Hydrogen on Mo xRe 1− x(100) – the impact of alloying on the adsorption structure

Abstract

Hydrogen adsorption at the (100) surfaces of the substitutionally disordered alloy Mo x Re 1− x was studied for three values of the bulk stoichiometry ( x=0.75, 0.85 and 0.95) by high resolution electron energy loss spectroscopy (HREELS) and quantitative low energy electron diffraction (LEED). The minority constituent Re is found to be completely depleted in the topmost layer and thus the local adsorption complex with hydrogen bridge-bonded to two substrate atoms is very similar to that retrieved on pure Mo(100). Nevertheless, the Re accumulation found for the second layer inhibits ( x=0.75, 0.85) or strongly modifies ( x=0.95) the development of ordered adsorption phases known from pure Mo(100) at submonolayer coverages. This behaviour might be due to the considerable substitutional disorder in the second layer. At maximum coverage, Θ H=2, LEED intensity analyses reveal a strong derelaxation of the uppermost two layer spacings. Surprisingly, however, the relaxations of deeper interlayer spacings remain practically unchanged with respect to those of the clean surfaces. This is interpreted to be caused by the frozen-in oscillatory concentration profile and thus reveals a further mechanism to drive surface relaxations.