| 1. |
- Andersen, J. N., et al.
(författare)
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Vibrational fine structure in the C 1s core level photoemission of chemisorbed molecules : Ethylene and ethylidyne on Rh(111)
- 1997
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Ingår i: Chemical Physics Letters. - 0009-2614. ; 269:3-4, s. 371-377
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Tidskriftsartikel (refereegranskat)abstract
- The origin of fine structure in the core-level photoemission spectra of the C2H4, C2D4, C2H3 and C2D3 molecules chemisorbed on Rh(111) is probed in a careful high-resolution study showing that this structure arises from internal molecular vibrations rather than from other chemically-shifted carbon atoms. It is shown by comparison of the adsorbate and gas-phase spectra that the underlying features are the same despite differences arising from adsorption. This new approach to the investigation of adsorbed molecules may prove to be useful in further studies of other systems and the possibility that such effects may exist could lead to the reinterpretation of other adsorbate systems.
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| 2. |
- Beutler, A., et al.
(författare)
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Coverage- and temperature-dependent site occupancy of carbon monoxide on Rh(111) studied by high-resolution core-level photoemission
- 1998
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Ingår i: Surface Science. - 0039-6028. ; 396:1-3, s. 117-136
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Tidskriftsartikel (refereegranskat)abstract
- High-resolution core-level photoemission is used to study structural aspects for the molecular adsorption of CO on the Rh(111) single-crystal surface, and in particular to derive the adsorption sites. The site sensitivity of the core-level binding energy and the fact that the core level photoemission signal is proportional to the adsorbate coverage make it possible to study quantitatively how the occupation of different sites changes with temperature and/or CO coverage. For the CO/Rh(111) adsorption system we find two sites (on-top and three-fold hollow) to be occupied by the CO molecules. At coverages up to 0.33 ML only on-top sites are occupied, whereas at higher coverages a mixture of three-fold hollow and on-top sites are found. The distribution between these two sites is found to depend strongly on temperature. Quantitative studies of these reversible, temperature-dependent site changes have been carried out for a number of CO coverages. For coverages between 0.33 and ∼0.54 ML, increasing the temperature results in part of the molecules moving from on-top to three-fold hollow sites. This change is strongest for a (4 × 4) structure formed at 0.5 ML where an order-disorder transition is observed at a temperature of 120 K. For coverages above ∼0.54 ML, increasing the temperature leads instead to a decrease of the relative occupation of the three-fold hollow sites. For coverages below 0.33 ML, the molecules occupy on-top sites at all temperatures.
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| 3. |
- Beutler, A., et al.
(författare)
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On the adsorption sites for CO on the Rh(111) single crystal surface
- 1997
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Ingår i: Surface Science. - 0039-6028. ; 371:2-3, s. 381-389
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Tidskriftsartikel (refereegranskat)abstract
- High resolution photoemission applied to the C 1s and Rh 3d core levels has been used to investigate the adsorption sites at low temperature of CO on the Rh(111) single crystal surface as a function of CO coverage. Two different sites are found to be occupied by the CO molecules. At coverages up to 0.5 monolayers the majority of the molecules are found to adsorb in on-top sites whereas at higher coverages three-fold hollow sites become increasingly populated. The different C 1s binding energy of the CO molecules in these two sites makes it possible to measure diffraction induced intensity variations versus photon energy in a site specific manner. The saturation (2 x 2)-3CO structure formed at a coverage of 0.75 monolayers is argued to contain one on-top and two three-fold hollow molecules per unit cell.
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| 4. |
- Heskett, D., et al.
(författare)
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Alkali-alkali interactions on Al(111) : segregation and islanding in Na-Rb mixtures
- 1994
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Ingår i: Surface Science. - 0039-6028. ; 316:3, s. 303-308
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Tidskriftsartikel (refereegranskat)abstract
- The low temperature coadsorption of Na and Rb on an Al(111) surface is examined with a combination of LEED and core level photoemission spectroscopy. We observe LEED pattern sequences and alkali core level binding energies which are consistent with a net total alkali coverage and can be explained by Na-Rb repulsion, segregation, and islanding, though other models cannot be definitely excluded.
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| 5. |
- Heskett, D., et al.
