Location of hydrogen adsorbed on palladium (111) studied by low-energy electron diffraction

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Abstract
Low-energy-electron-diffraction (LEED) I-V curves have been measured for H adsorbed on Pd(111) in the (√3 × √3 )R30°-2H structure, which occurs at a coverage Θ=(2/3. The measurements are compared to calculations using dynamical LEED techniques. Twenty-one distinct geometry types were investigated, each one with metal-interlayer relaxations allowed and the distance of the H atom above (or below) the top Pd layer varied. The metal-interlayer spacings that we find are within error bars of those of the clean surface. Agreement between theory and experiment was achieved for structures with one of the two H atoms in the unit mesh in a threefold hollow surface site, above third-layer metal atoms (denoted ‘‘A+’’). The second H resides either in the same type of site or in another type of threefold site, such as the hollow surface site above second-layer metal atoms B+ or the subsurface sites between first- and second-layer metal atoms, A- or B-. We find that good reliability factors R are achieved for subsurface occupation fractions up to 60%. The results are consistent with the embedded-atom-method prediction of substantial subsurface H and with previous electron-stimulated desorption measurements.