Location of hydrogen adsorbed on Rh(111) studied by low-energy electron diffraction and nuclear reaction analysis

The structures of clean and hydrogen-adsorbed Rh(111) surfaces were investigated by dynamical low-energy electron-diffraction (LEED) analysis. Exposure of D₂ induced no additional LEED patterns except for (1×1). Surface-layer relaxation occurs vertically on both clean and D-saturated surfaces. On the clean surface, the interlayer distance between the first and second layers (d₁₂) is smaller by 1.2(±0.6)% than the corresponding bulk distance of 2.1940.3em0ex. On the other hand, the contraction of d₁₂ is removed on the D-saturated surface. Detailed LEED analysis demonstrates that the D atoms are adsorbed on the fcc threefold hollow sites. The absolute saturation coverage of H on Rh(111) was determined to be 0.84 ML by nuclear reaction analysis (NRA). Moreover, the zero-point vibrational energy of H was derived from the analysis of the NRA resonance profile, which is discussed in comparison with the results of high-resolution electron-energy-loss spectroscopy.