(författare)
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Core-level satellite excitations of K/Al(100) and K/Al(111)
- 1995
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Ingår i: Physical Review B (Condensed Matter). - 0163-1829. ; 52:16, s. 12366-12371
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Tidskriftsartikel (refereegranskat)abstract
- Alkali-metal-induced satellite peaks associated with the K 3p and Al 2p core levels have been measured with photoemission for K/Al(111) and K/Al(100) under both low- and room-temperature preparation conditions. For low-temperature deposition we observedddd loss peaks in good agreement with electron-energy-loss spectroscopy studies of analogous systems which we assign to the excitation of collective plasmonlike modes in the alkali-metal overlayer. For room-temperature preparation conditions, we observed significant changes in the satellite loss structure which we attribute to a decrease or loss of metallic behavior in the alkali-metal layer. We account for some of our results as a change in bonding configuration of the K atoms from on-top-of surface to substitutional adsorption for low versus room-temperature preparations.
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| 6. |
- Jaworowski, A. J., et al.
(författare)
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Adsorption sites in O and CO coadsorption phases on Rh(111) investigated by high-resolution core-level photoemission
- 1999
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Ingår i: Surface Science. - 0039-6028. ; 431:1, s. 33-41
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Tidskriftsartikel (refereegranskat)abstract
- High-resolution core-level spectroscopy is used in combination with low-energy electron diffraction (LEED) and photoelectron diffraction to identify the adsorption sites for three different coadsorbed phases consisting of ordered overlayers of oxygen coadsorbed with CO on the Rh(111) single-crystal surface. The three ordered overlayer structures, which may be denoted as 2O + CO/Rh(111), O + CO/Rh(111) and O+2CO/Rh(111), all show (2 × 2) LEED patterns. In the 2O + CO and O + CO phases the CO molecules are found to occupy only on-top sites while the O + 2CO phase shows CO molecules in both on-top and three-fold hollow sites. In all cases the oxygen atoms are found in three-fold hollow sites. For the O + CO and O + 2CO phases our results confirm previous determinations by LEED, while the 2O + CO phase has not been observed before on Rh(111). The core-level binding energies of the C 1s and O 1s core levels for both adsorbates are characteristics of the adsorption site and are very close to the binding energies found for the pure cases of only oxygen or CO adsorbed on Rh(111). In the coadsorption phases we find that the interaction between the adsorbates has only a minor influence on the core-level binding energies. For the O + 2CO/Rh(111) coadsorption phase we find that a full CO coverage is not obtained; less than 80% of the unit cells contain two CO molecules, in line with previous findings.
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| 7. |
- Lundgren, E., et al.
(författare)
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Alkali core level binding energy shifts from buried interfaces between alkali films and metallic substrates with different surface index
- 1995
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Ingår i: Vacuum. - 0042-207X. ; 46:8-10, s. 1159-1163
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Tidskriftsartikel (refereegranskat)abstract
- Core level binding energy shifts from approximately three layers of different alkali metals deposited on Pd(111), Pd(100), Rh (111), and Rh (110) are presented in order to demonstrate how the packing of the substrate surface affects the layer-resolved alkali core level binding energy shifts. It is found that a more open surface induces a larger alkali core level binding energy shift. It is shown that this behaviour can be explained as a consequence of the higher coordination of the alkali metal atom to the substrate on a more open surface than on a more closed packed surface.
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| 8. |
- Lundgren, E., et al.
(författare)
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Alkali core-level binding-energy shifts in alkali/4d-metal interface systems
- 1994
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Ingår i: Physical Review B (Condensed Matter). - 0163-1829. ; 50:7, s. 4711-4717
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Tidskriftsartikel (refereegranskat)abstract
- Photoemission from different alkali core levels (Na 2p, K 3p, Rb 4p, and Cs 4d) has been studied for thin alkali films deposited on four different 4d metal surfaces: Mo(110), Rh(111), Rh(110), and Pd(100). Alkali atoms at the interface, the bulk, and at the surface of the adsorbed alkali film are found to have different core-level binding energies. It is found that the core-level binding-energy shifts of the alkali metals induced by the 4d-metal substrate increase with increasing atomic number of the alkali metal and with increasing atomic number of the 4d metal. Thermodynamical quantities such as interface segregation energies and adhesion energies are deduced from the layer-resolved shifts. Estimates of the experimental binding-energy shifts are given using semiempirical calculations, and certain complications in doing that are discussed.
