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Physical Review B





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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*≥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.


Published by American Physical Society in Physical Review B. Publisher PDF is available for download through link above.

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