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Size-selected Mon-, Wn-, and Fen - cluster anions are deposited on a weakly interacting substrate (highly oriented pyrolytic graphite) and studied ex-situ using atomic force microscopy. Depending on size, three growth modes can be distinguished. Very small clusters consisting of less than 10-30 atoms behave similar to atoms and coalesce into 3-dimensional bulk-like islands. Medium sized clusters consisting of hundreds of atoms do not coalesce and follow a Stanski-Krastanov growth pattern. At low coverage, an almost perfect monolayer is formed. This is a new finding different from all previous studies on deposited metal clusters. For clusters with several thousands of atoms, the growth pattern again changes. At low coverage, the substrate is dotted with individual clusters, while at high coverage, the surface becomes extremely rough. [ABSTRACT FROM AUTHOR]

Thins films are synthesized by deposition of size-selected Mon– cluster anions on an inert substrate. Scanning tunneling microscopy pictures indicate that the deposited material consists of individual particles with diameters corresponding to the size of the preformed clusters from the gas phase. Previous attempts to manufacture cluster materials from metals failed since these clusters coalesced at room temperature. Our data suggest the possibility to synthesize new nanomaterials from clusters of high fusing metals. This may prove to be the key to harness size-dependent and tuneable properties of clusters for creating novel classes of functional tailor-made materials. [ABSTRACT FROM AUTHOR]

The total cross section has been measured for the electron induced dissociation of trimethyl (methylcyclopentadienyl) platinum (IV) [MeCpPt(IV)Me3], a Pt precursor often used in focused electron beam induced processing (FEBIP), for incident electron energies ranging between 3–3 keV. Measurements were performed for the precursor in the adsorbed state under ultrahigh vacuum conditions. The techniques used in this study were temperature programmed desorption, x-ray photoelectron spectroscopy and mass spectrometry. Two surfaces were used in these experiments, amorphous carbon overlayers containing embedded Pt atoms (a:C-Pt), formed by the electron decomposition of the Pt precursor, and atomically clean Au. The results from these three experiments revealed a comparatively low total cross section at 8 eV (4.2±0.3×10-17 cm2 on the a:C-Pt and 1.4±0.1×10-17 cm2 on the Au) that increases with increasing incident electron energy, reaching a maximum at around 150 eV (4.1±0.5×10-16 cm2 on the a:C-Pt and 2.3±0.2×10-16 cm2 on the clean Au), before decreasing at higher incident electron energies, up to 3000 eV. Differences in the measured cross sections between Au and a:C-Pt surfaces demonstrate that the substrate can influence the reaction cross section of adsorbed species. Temperature programmed desorption was also used to measure the adsorption energy of MeCpPt(IV)Me3, which was found to depend on both the substrate and the adsorbate coverage. The work in this paper demonstrates that surface science techniques can be used to quantitatively determine the total cross section of adsorbed FEBIP precursors for electron induced dissociation as a function of incident electron energy. These total cross section values are necessary to obtain quantitatively accurate information from FEBIP models and to compare the reaction efficiencies of different precursors on a quantitative basis. [ABSTRACT FROM AUTHOR]

The electron stimulated reactions of methyl iodide (MeI) adsorbed on and suspended within amorphous solid water (ice) were studied using a combination of postirradiation temperature programmed desorption and reflection absorption infrared spectroscopy. For MeI adsorbed on top of amorphous solid water (ice), electron beam irradiation is responsible for both structural and chemical transformations within the overlayer. Electron stimulated reactions of MeI result principally in the formation of methyl radicals and solvated iodide anions. The cross section for electron stimulated decomposition of MeI is comparable to the gas phase value and is only weakly dependent upon the local environment. For both adsorbed MeI and suspended MeI, reactions of methyl radicals within MeI clusters lead to the formation of ethane, ethyl iodide, and diiodomethane. In contrast, reactions between the products of methyl iodide and water dissociation are responsible for the formation of methanol and carbon dioxide. Methane, formed as a result of reactions between methyl radicals and either parent MeI molecules or hydrogen atoms, is also observed. The product distribution is found to depend on the film’s initial chemical composition as well as the electron fluence. Results from this study highlight the similarities in the carbon-containing products formed when monohalomethanes coadsorbed with amorphous solid water are irradiated by either electrons or photons. [ABSTRACT FROM AUTHOR]

