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

We present an experimental study of the broadband IR reflectance change Delta R/R induced by CO adsorption on a Pt(111) single crystal, over the frequency range 425-2800 cm(-1). CO's 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 Delta R/R, 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 Angstrom(2) at low coverage. After increasing with coverage for low CO coverages, the magnitude of Delta R/R peaks around a coverage of theta = 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 Delta R because of the increase in the electron mean-free-path. We attribute the decrease in \Delta R/R\ 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.