HE-DIFFRACTION INVESTIGATIONS OF THE (2 X 2)-PHASES OF OXYGEN AND HYDROGEN ON NI(111)

He-diffraction results are reported for the clean Ni(111)-surface as well as for the (2 x 2)-chemisorption phases of oxygen and hydrogen. Intensity analyses confirm the early LEED-results concerning both the oxygen overlayer with one adatom and the honeycomb arrangement of the hydrogen overlayer with two adatoms per (2 x 2)-unit cell. Charge density calculations to reproduce the measured amplitudes of the adatom corrugation hills yield bond lengths for both species to the Ni-substrate atoms in good agreement with LEED. For the (2 x 2)2H-case the best-fit corrugation suggests a slight outward relaxation of those Ni-atoms, which lie in the centers of the H-honeycomb arrangement and are thus not bound directly to the H-adatoms; the possibility of a H-induced substrate reconstruction was not considered in the previous LEED-analyses.