Coverage-, temperature-, and frequency-dependent broadband infrared reflectance change induced by CO adsorption on Pt(111) Original Research Article

We present an experimental study of the broadband IR reflectance change ΔRR induced by CO adsorption on a Pt(111) single crystal, over the frequency range 425–2800 cm−1. COʹs modification of the near-surface conductivity of the metal causes an increase in the absorption of light by the metal. Coverage-, temperature-, and frequency-dependent data are compared to a model of conduction electron scattering from randomly positioned adsorbates. The maximum magnitude of ΔRR, at room temperature and high frequency, is 0.28% ± 0.03%, a factor of three smaller than is observed for oxygen or CO on Cu(100). The scattering cross section per CO molecule is about 1 Å2 at low coverage. After increasing with coverage for low CO coverages, the magnitude of ΔRR peaks around a coverage of ϑ = 0.33 monolayers, and then decreases toward saturation coverage. Possible explanations for the nonmonotonic dependence include coherent scattering from a partially ordered overlayer or a coverage-dependent scattering cross section of the CO. The fractional reflectance change decreases when the temperature is lowered to 90 K, even though theory predicts an increase in ΔR because of the increase in the electron mean-free-path. We attribute the decrease in |ΔRR| to an increase in the clean-surface reflectance R, which is in agreement with theory. The frequency dependence is roughly consistent with predictions, but a rigorous test was not possible.