Publications (8)0 Total impact
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Article: X-ray and UV photoemission studies of mono-, bi- and multilayers of physisorbed molecules: O2 and N2 on graphite
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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 . 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 O2 level, 1Π−1g, 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.Surface Science. -
Article: Orientation of a molecular precursor: a NEXAFS study of O2/Ag(110)
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ABSTRACT: 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.Surface Science. 278(3):239-245. -
Article: Distinction between different adsorption states: chemisorbed and physisorbed Ar
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ABSTRACT: We introduce a procedure to distinguish between physical and chemical adsorption on surfaces using core level spectroscopies. Based on Ar 2p X-ray absorption and autoionization spectra Ar is proposed to be physisorbed on graphite but chemisorbed on Ag(110).Surface Science Letters 293. -
Article: Core level spectroscopy of physisorbed molecules on graphite
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ABSTRACT: Several recent applications of high-resolution core-level spectroscopies to the investigation of physisorbed molecules (N2 and O2 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 O2 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 N2 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 N2.Surface Science. -
Article: O/Cu(100) studied by core level spectroscopy
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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.Surface Science. -
Article: Chemisorption of CO on Cu(100), Ag(110) and Au(110)
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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.Surface Science. -
Article: Physisorbed, chemisorbed and dissociated O2 on Pt(111) studied by different core level spectroscopy methods
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ABSTRACT: For 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 1πg 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.Surface Science. -
Article: Overlayer structure from adsorbate and substrate core level binding energy shifts: CO, CCH3 and O on Pt(111)
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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.Surface Science.
