LEED and HREELS investigation of the hydrogen-covered Mo(110) surface

The adsorption of hydrogen on the Mo(110) surface has been investigated by both low-energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS). Results fall into three regimes. For low coverages, the LEED pattern observed remains (1 × 1), similar to the clean surface. At hydrogen coverages near 0.5 ML, a substrate reconstruction alters the observed pattern to (2 × 2). For higher coverages, fractional-order diffraction beams are extinguished, leaving a (1 × 1) pattern. Two vibrational peaks are observed in HREELS spectra for both low- and high-coverage regimes at loss energies of 88 and 136 meV, and at 99 and 152 meV, respectively. A quasi-trigonal adsorption site is consistent with the vibrational energies and diffraction patterns observed at low and saturation coverages. At intermediate coverages, a vibrational peak with differing symmetry indicates the occupation of a long bridge-site during the transition from (2 × 2) to the hydrogen-saturated (1 × 1). No evidence is found for the hydrogen “liquid” phase reported by Balden et al. [Phys. Rev. Lett. 73 (1994) 854] for H W (110) ; consequently this phase is not responsible for the giant phonon anomalies observed in both systems. Published by Elsevier Science B.V.