The kinetics of decomposition and subsequent chemistry of adsorbed CF2Cl2, activated by low-energy electron irradiation, have been examined and compared with CCl4. These molecules have been adsorbed alone and coadsorbed with water ice films of different thicknesses on metal surfaces (Ru; Au) at low temperatures (25 K; 100 K). The studies have been performed with temperature programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), and x-ray photoelectron spectroscopy (XPS). TPD data reveal the efficient decomposition of both halocarbon molecules under electron bombardment, which proceeds via dissociative electron attachment (DEA) of low-energy secondary electrons. The rates of CF2Cl2 and CCl4 dissociation increase in an H2O (D2O) environment (2–3×), but the increase is smaller than that reported in recent literature. The highest initial cross sections for halocarbon decomposition coadsorbed with H2O, using 180 eV incident electrons, are measured (using TPD) to be 1.0±0.2×10-15 cm2 for CF2Cl2 and 2.5±0.2×10-15 cm2 for CCl4. RAIRS and XPS studies confirm the decomposition of halocarbon molecules codeposited with water molecules, and provide insights into the irradiation products. Electron-induced generation of Cl- and F- anions in the halocarbon/water films and production of H3O+, CO2, and intermediate compounds COF2 (for CF2Cl2) and COCl2, C2Cl4 (for CCl4) under electron irradiation have been detected using XPS, TPD, and RAIRS. The products and the decomposition kinetics are similar to those observed in our recent experiments involving x-ray photons as the source of ionizing irradiation. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

CuO[sub x]/Al exothermic reactions in multilayer foils were studied to identify reaction paths and reaction kinetics. Heating samples at a slow, controlled rate in a differential thermal analyzer showed that the reduction of CuO[sub x] and the oxidation of Al proceeded via two separate exotherms. To analyze this reaction pathway, samples were heated to various temperatures within these exotherms, quenched, and characterized with x-ray diffraction, Auger depth profiling, and transmission electron microscopy. Experimental evidence indicates that in the first reaction, CuO[sub x] is reduced to a mixture of CuO and Cu[sub 2]O and an interfacial layer of Al[sub 2]O[sub 3] grows to coalescence; the final products of the second exotherm are Cu, Al[sub 2]O[sub 3], and Cu[sub 2]O. The first exotherm was believed to be controlled by the two-dimensional, interface-limited growth of the Al[sub 2]O[sub 3] layer, while the second exotherm was believed to be controlled by both the diffusion-limited one-dimensional growth of the Al[sub 2]O[sub 3] and the interface-controlled growth of the Cu due to the reduction of Cu[sub 2]O. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies were conducted on Cuox/ Al exothermic reactions in multilayer foils to identify reaction paths and reaction kinetics. The results identified that in the first exothermic reaction, the lateral growth of Al2O3 nuclei appears to control the rate of heat generation and therefore the reaction rate whereas in the second reaction, diffusion of O through the Al2O3 controls the reaction rate in the first half of the exotherm.

The 257 nm photochemistry of CD3I adsorbed on MgO(100) has been investigated. The photofragments ejected along the surface normal were probed using resonantly enhanced multiphoton ionization spectroscopy coupled with time-of-flight mass spectrometry. Methyl radicals and atomic iodine fragments in both the ground I(2P3/2)≡I and spin–orbit excited I(2P1/2)≡I* states were observed as direct photofragments produced at the surface. Laser-induced desorption of methyl iodide competes with dissociation at the surface. The total cross section for removal of adsorbed CD3I from the surface at 257 nm is comparable to the gas phase cross section. A significant portion of the atomic iodine fragments remain trapped at the surface with preferential trapping of I*. Additionally, nonresonant I+ and I+2 signals were detected and initially observed to increase as a function of the irradiation time and then decrease with further irradiation. This behavior is shown to be indicative of photoinduced production and photoinduced removal of a surface intermediate. A surface reaction mechanism and the existence of a common precursor resulting from the interaction of adsorbed atomic iodine with adsorbed methyl iodide or methyl iodide clusters is postulated. [ABSTRACT FROM AUTHOR]

Presents a study which examined the adsorption of triethylgallium on the gallium arsenide surface. Experimental details; Results and discussion; Conclusion.

