ANOMALOUS HYDROGEN ADSORPTION SITES FOUND FOR THE C(2X2)-3H PHASES FORMED ON THE RE(10(1)OVER-BAR-0) AND RU(10(1)OVER-BAR-0) SURFACES

Hydrogen adsorption on the (10 (1) over bar 0) surfaces of Ru and Re l eads to the formation of c(2x2)-3H phases. As determined by quantitati ve low-energy electron diffraction (LEED) and density functional theor y calculations, hydrogen atoms, as expected, occupy threefold coordina ted hcp sites along the densely packed rows and the unexpected short-b ridge sites along the ridges in both c(2x2) phases. The Ru and Re subs trates reconstruct only weakly and in a very similar fashion under hyd rogen chemisorption. Most notably, there is a buckling in the third su bstrate layer of about 0.06 Angstrom. Probably (though not outside the limits of error), there are also slightly lateral displacements (0.02 Angstrom) of top-layer substrate atoms which are bridge-coordinated t o hydrogen. The metal-hydrogen bond lengths determined for both surfac es correspond to hydrogen radii in the expected range of 0.4-0.7 Angst rom. (C) 1998 American Institute of Physics. [S0021-9606(98)01320-8].