Temperature dependence (90-440 K) of the vibrational spectra of CO adsorbed on platinum(111) studied by sum-frequency generation

Using optical infrared-visible sum-frequency generation (SFG) the temperature dependence of the stretching vibrational spectrum of CO terminally adsorbed on platinum was studied in the substrate temperature range 90-440 K. Measurements were carried out on a disordered Pt(111) substrate which was prepared by Ar+ sputtering and exposed to CO at T = 90 K until saturation coverage was reached. From the measured SFG spectra the frequency of the CO vibrationally resonant contribution could be determined as a function of temperature. At low temperature (90 K) a center-frequency of 2091 +/- 4 cm(-1) was determined which is characteristic of the stretching vibration of CO 'on top'-bound at Pt terrace sites. At a temperature of 360 K a center-frequency of 2074 +/- 5 cm(-1) was obtained which is characteristic of the stretching vibration of CO bound on step sites of the disordered Pt(111) substrate. Although the quality of the SFG signal markedly decreased at higher temperatures due to rapid desorption, it was possible to detect adsorbed CO up to a substrate temperature of 440 K. (C) 1997 Elsevier Science B.V.