A time resolved electron energy loss spectroscopy study of CO on Pt(111): Adsorption site occupations versus coverage and temperature

Abstract

The intensities of the intramolecular vibrational modes obtained by electron energy loss spectroscopy (EELS) for CO adsorbed in top and bridge sites on Pt(111) are presented as a function of coverage and temperature. The top site is predominantly occupied at low coverages. A well ordered half top–half bridge structure is formed with a c(4×2) low-energy electron diffraction (LEED) pattern following adsorption of 0.5 monolayers (ML) at 95 K. The ratio of the EELS cross section of the C–O stretch in the top site to that in the bridge site is found to be σt/ σb =1.8±0.2. The occupations of bridge and top sites are observed to vary reversibly during temperature programmed EELS (TP-EELS) when the temperature is linearly ramped up from 95 K to just below the onset of desorption and back down for coverages from 0.09 to 0.44 ML; the difference in the adsorption enthalpy between top and bridge sites is found to vary with coverage from ΔH=0.62±0.05 kcal mol−1 at 0.09 ML to −0.14±0.05 kcal mol−1 at 0.44 ML, and returns to near zero (−0.03±0.05 kcal mol−1) at 0.50 ML. The ratio of the preexponential factors for site change is νb→t/ νt→b =2.0±0.3. The utility of modulating the EEL spectrometer lens potentials to minimize the effects of changes in surface order and work function changes on the observed normalized loss intensities is investigated and discussed. The possible effects of the low frequency frustrated translational and rotational vibrations on the measurement of the enthalpy difference derived from the EELS intensities are examined.