H. Tillborg

Overlayer structure from adsorbate and substrate core level binding energy shifts: CO, CCH3 and O on Pt(111)

Abstract By combining high resolution photoemission measurements of adsorption induced binding energy shifts of both adsorbate and substrate core levels the nature and distribution of adsorption sites in the CO/Pt(111) system can be determined. The existence of different surface shifted components demonstrates the local character of the surface core level shift. This is used to study the O/Pt(111) (2 × 2) and CCH3/Pt(111) “(2 × 2)” phases. The intensity relations of the different surface peaks suggest O/Pt(111) to be a true (2 × 2) phase, while the CCH3/Pt(111) “(2 × 2)” phase is proposed to consist of three equivalent (2 × 1) domains.

Photoabsorption and the unoccupied partial density of states of chemisorbed molecules

The 1s to 2π* resonances have been studied for CO strongly chemisorbed on Ni(100) and weakly chemisorbed on Cu(100) using high resolution X-ray absorption spectroscopy. We show that the spectra can be consistently described in terms of the local density of unoccupied 2π* electron states with the Fermi level obtained from corresponding X-ray photoelectron spectra. The core hole is demonstrated to induce large shifts in the unoccupied 2π* density of states.

Shake-up and shake-off structures in core level photoemission spectra from adsorbates

Abstract Previous studies of the satellites in adsorbate core level spectra are reviewed. New or recently published high resolution X-ray photoemission spectra are presented. The results represent systems which cover a range of different adsorption bond strengths, from physisorption (O2/graphite) and weak chemisorption (CO/Cu(100), N2/Ni(100)) to strong chemisorption (CO/Ni(100)). The shake-up spectra reveal some new details, and are discussed in the light of previous models and interpretations. By extended shake-up/shake-off measurements, it is shown that the relative spectral weight of the main line in many cases is lower than previously suggested. The shake-up structures are discussed in terms of valence excitations in the substrate-adsorbate complex, and the role of the core hole is stressed.

Studies of the COH,H2Ni(100) system using photoelectron spectroscopy

Abstract The coadsorbate system of CO and H on Ni(100) has been studied by means of X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). Three mixed phases below room temperature have been considered, two ordered phases, c(2 × 2) and c(2√2 × √2)R45°, and a disordered phase. Large shifts in the adsorbate core-levels make it possible to distinguish between different adsorption geometries for the CO molecules. Three CO adsorption sites, on top, bridge and hollow, are populated. X-ray photoelectron diffraction (XPD) indicate that the molecules adsorb in a perpendicular geometry in all adsorption sites. Shake-up energies and intensities provide information about the electronic structure and bond strength in the adsorbate-substrate complex. The CONi interaction energy is lowered in the presence of hydrogen. At higher CO coordination to the surface, a larger population of the 2π ∗ derived level can be deduced from the satellite intensity. This leads to a weakened CO intramolecular bond. The UP spectra confirm the previously reported shift of the 4σ level towards higher binding energy for the c(2 × 2) phase. No satellite associated with this state is observed.

X-ray and UV photoemission studies of mono-, bi- and multilayers of physisorbed molecules : O2 and N2 on graphite

Abstract X-ray and UV photoelectron spectroscopy (XPS and UPS) are used to study oxygen and nitrogen molecules physisorbed on graphite. The photoemission spectra from the physisorbed molecules resemble the corresponding spectra for the free molecule. However, the XPS (UPS) binding energies for the monolayer are lowered by about 1.4 ± 0.1 eV (1.2 ± 0.2 eV) due to external screening from the polarizable surface and neighbouring molecules. The growth mode of mono-, bi- and multilayers of oxygen can be understood from the layer-dependent binding energy shifts of the adsorbate levels in the XP and UP spectra. The results are compared to XP spectra recorded for N 2 graphite . It is found that the binding energy shifts between the first and second layer is 0.50 ± 0.05 eV for XPS and 0.3 ± 0.1 eV for UPS, indicating that the final state photohole distribution is of importance. The vibrational progression in the UP spectra from the outermost O 2 level, 1Π −1 g , is observed to be slightly modified for the physisorbed molecules. The modification is more pronounced for the layer closest to the surface than for the outermost layers in the case of multilayers.

Core level spectroscopy of physisorbed molecules on graphite

Abstract Several recent applications of high-resolution core-level spectroscopies to the investigation of physisorbed molecules (N 2 and O 2 on graphite) are reviewed. The ordered arrangements of the adsorbed molecules allow the distinction between σ and π molecular states revealing for instance a complex nature of the so called σ-resonance region. The molecular Rydberg states are found to persist in the adsorbates. An orientational phase transition of O 2 on graphite has been investigated using polarization dependent XAS. This phase transition also gives rise to pronounced changes in the XPS line profile due to a position dependent screening response from the substrate. The Auger spectrum of N 2 on graphite shows features corresponding to the decay of ionic as well as neutral states making it possible to determine the charge transfer rate for the neutralization process. The new possibility of recording autoionization spectra with vibrational selection in both the excitation and decay steps are demonstrated for adsorbed N 2 .

On the growth of Ni on Cu(100)

The growth of Ni on Cu(100) has been studied by X-ray photoelectron spectroscopy (XPS). The thermal stability of the submonolayer phase was investigated using XPS forward scattering measurements. The results suggest that the surface alloying starts already around 260–300 K. The variations of Ni2p32 binding energy and line profile with coverage were studied in order to obtain information about the Ni growth mode and lateral morphology of the Ni overlayer. Furthermore, CO titration was used to study the distribution of Ni atoms in the initial stages of overlayer growth. The experiments lend support to a model of Ni monolayer development based on the gradual compression of isolated Ni adatoms.

Local probing of adsorbate electronic structure using soft X-ray emission ; atomic nitrogen on Ni(100) and Cu(100)

Abstract The N 2p partial density of states of p4g-N/Ni(100) and c(2 × 2)-N/Cu(100) overlayers have been studied by soft X-ray emission spectroscopy (SXES) and comparisons with corresponding UV photoelectron spectra are made. Broad states (∼10 eV) are observed due to the hybridisation between the N 2p and the substrate 3d and 4sp bands. For both N/Cu and N/Ni the intensity stretches out to the Fermi level, where a prominent peak for N/Cu is observed. These states close to the Fermi level are interpreted as antibonding 2p-3d hybridised states. The higher occupancy of these states in N/Cu is expected due to the higher binding energy of the 3d band in Cu than in Ni. For a complete interpretation of the SXE spectra, especially for N/Cu where many features are observed, further theoretical studies are required.

O/Cu(100) studied by core level spectroscopy

Abstract The chemisorption of oxygen on Cu(100) has been investigated by X-ray photoelectron spectroscopy. From the O 1s binding energy shifts, it is concluded that different adsorption sites are involved in the unreconstructed, disordered and reconstructed (2√2 × √2)R45° phases, respectively. There is an overall binding energy shift of 0.6 eV between θo = 0.13 and θo = 0.50 and the shift is towards higher binding energy for increasing coverage. A strongly asymmetric broadening of the core electron line is observed in the spectra recorded from the (2√2 × √2)R45° phase at elevated temperatures. The full width at half maximum is 1.2 eV at 770 K, compared to 0.80 eV at 300 K. The temperature effects are interpreted as due to vibrational excitations of the adsorbate-substrate complex. No adsorbate-induced ordered structure was observed by LEED except for the (2√2 × √2)R45° pattern.