Adsorption study of hydrogen on a stepped Pt (997) surface using low energy recoil scattering

Information on the binding energy of hydrogen adsorbed on a stepped Pt(S)-[9(111)x(111)] surface can be obtained by bombarding this surface with low energy noble gas ions and by measuring the yield of recoil hydrogen. The angle between the incident ions and the detected recoil hydrogen particles is fixed at 300. The applied technique appears to be so sensitive that the dose of primary ions required for measuring an energy spectrum amounts to only 2x10-4 of a monolayer. By a suitable choice of the scattering geometry one may distinguish between the adsorption properties of step edges and terraces. Step edge properties are probed preferentially by directing the ion beam step-upwards at glancing incidence, whereas terrace sites are probed mainly under specular conditions (e.g. parallel to the steps). The yield of recoil hydrogen is measured at constant equilibrium hydrogen pressure as a function of the sample temperature for both geometries. The shapes of the isobars are distinctly different. For step edge sites a coverage independent binding energy of 93 kJ/mol is measured and for terrace sites we deduce an initial value of 75 kJ/mol gradually decreasing with increasing coverage to 58 kJ/mol at theta=0.45. (orig.)