P. Bennich

Physisorbed, chemisorbed and dissociated O2 on Pt(111) studied by different core level spectroscopy methods

Abstract For O 2 Pt (111) we have found four different adsorption phases which are formed at different substrate temperatures. At about 25 K the oxygen molecules physisorb on the surface. Two chemisorbed phases are observed at 90 and 135 K, respectively. An atomic phase, characterized by a sharp (2 × 2) LEED pattern, exists at a temperature above 150 K. Different spectroscopic techniques have been used to characterize the four different adsorption states: XPS studies of adsorbate and surface core level shifts, UPS, NEXAFS, autoionization and Auger spectroscopy. We conclude that oxygen adsorbs in two different molecular chemisorbed states which can be considered to be precursors for the thermally activated atomization process. The first of these molecular states is weakly chemisorbed at 90 K. It is adsorbed in a hollow site with a saturation coverage of 0.23 (molecules per Pt surface atom). We have identified this phase as a superoxo-like configuration. The second phase is more strongly bonded to the Pt substrate. It is characterized by a longer and weaker molecular σ bonding due to more charge transfer from the metallic substrate to the antibonding molecular 1 π g orbitals than for the first chemisorbed phase. With a coverage of 0.15 the oxygen molecules seem to be adsorbed in hollow or hollow-bridge sites. We have characterized this phase as a peroxo-like configuration of the oxygen molecule. For atomic oxygen on platinum we have found a coverage of 0.25 (oxygen atoms per Pt surface atom) and a threefold adsorption site, in agreement with previous studies. We discuss the XAS results according to a model for the density of states induced by the hybridization of the 2p atomic orbitals with the 6sp states and 5d band of the metal.

The bonding of CO to metal surfaces

The atom and symmetry specific properties of x-ray emission spectroscopy have been applied to the investigation of CO adsorbed on Ni(100) and Cu(100) surfaces. In comparison to ab initio electronic structure calculations, obtained in density functional theory, we develop a consistent electronic structure model of CO adsorption on transition and noble metals and extend to a conceptual model of the surface chemical bond. A strong CO–substrate interaction is found, characterized by significant hybridization of the initial CO orbitals and the metal bands. In the π system an allylic configuration is found as the result of orbital mixing between the CO 1π, 2π* and the metal dπ-band which is manifested experimentally in the observation of an oxygen lone-pair state. In the σ system experimental evidence of equally strong orbital mixing has been found. Energetically, the adsorbate–substrate complex is stabilized by the π-interaction but is destabilized by the σ-interaction. Furthermore, the internal C–O bond carri...

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.

Chemisorption of CO on Cu(100), Ag(110) and Au(110)

Abstract The adsorption of CO on Cu, Ag and Au is studied using core and valence photoemission, X-ray absorption and autoionization of core excited states. The purpose is to investigate the nature of the adsorption bond starting out from the well-established chemisorption system CO/Cu(100)-c(2 × 2), and from the results we suggest that CO forms chemisorbed phases also on Ag(110) and Au(110). The photoemission spectra show strong shake-up satellites both for the valence levels and the core levels. The separation to the satellite appearing closest to the main line is observed to follow the position of the substrate d-band relative to the Fermi level. The CO adsorption strength for the noble metals is deduced to decrease in the order Cu-Au-Ag. This is based on the widths of the XA resonances, which are related to the adsorbate-substrate interaction strength of the core excited states, and the relative shake-up intensities, which are expected to increase with a decreasing adsorption strength in the ground state. The same trends regarding the shake-up intensities are observed both for the valence and core levels.

Nature of the ns-derived states for an isolated alkali atom on a surface

It has recently been shown (for K/graphite) that experimental core level spectroscopy data for adsorbed Ar combined with calculations is a most valuable tool in order to obtain information about th ...

A photoemission and XAS study of oxygen coadsorbed with a (2x2) layer of K on graphite

We have studied the coadsorption of oxygen with a (2 × 2) monolayer of K on graphite. At least three different adsorption phases for oxygen have been found. Different spectroscopic techniques have been used in order to identify the different oxygen species adsorbed on the surface. For low oxygen coverage, the molecules dissociate and form a K2O species. At higher oxygen doses, K2O2 and KO2 are present on the surface up to saturation coverage. XAS yields an O2 bond length of about 1.39 A for the KO2 in this phase. This species is found to form an ionic complex which is weakly bound to the graphite.

XPS and XAS study of oxygen coadsorbed with a dispersed phase of K on graphite

The adsorption of O2 onto a dispersed (low coverage) phase of K on graphite has been characterized by X-ray photoemission and X-ray absorption spectroscopy. We identify two oxygen adsorption phases before physisorbed oxygen appears. The first species dominates at low O2 exposures whereas the second one, which is probably due to oxygen adsorbed in a superoxo-like configuration (O−δ2, δ⩽1), is dominant at higher oxygen coverage. As a function of increasing oxygen dose, we confirm that the charge, initially donated by the potassium adlayer to the graphite, is gradually withdrawn from the substrate.

New final state in the autoionization decay detected for N2/(2×2)K/graphite: Relevance for the affinity level of NO on a K monolayer

We have observed a new type of charge transfer state in the autoionization spectrum for N2/(2×2)K/graphite. It can be viewed as a consequence of an important covalent component to the bonding with the surface in the core hole excited state for N2/(2×2)K/graphite or equivalently for the Z+1 system, “NO”/(2×2)K/graphite.