Surface-reconstruction-induced changes in free-carrier scattering from the W(100) surface: An infrared surface-electromagnetic-wave study

Using the infrared-absorption technique of surface-electromagnetic-wave spectroscopy (SEWS) we have studied the effects of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ chemisorption on the SEW attenuation coefficient α in the 10-μm region between 165 and 350 K. The change in α at room temperature has also been measured for ${\mathrm{N}}_2$, ${\mathrm{O}}_2$, and CO adsorption. The coverage (CTHETA) dependence of α for ${\mathrm{N}}_2$, ${\mathrm{O}}_2$, and CO is fairly simple [either a monotonic increase (${\mathrm{N}}_2$) or a peak at some intermediate coverage (${\mathrm{O}}_2$, CO)] and points to changes in free-carrier (FC) surface scattering as the dominant α-changing mechanism. Much richer structure in α(CTHETA) upon ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ adsorption is observed: A small peak followed by a sharp dip at CTHETA=0.42 monolayer (ML) (CTHETA=2 monolayers ≡saturation), a broad maximum at CTHETA=1.28 ML, and a value at CTHETA=2.0 ML higher than for the clean surface. For CTHETA>0.42 ML the variations are dominated by reconstruction-induced changes in FC scattering from the surface. The difference in α between CTHETA=1.28 ML and saturation indicates that the CTHETA=1.28 ML disordered surface phase and the CTHETA=2.0 ML p(1×1)-D phase are temperature independent between 165 and 300 K. Adsorbate-induced quenching of the W(100) 0.3-eV intrinsic surface states also contributes to the changes in α; the associated loss of oscillator strength is most apparent in a strong frequency dependence of α relative to the clean surface for ${\mathrm{D}}_2$ adsorption. In addition, it is possible that a decrease in relaxation of the W(100) surface as CTHETA approaches 0.42 ML for ${\mathrm{H}}_2$ or ${\mathrm{D}}_2$ has some effect on the SEW attenuation by influencing the amount of FC interband scattering at the metal-vacuum interface.