Physical Review B
The adsorption of hydrogen on the c(2×2) β-N-covered W(100) surface has been studied with infrared and thermal-desorption spectroscopies. A new dipole-active vibrational absorption due to chemisorbed hydrogen has been discovered. Its center frequency (1738 cm-1 for minimal H2 adsorption), isotopic dependence (1252 cm-1 for D2 adsorption and the existence of both lines for HD adsorption), absorption strength versus β-N coverage, and effective dynamic charge e*/ε∞≥0.12e lead to the assignment of the W-H stretch associated with a top-bonded H species. The vibration has been studied in detail on the highly ordered surface characterized by a β-N coverage of 0.5 monolayer: In the zero-coverage limit vibrational decay due to electron-hole pair excitations may provide the dominant contribution to the full width at half maximum of 12 cm-1. Away from this limit inhomogeneous broadening, caused by coadsorption of molecular and other atomic species, appears to increasingly contribute to the width, which attains a maximum of 35 cm-1 at saturation. Isotopic dilution and H-coverage experiments reveal a dynamical shift of +14 cm-1 in going from the isolated adatom to full layer limit which is explained by a combination of direct dipole-dipole and indirect electron-mediated interactions. With increasing coverage two chemical shifts have also been discerned: a positive one (+21 cm-1) attributed to electron-density competition among the adsorbed atomic species and a negative one (-17 cm-1) due to molecular-species coadsorption. Low-temperature annealing produces an increase in barrier height to parallel motion from the on-top site, apparently caused by the filling of more tightly bound sites near the remaining on-top H. The concurrent constancy of the W-H vibrational frequency indicates that the potential well perpendicular to the surface is not drastically altered by this rearrangement of adatoms.
"Hydrogen Adsorption on the β N-Covered W(100) Surface: An Infrared Study of the W–H Stretch," D. M. Riffe and A. J. Sievers, Phys. Rev. B 41, 3406 (1990).