Photoemission and electron-stimulated desorption studies of H on W(110): Single- versus two-binding-site models

The chemisorption of H on W (110) at room temperature is studied with the use of angle-integrated photoemission and electron-stimulated desorption (ESD). The ESD cross sections of ${\mathrm{H}}^{+}$ are found to be so low that no significant ${\mathrm{H}}^{+}$ signals with meaningful ion energy distributions are observed. The photoemission results show, however, two types of H adatoms, referred to as ${\beta }_2$ and ${\beta }_1$ states, for this chemisorptive system. Both states are found to appear simultaneously rather than sequentially as suggested by previous studies, and exhibit a simple $1-\Theta$ adsorption kinetics with different initial sticking coefficients. The ${\beta }_2$ state induces two binding energy levels at -2.0 and -6.0 eV, respectively, whereas the ${\beta }_1$ state induces a level at -3.8 eV. The work-function change (with a maximum value of -0.45 eV) is found to follow exactly with the intensity of the ${\beta }_2$ state. These results are found to be compatible with the two-binding-site model, inherently suggested by the reflection high-energy electron-diffraction data. However, the results can also be consistent with a single-binding-site model suggested by a recent angle-resolved photoemission and inelastic electron scattering study. A model based on the present results is proposed and critically compared with previous studies. Unresolved problems associated with both single- and two-binding-site models are also discussed.