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| 9. |
- Lundgren, E., et al.
(författare)
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Na and K on Al(100) studied by low-energy electron diffraction and high-resolution core-level spectroscopy
- 1997
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Ingår i: Surface Science. - 0039-6028. ; 370:2-3, s. 311-323
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Tidskriftsartikel (refereegranskat)abstract
- Experimental results for Na and K deposited at 100 K on the A1(100) surface using low-energy electron diffraction and high-resolution core-level spectroscopy are presented. Our results show that from a coverage of 0.20 monolayers (ML), Na condenses into dense islands with a local Na coverage of 0.50 ML. In the case of K, the overlayer condenses into islands at a coverage of 0.18 ML, with a local coverage of 0.30 ML. A compilation of various geometric parameters of the present systems and of alkalis on Al(111) suggests that the condensation of K on Al(100) differs from that of other alkalis on Al systems which undergo condensation into two-dimensional islands. Furthermore, all the island-forming structures display an alkali-alkali distance which is expanded relative to the nearest-neighbour distance in the respective alkali metal. The reasons for and implications of this are discussed.
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| 10. |
- Lundgren, E., et al.
(författare)
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On the temperature induced transformation between the two Al(111)-(√3 × √3)R30°-Rb structures
- 1995
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 343:1-2, s. 37-43
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics of the temperature induced transformation between low and high temperature adsorption phases of Rb on Al(111) has been followed by high resolution core level spectroscopy. Particular emphasis has been paid to the order-preserving transformation between the low and high temperature Al(111)-Rb-(√3 × √3)R30° structures in which Rb atoms occupy on-top and substitutional sites, respectively. For a Rb coverage of 0.33 monolayer the rate of conversion from on-top ti substitutional sites is found to increase strongly at 250 K. For Rb coverages below 0.33 but above approximately 0.14 monolayers the conversion occurs at lower temperatures. Increasing the coverage above 0.33 monolayers increases the conversion temperature. The possibility to detect the Al atoms which are expelled from the first Al layer during the conversion is demonstrated.
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| 11. |
- Lundgren, E., et al.
(författare)
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Unified behavior of alkali core-level binding-energy shifts induced by sp metals
- 1997
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Ingår i: Physical Review B (Condensed Matter). - 0163-1829. ; 56:19, s. 12560-12565
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Tidskriftsartikel (refereegranskat)abstract
- Thin overlayers of Na, K. Rb, and Cs on different sp-metal substrates have been investigated using photo-electron spectroscopy. The alkali core levels show clearly resolved binding-energy shifts between the surface layer, the intermediate layer(s), and the interface layer. The magnitude of these shifts depends on sp metal and on alkali metal. The layer-resolved core-level binding-energy shifts are well reproduced by models based on a thermodynamical description. For three-layer alkali films the core-level binding energy of the intermediate layer is found to exhibit a small but significant shift between different sp-metal substrates. A simple relationship between the core-level binding-energy shift for the interface layer and the difference in rs value between the sp substrate and the adsorbate is shown to exist.
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| 12. |
- Strisland, F., et al.
(författare)
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Adsorption sites in coadsorption systems determined by photoemission spectroscopy : K and CO coadsorbed on Rh(111)
- 1998
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Ingår i: Surface Science. - 0039-6028. ; 410:2-3, s. 330-343
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption sites of coadsorbed K and CO on the Rh(111) surface have been determined using high-resolution core-level spectroscopy, low-energy electron diffraction and site-resolved photoelectron diffraction. For both a (2 × 2)-2CO-1K and a (2√3 × 2√3)-6CO-1K structure, we find that the CO molecules occupy threefold hollow sites and the K atoms on-top sites, contrary to the adsorption sites of K (threefold hollow site) and CO (on-top site below 0.5 monolayers) if adsorbed alone on Rh(111). Deposition of K onto a CO precovered surface is found to induce large shifts towards lower binding energy of the C and O 1s core levels (∼0.7 eV for C 1s and ∼1.5 eV for O 1s). The major part of these shifts is shown to arise from the K-induced site change of the CO molecules. This finding may be of importance in the interpretation of XPS data of related co-adsorption systems. Finally, it is suggested that the C and O 1s binding energies provide useful fingerprints of the CO adsorption site also for co-adsorption systems.
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