P.A. Montano

X-ray absorption fine structure study of small metal clusters isolated in rare-gas solids

Abstract The use of extended X-ray absorption fine structure (EXAFS) to determine the interatomic distances in small metal clusters is of considerable scientific as well as technical importance, we have used synchrotron radiation to study the near edge structure as well as the EXAFS spectra of metal clusters isolated in rare-gas matrices, by studying such small metal clusters in inert-gas matrices, it is possible to investigate metal-metal interactions without the interference of strongly interacting supports. Considerable contraction in small metal clusters of iron and chromium consisting of a few atoms has been observed. Associated with this contraction is a large variation in the electronic properties as detected by the isomer shift and magnetic hyperfine field at the 57 Fe nucleus. The chromium results are compared with recent gas-phase measurements reported in the literature. EXAFS was also used to determine the interatomic distance for small metal particles of silver isolated in solid argon.

Leed, auger, and electron energy loss studies of Ni epitaxially grown on Cu(100)

Abstract Nickel was epitaxially deposited onto a clean, flat Cu(100) surface. Low energy electron diffraction I ( E ) curves were recorded for 0.6, 1.1, and 2.7 monolayer (ML) Ni coverage. Multilayer relaxation was considered in theoretical calculations, which were compared with experiment by means of the R | ΔE | factor. The estimated relaxations of the first and second interlayer spacings are estimated to be − 2% and + 1.5% for clean Cu(100), − 2% and − 1.5% for 1 ML Ni coverage, relative to the bulk Cu interlayer spacing of 1.81 A, and −1% and 0% for 3 ML Ni coverage, relative to the bulk Ni spacing of 1.76 A. Decreasing the surface Debye temperature of the Ni layer to 268 K from the bulk value of 440 K improves the agreement between theory and experiment. The optimum inner potential values are − 9 and − 10 eV for clean Cu(100) and Ni on Cu(100), respectively. Auger electron spectroscopy was used to determine the thickness of the Ni films, and LEED indicates layer-by-layer growth until about 4 layers, when the LEED spots begin to spread, indicating island formation. Electron energy loss spectra were obtained with primary electron energies of 150 and 300 eV. The 3p core ionization transition was clearly observed after 0.5 ML Ni coverage. Peaks at 3.8 and 7.5 eV for clean Cu are ascribed to interband transitions, and shift to higher energy with Ni coverage. Peaks at 10 and 16 eV for clean Cu (ascribed to an interband transition and a surface plasmon, respectively) disappear with Ni coverage. Bulk plasmon peaks at 19 and 27 eV remain unshifted with Ni coverage. The effect of 0.9 and 1.3 ML Ni coverage of Cu(100) on the chemisorption of Co and oxygen was also studied. The behavior of the surface towards oxygen chemisorption was similar to that of the pure Ni surface. For a large exposure of oxygen (50 L and more) the EEL and Auger spectra are very similar to those observed for NiO. In the case of CO, for submonolayer Ni coverage, the surface shows a more Cu-like behavior, while for larger Ni coverage (a monolayer and more) there is a great similarity with the behavior of the pure Ni(100) surface.

EXAFS, XANES and TEM studies of Pt-Ni bimetallic catalysts

Abstract Pt-Ni bimetallic particles have been studied by EXAFS and XANES techniques and electron diffraction. The near edge absorption measurements show conclusively that an alloy is formed in this system. By plotting the change in nearest neighbor distance as a function of concentration, it is shown that the linear relationship between concentration and interatomic distance (Vegards law) which is commonly observed in bulk alloys is not followed in the case of small particles.

Electron energy loss and Auger study of epitaxially grown Cu on Ni(100)

Abstract Auger and electron energy loss spectra have been measured on films of Cu epitaxially grown on Ni(100). The films were prepared under UHV conditions using a quartz crystal for monitoring the deposition rate. LEED measurements were taken to determine the orientation of the films. The presence of a monolayer of Cu on Ni(100) is enough to suppress the 3p-3d transition on the surface of the sample. The electron energy loss spectra were studied as a function of the primary electron energy (50 to 300 eV). The experimental results were qualitatively analyzed using recent theoretical calculations of Cooper and co-workers. The effect of a small Cu coverage on Ni(100) on the chemisorption of CO and O 2 was also studied. A strong suppression of CO chemisorption at room temperature was observed. In the case of O 2 , large exposures are necessary in order to observe a significant amount of oxygen on the surface. The absence of any appreciable chemisorption on the surface of the metal is attributed to the lack of empty d-surface states.

