Hydrogen adsorption on Al(100)

Adsorption studies of hydrogen and deuterium on the clean (100) surface of aluminum are reported. The probability for ${\mathrm{H}}_2$ (${\mathrm{D}}_2$) dissociation and subsequent metal-hydrogen bond formation is less than 1:${10}^4$. This prompted the use of a beam of atomic hydrogen (deuterium). We have adsorbed between 0.01 and 1.0 monolayer and studied annealing sequences between 80 and 750 K with electron-energy-loss spectroscopy (EELS) and thermal-desorption spectroscopy (TDS). At <90 K we observe surface hydrogen coordinated in bridge positions, characterized by an Al-H (-D) vibrational band at 1125 (800) ${\mathrm{cm}}^{\mathrm{-}1}$. Annealing to 150 K causes dominant features to appear at 1750 (1250) and 750 (550) ${\mathrm{cm}}^{\mathrm{-}1}$. These losses are interpreted as the stretching and bending and/or scissoring modes of a terminally coordinated species. Further annealing to 280 K sees the growth of an intense band at 1825 (1325) ${\mathrm{cm}}^{\mathrm{-}1}$, with no correlated low-frequency band. This last annealing signals the formation of another terminally coordinated species, tentatively assigned as ‘‘p(1×1) islands.’’ Hydrogen recombination and molecular desorption is observed between 340 and 360 K. We have no indication of any scattering resonance in the elastic energy range 1–9 eV, nor do the angular profiles of the above losses deviate from the profile of the elastic peak. All vibrational modes are thus concluded to be dominated by dipole excitations.