Hydrogen adsorption on the β-N-covered W(100) surface: An infrared study of the W-H stretch

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 ${\mathrm{cm}}^{\mathrm{-}1}$ for minimal ${\mathrm{H}}_2$ adsorption), isotopic dependence (1252 ${\mathrm{cm}}^{\mathrm{-}1}$ for ${\mathrm{D}}_2$ adsorption and the existence of both lines for HD adsorption), absorption strength versus β-N coverage, and effective dynamic charge $e^{\mathrm{*}}$/${\epsilon }_{\infty }$≥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 ${\mathrm{cm}}^{\mathrm{-}1}$. 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 ${\mathrm{cm}}^{\mathrm{-}1}$ at saturation. Isotopic dilution and H-coverage experiments reveal a dynamical shift of +14 ${\mathrm{cm}}^{\mathrm{-}1}$ 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 ${\mathrm{cm}}^{\mathrm{-}1}$) attributed to electron-density competition among the adsorbed atomic species and a negative one (-17 ${\mathrm{cm}}^{\mathrm{-}1}$) 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.