An xps study of the interaction of SO2 with CaO(100): Effect of temperature and metal adsorbates (Fe, Na)

Using X-ray photoelectron spectroscopy (XPS), the chemisorption of SO2 on CaO(100) has been studied in the temperature range from 25 to 400° C. The chemical state of sulfur has been identified as that of sulfate. The kinetics of SO2 chemisorption on CaO are discussed. The binding states of Fe and Na on CaO were determined to be Fe2+ and Na+ respectively. The effect of the metal adsorbates (Fe, Na) on the chemisorption of SO2 was also studied. In the presence of Fe (coverage ⩾ 0.5 monolayer), both sulfate and sulfide were observed; in the presence of Na, however, only sulfate was observed. The saturated amount of sulfate formed depends on the Fe coverage, but it is independent of the Na coverage. At low Fe coverages (< 0.5 monolayer), there is a large increase in the rate of sulfate formation at low SO2 exposures (< 3 L). This increase is explained by the “activation” of SO2 chemisorption sites by Fe adatoms. At increased Fe coverage, the rate of sulfate formation decreases due to the blocking effect of the Fe islands formed on the surface. Similarly, the effect of Na adatoms on SO2 uptake can also be explained by the same mechanism. However, the inability of the Na islands to block the SO2 chemisorptive sites and the lack of chemisorptive bond formation between SO2 and Na are explained by the diffusion of SO2 through the Na adlayer.

EXAFS study of Ag, Fe, and Ge microclusters

Abstract We have determined the structure of microclusters of Ag, Fe, and Ge using EXAFS. The measurements were performed over a wide range of cluster sizes. The clusters were prepared using the gas aggregation technique and isolated in solid argon. The measurements were performed at beam line X-18B at the National Synchrotron Light Source (NSLS).

An electron energy loss study of the chemisorption of oxygen and carbon on iron

Abstract Electron Energy Loss Spectroscopy (EELS) was used to investigate the chemisorption of oxygen and carbon on iron. Compared with those of clean iron, the EELS spectra of oxidized iron show characteristic features with strong enhancement of the interband transitions involving the Fe-3d band (4.6 and 7.5 eV) and moderate enhancement of the M 2,3 transition doublet (54.4 and 58.2 eV). Interband transitions involving the O-2p (7.5 and 12.5 eV) and O-2s (19.8 eV) bands appear. Plasmon peaks (17.3 and 23.5 eV) of the oxides were also identified. Prolonged sputtering of the bulk iron oxides results in enrichment of metallic iron on the oxide surfaces. The EEL spectra show a pronounced increase in the intensity of the peak at 12.6 eV and a shift of the peak at 17.3 to 15.2 eV. The changes in the electron energy loss structures with an overlayer of graphitic or carbidic carbon were also investigated. The electron energy loss peaks are compared with those of bulk graphite and Fe3C.

EXAFS and Mössbauer Study of Small Metal Clusters Isolated in Rare-Gas Solids

The determination of the interatomic separation in small metal clusters is of great importance. For instance, the knowledge of this parameter permits one to carry out theoretical calculations on small clusters to predict their experimentally measured properties. There have been several electron diffraction studies to determine the lattice parameter for various cluster sizes [1]. However, a simple interpretation of the electron diffraction data for small clusters is often unreliable and this obscures the determination of the atomic separation [1]. On the other hand, EXAFS provides a unique approach to such measurements on small clusters.

LEED intensity measurements of Cu epitaxially grown on Ni(100)

LEED intensity measurements are reported for pure Ni(100), and for one monolayer, four and nine layers of Cu on Ni(100). The intensities of the (00), (10), (11), (20) and (22) sets of spots were measured using a spot photometer. The largest variation in the intensity pattern with coverage was observed for the (00) spot. Dynamical calculations were performed for the specular and non-specular beams. It is found that the clean Ni(100) surface has the same interlayer distance as bulk nickel, while one monolayer of Cu on Ni(100) is found to be expanded by about 1.7%.