HYDROGEN ON MOXRE1-X(100) - THE IMPACT OF ALLOYING ON THE ADSORPTION STRUCTURE

Hydrogen adsorption at the (100) surfaces of the substitutionally diso rdered alloy MoxRe1-x was studied for three values of the bulk stoichi ometry (x=0.75, 0.85 and 0.95) by high resolution electron energy loss spectroscopy (HREELS) and quantitative low energy electron diffractio n (LEED). The minority constituent Re is found to be completely deplet ed in the topmost layer and thus the local adsorption complex with hyd rogen bridge-bonded to two substrate atoms is very similar to that ret rieved on pure Mo(100). Nevertheless, the Re accumulation found for th e second layer inhibits (x=0.75,0.85) or strongly modifies (x=0.95) th e development of ordered adsorption phases known from pure Mo(100) at submonolayer coverages. This behaviour might be due to the considerabl e substitutional disorder in the second layer. At maximum coverage, O- H=2, LEED intensity analyses reveal a strong derelaxation of the upper most two layer spacings. Surprisingly, however, the relaxations of dee per interlayer spacings remain practically unchanged with respect to t hose of the clean surfaces. This is interpreted to be caused by the fr ozen-in oscillatory concentration profile and thus reveals a further m echanism to drive surface relaxations. (C) 1998 Elsevier Science B.V. All rights reserved.