Pressure Dependence (10−10–700 Torr) of the Vibrational Spectra of Adsorbed CO on Pt(111) Studied by Sum Frequency Generation

The vibrational spectrum of CO on Pt(111) was monitored over 13 orders of magnitude in CO pressure by using infrared-visible sum frequency generation. Surprising results at high pressures indicate a pressure-induced major reorganization of CO on Pt(111). At low pressures ( ${10}^{-10\mathrm{}}–{10}^{-1\mathrm{}}\mathrm{Torr}$) CO bonds at atop and bridge sites, but above 10 Torr new features appear, and by 700 Torr they dominate the spectrum. They suggest the presence of an incommensurate CO overlayer together with some terminally bound CO at atop defect sites.