| 11. |
- Sandell, A., et al.
(författare)
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Observation of a low-energy adsorbate core-level satellite for CO bonded to palladium : Coordination-dependent effects
- 1998
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Ingår i: Physical Review B (Condensed Matter). - 0163-1829. ; 57:20, s. 13199-13208
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Tidskriftsartikel (refereegranskat)abstract
- A strong low-energy shake-up satellite for CO adsorbed on Pd is observed. The occurrence of the satellite is established for the CO/1 ML Pd/Mo(110) system at a coverage where CO adsorbs exclusively on-top. Comparisons with CO adsorbed on Pd single-crystal surfaces and small supported Pd particles indicate that the strongly increased satellite intensity is due to the decreased CO-Pd interaction strength for on-top adsorbed CO. This can be used to get further insight into the structure and bonding properties of the adsorbate system. Since a low-energy shake-up feature may be misinterpreted as a chemically shifted component, the conclusion is that great care has to be taken in the evaluation of adsorbate core-level spectra for systems with large variations in adsorption strength depending on the adsorbate sites. Large variations in the CO site distribution may furthermore occur depending on the nature of the Pd substrate: Adsorption of CO on 1 ML Pd/Mo(110) leads to an overlayer dominated by an on-top species and, likewise, the CO overlayer formed on small Pd particles after large doses has a large fraction of on-top bonded species. This is in strong contrast to Pd single-crystal surfaces, where CO adsorbed in more highly coordinated sites is abundant.
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| 12. |
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| 13. |
- Beutler, A., et al.
(författare)
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Adsorption properties of a mixed surface studied by high resolution core level photoemission : CO/0.5 ML Pd/Rh(111)
- 1998
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Ingår i: Surface Science. - 0039-6028. ; 411:1-2, s. 111-122
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Tidskriftsartikel (refereegranskat)abstract
- The coverage-dependent adsorption properties of a laterally heterogeneous bimetallic surface have been investigated by high resolution core level photoemission and low energy electron diffraction. The specific system under study was CO adsorbed on a Rh(111) surface onto which 2D Pd islands (coverage 0.5 ML) were formed by vapor deposition. The CO adsorption properties of the heterogeneous surface were compared with CO adsorption on a Rh(111) surface covered with a full Pd monolayer and with previous results for the CO/Rh(111) system. For low exposures CO is only found on the Rh(111) patches which can be explained by diffusion of CO from the Pd islands onto Rh parts in the adsorption process. At higher exposures CO diffusion from Rh to Pd is indicated. The origin of the diffusion processes can be found in the different coverage-dependent CO adsorption energies on the two surface parts.
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| 14. |
- 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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| 15. |
- Beutler, A., et al.
(författare)
-
The influence of preadsorbed oxygen on the adsorption of CO on two-dimensional Pd islands on a Rh (111) surface
- 1998
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Ingår i: Surface Science. - 0039-6028. ; 418:2, s. 457-465
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Tidskriftsartikel (refereegranskat)abstract
- The influence of preadsorbed oxygen on the CO adsorption properties of a laterally heterogeneous bimetallic surface consisting of Pd islands on Rh(111) at a Pd coverage of 0.5 monolayers has been studied by high resolution core level photoemission. A surface consisting of clean Pd islands surrounded by oxygen-covered Rh(111) patches was prepared by predosing oxygen at room temperature. By applying core level photoemission to the Pd 3d5/2 and the C 1s levels, the adsorption of CO on this surface was studied with particular attention being paid to possible CO diffusion between the two surface parts. The CO molecules are found to diffuse from the oxygen-covered Rh(111) patches onto the Pd islands for low to medium CO exposures. This diffusion direction is opposite of that found previously for the 0.5 ML Pd on Rh(111) system with no oxygen predosing. This reversal of the diffusion direction is argued to be due to a large reduction of the CO adsorption energy on the Rh patches of the surface caused by the preadsorbed oxygen.
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| 16. |
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| 17. |
- Caidahl, Kenneth, 1949, et al.
(författare)
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New concept in echocardiography: harmonic imaging of tissue without use of contrast agent.
- 1998
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Ingår i: Lancet (London, England). - : Elsevier BV. - 0140-6736. ; 352:9136, s. 1264-70
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Tidskriftsartikel (refereegranskat)abstract
- Endocardial border detection is important for echocardiographic assessment of left-ventricular function. Second harmonic imaging of contrast agents enhances this border detection. We discovered that harmonic imaging improves tissue visualisation even before contrast injection. We therefore sought objectively to demonstrate the degree of enhancement of endocardial and myocardial visualisation.An ATL HDI-3000 scanner with software for contrast harmonic imaging was used to record short-axis images of the left ventricle in 27 patients with possible myocardial disease and 22 controls, in the fundamental mode and with harmonic imaging. A computer program measured the relative grey-scale values within six segments of the endocardium and myocardium. An Acuson Sequoia scanner equipped with software for tissue harmonic imaging was used to investigate the reproducibility of ejection-fraction calculations in 22 patients with ischaemic heart disease.Harmonic imaging produced brighter endocardium within each segment. Relative to the mean grey value of the total imaging sector, the values for harmonic and fundamental imaging were 171.5 vs 85.6% (p<0.0001) in end diastole and 194.1 vs 106.7% (p<0.0001) in end systole. Results for the myocardial segments were also significantly better for harmonic imaging. Structure enhancement of similar magnitude was seen among patients and healthy controls. Use of harmonic imaging reduced the proportion of unacceptable images by 14-46% in different views and improved the reproducibility of biplane ejection-fraction measurements.In comparison with fundamental imaging, the relative endocardial and myocardial brightness is enhanced by harmonic imaging.
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| 18. |
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| 19. |
- 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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| 20. |
- 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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