Surface reflectance spectroscopy studies of chemisorption on W(100)

Surface reflectance spectroscopy (SRS) has been used to study chemisorption-induced changes in the electronic structure of a clean W(100) surface. The relative changes in optical reflectance, $\frac{\Delta R}{R}$, caused by chemisorption of ${\mathrm{H}}_2$ CO, and ${\mathrm{O}}_2$ have been measured as a function of exposure and photon energy $\hslash \omega$ in the range $0.6<\mathrm{}\hslash \omega <4.8\mathrm{}$ eV for adatom coverages up to ∼1 monolayer. Structure in the exposure and coverage dependence of $\frac{\Delta R}{R}$ displays the various adatom binding states and also indicates the presence of adatom-adatom interactions within a given binding state. The model of McIntyre and Aspnes (MA) is adopted to relate the observed $\frac{\Delta R}{R}\left(\hslash \omega \right)$ to changes in an effective complex surface dielectric function $\Delta {\hat{\epsilon }}^s\left(\hslash \omega \right)$; by assuming a simple form for $\Delta {\hat{\epsilon }}^s\left(\hslash \omega \right)$, we fit $\frac{\Delta R}{R}\left(\hslash \omega \right)$ spectra calculated using the MA model to the experimental data and thereby obtain difference spectra $\Delta {\hat{\epsilon }}^s\left(\hslash \omega \right)$ which show the chemisorption-induced changes in the optical response of the surface region. The positions of prominent peaks and dips in $\Delta {\epsilon }_2^s(=\mathrm{Im}\Delta {\hat{\epsilon }}^s)$ give the energies of important optical transitions associated with the surface electronic structure which are produced or quenched by chemisorption. These energies show a strong correlation with the position of surface levels (relative to the Fermi energy $E_F$) seen in ultraviolet photoemission and field-emission spectroscopies. As a result, the excitations obtained from SRS are attributed to optical transitions from intrinsic surface states and adsorbate-induced surface orbitals to final states concentrated primarily at $E_F$, which are most likely the tails of extended (Bloch) states of the metal as modified at the surface.