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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.

The adsorption of D2O on Pt(111) and the coadsorption with CO have been revisited by quantitative and in situ high-resolution X-ray photoelectron spectroscopy (XPS). During D2O adsorption at 110 K, O 1s spectra clearly show the known formation of a bilayer and multilayers. As indicated by a binding energy change of the multilayer peak in the XP spectra recorded during heating, a structural change in the D2O multilayer at the onset of multilayer desorption (∼145 K) is observed. Coadsorption of CO and D2O leads to occupation of bridge and on-top sites by CO, with the order in site occupation reversed as compared to the clean surface, as previously observed. In addition, a new CO species is clearly resolved in C 1s spectra, which may be identified with CO on hollow sites. Upon D2O desorption, this new species disappears and the CO site occupation changes to the value observed on the clean Pt(111). Induced by CO coadsorption, partial replacement of D2O from the surface into higher layers is observed, indicated by a reduced desorption temperature of water and an O 1s binding energy shift toward the value of the multilayer signal. Our results suggest an intermixed first layer of CO and D2O molecules; D2O on top of this layer also influences the CO site distribution. On a surface precovered with 0.5 monolayer (ML) of CO, D2O molecules can only adsorb on top of the CO layer and no mixed layer is formed; hence, no site change is observed. The preadsorbed D2O bilayer lowers the initial CO sticking coefficient by a factor of ∼13 as compared to the clean surface; this reduction is larger than previously detected.

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The adsorption and thermal evolution of ethene (ethylene) on clean and oxygen precovered Ni(111) was investigated with high resolution x-ray photoelectron spectroscopy using synchrotron radiation at BESSY II. The high resolution spectra allow to unequivocally identify the local environment of individual carbon atoms. Upon adsorption at 110 K, ethene adsorbs in a geometry, where the two carbon atoms within the intact ethene molecule occupy nonequivalent sites, most likely hollow and on top; this new result unambiguously solves an old puzzle concerning the adsorption geometry of ethene on Ni(111). On the oxygen precovered surface a different adsorption geometry is found with both carbon atoms occupying equivalent hollow sites. Upon heating ethene on the clean surface, we can confirm the dehydrogenation to ethine (acetylene), which adsorbs in a geometry, where both carbon atoms occupy equivalent sites. On the oxygen precovered surface dehydrogenation of ethene is completely suppressed. For the identification of the adsorbed species and the quantitative analysis the vibrational fine structure of the x-ray photoelectron spectra was analyzed in detail.