dR30° H-Be(0001) chemisorption structure, in which 1/3 of the Be top-layer atoms are removed to form a honeycomb structure of Be vacancies. Upon adsorption above 300 K a s133d superstructure forms, which has been proposed to exhibit a missing-row reconstruction of the Be top layer. Our hydrogen-induced SCLS measurements reveal that beryllium vacancies are forming already at small H coverages of 0.2 monolayers.