Recent research in heterogeneous catalysis, especially on size-selected model systems under UHV conditions and also in realistic catalytic environments, has proved that it is necessary to think in terms of the exact number of atoms when it comes to catalyst design. This is of utmost importance if the amount of noble metal, gold in particular, is to be reduced for practical reactions like CO oxidation. Here it is shown that on TiO2 only Au6 and Au7 clusters are active for CO oxidation which holds for the single crystal, thin films, and titania clusters deposited on HOPG. Size-selected cluster deposition and TPD methods have been employed to investigate the CO oxidation activity of Aun/TiO2 systems which are compared to recent results reported by Lee et al. to form a consistent picture in which only two species can be regarded as “active”. The efficiency of investigated Aun/(TiO2)93/HOPG composite materials is attributed to carbon-assisted oxygen spillover from gold to support particles and across grain boundaries. [ABSTRACT FROM AUTHOR]

The kinetics of decomposition and subsequent chemistry of adsorbed CF2Cl2, activated by low-energy electron irradiation, have been examined and compared with CCl4. These molecules have been adsorbed alone and coadsorbed with water ice films of different thicknesses on metal surfaces (Ru; Au) at low temperatures (25 K; 100 K). The studies have been performed with temperature programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), and x-ray photoelectron spectroscopy (XPS). TPD data reveal the efficient decomposition of both halocarbon molecules under electron bombardment, which proceeds via dissociative electron attachment (DEA) of low-energy secondary electrons. The rates of CF2Cl2 and CCl4 dissociation increase in an H2O (D2O) environment (2–3×), but the increase is smaller than that reported in recent literature. The highest initial cross sections for halocarbon decomposition coadsorbed with H2O, using 180 eV incident electrons, are measured (using TPD) to be 1.0±0.2×10-15 cm2 for CF2Cl2 and 2.5±0.2×10-15 cm2 for CCl4. RAIRS and XPS studies confirm the decomposition of halocarbon molecules codeposited with water molecules, and provide insights into the irradiation products. Electron-induced generation of Cl- and F- anions in the halocarbon/water films and production of H3O+, CO2, and intermediate compounds COF2 (for CF2Cl2) and COCl2, C2Cl4 (for CCl4) under electron irradiation have been detected using XPS, TPD, and RAIRS. The products and the decomposition kinetics are similar to those observed in our recent experiments involving x-ray photons as the source of ionizing irradiation. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The kinetics of decomposition and subsequent chemistry of adsorbed CF(2)Cl(2), activated by low-energy electron irradiation, have been examined and compared with CCl(4). These molecules have been adsorbed alone and coadsorbed with water ice films of different thicknesses on metal surfaces (Ru; Au) at low temperatures (25 K; 100 K). The studies have been performed with temperature programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), and x-ray photoelectron spectroscopy (XPS). TPD data reveal the efficient decomposition of both halocarbon molecules under electron bombardment, which proceeds via dissociative electron attachment (DEA) of low-energy secondary electrons. The rates of CF(2)Cl(2) and CCl(4) dissociation increase in an H(2)O (D(2)O) environment (2-3x), but the increase is smaller than that reported in recent literature. The highest initial cross sections for halocarbon decomposition coadsorbed with H(2)O, using 180 eV incident electrons, are measured (using TPD) to be 1.0+/-0.2 x 10(-15) cm(2) for CF(2)Cl(2) and 2.5+/-0.2 x 10(-15) cm(2) for CCl(4). RAIRS and XPS studies confirm the decomposition of halocarbon molecules codeposited with water molecules, and provide insights into the irradiation products. Electron-induced generation of Cl(-) and F(-) anions in the halocarbon/water films and production of H(3)O(+), CO(2), and intermediate compounds COF(2) (for CF(2)Cl(2)) and COCl(2), C(2)Cl(4) (for CCl(4)) under electron irradiation have been detected using XPS, TPD, and RAIRS. The products and the decomposition kinetics are similar to those observed in our recent experiments involving x-ray photons as the source of ionizing irradiation., ((c) 2004 American Institute of Physics.)