Physical Review B
Using the infrared-absorption technique of surface-electromagnetic-wave spectroscopy (SEWS) we have studied the effects of H2 and D2 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 N2, O2, and CO adsorption. The coverage (CTHETA) dependence of α for N2, O2, and CO is fairly simple [either a monotonic increase (N2) or a peak at some intermediate coverage (O2, CO)] and points to changes in free-carrier (FC) surface scattering as the dominant α-changing mechanism. Much richer structure in α(CTHETA) upon H2 and D2 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 D2 adsorption. In addition, it is possible that a decrease in relaxation of the W(100) surface as CTHETA approaches 0.42 ML for H2 or D2 has some effect on the SEW attenuation by influencing the amount of FC interband scattering at the metal-vacuum interface.
"Surface-Reconstruction-Induced Changes in Free-Carrier Scattering from the W(100) Surface: An Infrared Surface-Electromagnetic-Wave Study," D. M. Riffe, L. M. Hanssen, and A. J. Sievers, Phys. Rev. B 34, 692 (1986).