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Using time-dependent high-resolution x-ray photoelectron spectroscopy at BESSY II, the adsorption and desorption processes of CO on stepped Pt(355) = Pt[5(111) x (111)] were investigated. From a quantitative analysis of C 1s data, the distribution of CO on the various adsorption sites can be determined continuously during adsorption and desorption. These unique data show that the terrace sites are only occupied when the step sites are almost saturated, even at temperatures as low as 130 K. The coverage-dependent occupation of on-top and bridge adsorption sites on the (111) terraces of Pt(355) is found to differ from that on Pt(111), which is attributed to the finite width of the terraces and changes in adsorbate-adsorbate interactions. In particular, no long-range order of the adsorbate layer could be observed by low-energy electron diffraction. Further details are derived from sticking coefficient measurements using the method devised by King and Wells [Proc. R. Soc. London, Ser. A 339, 245 (1974)] and temperature-programmed desorption. The CO saturation coverage is found to be slightly smaller on the stepped surface as compared to that on Pt(111). The initial sticking coefficient has the same high value of 0.91 for both surfaces.

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The activated adsorption of methane was investigated on two stepped Pt(355) and Pt(322) surfaces by timedependent in situ high-resolution X-ray photoelectron spectroscopy (XPS) combined with a supersonic molecular beam and was compared to corresponding results on Pt(111). Both stepped surfaces have five atom row wide (111) terraces but a different step orientation, namely, ( 111) and ( 100), respectively. Independent of the kinetic energy (0.45-0.83 eV) of impinging methane (CH 4 or CD 4 ), methyl is formed on all surfaces upon adsorption at 120 K. For the stepped surfaces, two different methyl species are identified from the XP spectra, which are attributed to adsorption at terrace sites and at step sites. The total initial sticking coefficients (for terrace + step sites) and the total coverages are very similar for all three surfaces. At low coverages, diffusion of methyl from the terraces to the steps is significantly stronger on Pt(355) than on Pt(322), and the step saturation coverage is higher on Pt(355). The thermal evolution of methyl was also investigated by in situ temperature-programmed XPS. Overall, an enhanced reactivity toward dehydrogenation to methylidyne is found for both terrace and step sites on the stepped surfaces, with the (111) steps of Pt(355) exhibiting the highest activity.

The vibrational fine structure of x-ray photoelectron (XP) spectra of a number of different small hydrocarbon molecules and reaction intermediates adsorbed on Pt(111) and Ni(111) has been investigated in detail. The data for methyl, methylidyne, acetylene, and ethylene can consistently be analyzed within the linear coupling model. The S factor, i.e., the intensity ratio of the first vibrationally excited to the adiabatic transition, is obtained to be 0.17+/-0.02 per C-H bond; for the deuterated species a value of 0.23+/-0.02 is obtained. Therefore, the vibrational fine structure can be used for fingerprinting in the analysis of XP spectra and for identifying unknown reaction intermediates. From the data, Deltar, the change of the minimum in the potential energy curve upon core ionization, is calculated within the linear coupling model using a first order correction. For all adsorbates, including the deuterated ones, a value of Deltar=0.060+/-0.004 A is obtained. Furthermore, from the binding energy of the adiabatic peak and from the energy of the vibrational excitation in the ionic final state some information on the adsorbate/substrate bond and the adsorption site can be derived.