B. Hernnäs

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.

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.

Orientation of a molecular precursor: a NEXAFS study of O2/Ag(110)

We present the results of a near-edge X-ray absorption fine structure (NEXAFS) study of physisorbed and chemisorbed molecular oxygen on Ag(110). The physisorbed species was found to lie down on the surface for the monolayer case (with a mean tilt of 29° ± 5° away from the surface) and with the molecular axis lying along the [001] direction; this is perpendicular to the conventionally held azimuthal orientation of the chemisorbed molecule. For the chemisorbed phase, our spectra show features not seen in earlier studies; these cannot be interpreted in terms of the orbitals of the free molecules and imply both the π and π∗ oxygen molecular orbitals are involved in bonding with the substrate.

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.

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.

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.

NEXAFS study of molecular orientation in physisorbed oxygen on graphite

Abstract The results of a near-edge X-ray adsorption fine structure (NEXAFS) study of physisorbed oxygen on graphite at 25 K are reported. Two monolayer phases were investigated (δ and ξ), together with a multilayer of condensed O2. As the oxygen coverage is increased, an orientational transition between the monolayer δ and ξ phases is clearly seen with the mean molecular tilt angle changing from 20° to the surface to 37 ± 10° to the surface normal. In multilayer growth the molecules tend to tilt closer to the surface normal with increased oxygen dosage. These changes are discussed in terms of static and dynamic tilting.

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.

The inner valence region of CO adsorbed on Pd(100)

The inner valence region of CO/Pd(100) p(2 square root 2* square root 2)R45 degrees has been studied by angular resolved photoemission at the Pd 4d Cooper minimum, and with resonant Auger spectroscopy at photon energies corresponding to the C 1s and O 1s X-ray absorption (XA) maxima of the unoccupied parts of the 2 pi *-Pd 4d hybrid (2 pi un). Previously unobserved inner valence states are revealed in the direct photoemission and are compared with resonant Auger results. The interpretation and assignment of the different spectral features to different main final state configurations are based on energy, symmetry and intensity arguments, as well as comparisons with previous results.