Hydrogen on W(100): temperature dependence of LEED and angle-resolved ESD

For hydrogen adsorption on W(100), the evolution of the c(2 × 2) LEED intensities and of the H⁺ ESD signal with H coverage have been investigated for various adsorption and annealing temperatures. Striking changes have been found for the half-order LEED intensities in the temperature range 140–360 K, in agreement with other workers, where the H⁺ signal showed only minor differences. The maxima of the LEED and the ESD intensities, however, occurred at the same exposure throughout this range (≈25% of saturation coverage). A temperature dependent variation of the height of the H⁺ maximum was observed which was reversible up to the desorption temperature of the β₂ hydrogen phase. The H⁺ ESDIAD lobe was found to have a polar FWHM of about 21°, independent of temperature between 140 and 450 K, and without any azimuthal dependence. These results provide evidence for the assumption that the observable H⁺ ions desorb from reconstructed sites. The number of these sites depends on temperature and hydrogen coverage, as shown by the change of the H⁺ current with these parameters. The transition from H on reconstructed to H on unreconstructed sites is of the order-order type; the energy difference between the two different adsorbate situations is about 135 meV/site at the quarter coverage. The consistency of the results and conclusions with a bridge-site model for H adsorption is shown. Elastic interactions lead to agglomeration of adsorbed H. The azimuthal isotropy of the ESDIAD lobes is interpreted by a superposition of emission from various types of bridge-sites which smear out the anisotropy expected for individual bridge-sites.