On the adsorption of CO on Pt(111)

Abstract The adsorption system Pt(111)-CO has been investigated with LEED, thermal desorption and vibrat on spectroscopy using electron energy losses (EELS). Upon adsorption around 100 K at low coverages (θ ≲ 0.33) spontaneously ordered overlayers of CO, not observed before, have been found. At higher coverages the superstructures exhibit a substantial disorder at low temperatures, but transform into well ordered structures with defined diffraction patterns if the sample is annealed and subsequently cooled down again. The structures have been characterized according to the binding types of CO by their vibrational spectra. Up to a coverage of about 0.17 CO is exclusively linearly bonded in on-top sites. At coverages above 0.17 bridge sites are occupied in addition. At 100 K the adsorbate exhibits an island structure. The growth of islands is reflected in the coverage dependence of the sticking probability which passes through a maximum. At 300 K the sticking probability is further influenced by a precursor state making the sticking probability almost coverage independent within a certain range. In the vibration spectra multiple losses, combination bands and overtones have also been observed. From the overtones of the “dipole forbidden” hindered rotations of the molecule the fundamental frequencies of the CO bending modes can be estimated to 410 and 360 cm −1 for linear- and bridge-bonded CO, respectively.