AVS1997 Session SS-TuP: Surface Science Poster Session II
Tuesday, October 21, 1997 5:30 PM in Room Exhibit Hall 1
Tuesday Afternoon
Time Period TuP Sessions | Topic SS Sessions | Time Periods | Topics | AVS1997 Schedule
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SS-TuP-1 Anomalous Thermal Expansion at the (0001) Surface of Beryllium; without Anharmonicity
K. Pohl (The University of Tennessee and Oak Ridge National Laboratory); E.W. Plummer (The University of Tennessee) Faced with the glaring discrepancy between first-principles calculations 1 and experiment 2 for the toplayer spacing of the close-packed Be(0001) surface (2.5% vs. 5.8%), we have carefully measured the temperature dependence of the surface relaxation using LEED I-V. Our data revealed two surprising results: 1) At 300 K the Be(0001) toplayer expansion is +4.3% and not 5.8%, and 2) Be(0001) exhibits a large thermal expansion of the first interlayer spacing even though HREELS shows no enhancement in the surface anharmonicity 3. The toplayer spacing expands from +3.1% at 110 K, +4.3% at 300 K, to +6.7% at 700 K resulting in a thermal expansion coefficient of 70x10-6 K-1, six times the bulk value. In addition, the rms vibrational amplitudes of the surface atoms are a factor of 2 to 3 larger than the bulk. In stark contrast to the general understanding, the thermal expansion is not the result of anharmonicity in the interlayer potential, instead, we believe results form the broken symmetry at the surface and the special electronic properties of the Be surface. Recent theoretical calculations of the temperature dependent surface structure suggest that a softening of the in-plane vibrations can lead to enhanced surface expansion 4. Finally, we will address the fundamental issue of the reliability of a LEED analysis for a metal and the importance of a careful treatment of the non-structural parameters and explain why we disagree with the previous room temperature LEED I-V study.
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SS-TuP-2 Nucleation and Growth of GaN Thin Films Deposited by Chemical Beam Epitaxy
E. Kim, I.E. Berishev, A. Bensaoula (University of Houston); A. Shultz, K. Waters (Ionwerks); S. Lee, S.S. Perry (University of Houston) Chemical Beam Epitaxy (CBE) is one of the few techniques used in the growth of III-V semiconductor heterostructures that permits precise control of the material composition and allows the use of in-situ characterization tools. Even though CBE growth technology of group III phosphides, arsenides and antimonides is well established, very little work has been published regarding CBE growth of group III nitrides. In our study, gallium nitride films were grown by CBE using tri-ethyl gallium and ammonia on sapphire (0001) substrates. Low temperature buffer layers were deposited using ECR generated activated nitrogen. During CBE growth of the final layers, the substrate temperature was varied from 500-900 deg. C, and different TEG to ammonia flow ratios were used. In-Situ characterization of the GaN surface by RHEED and low energy ion scattering mass spectroscopy of recoil ions (MSRI) was performed at each step of the growth process. We found that the low temperature buffer layer has great influence not only on the quality of the final layer, but also on the initialization of the growth. Nucleation of GaN layers on sapphire by a pure CBE process (TEG and ammonia) was found to be difficult at all growth conditions. The growths conducted on ECR buffer layers give growth rates between 1000-3000 Å/h and depend strongly on the TEG and ammonia flows. RHEED from the final layers varied from textured polycrystal to single crystal 2D patterns depending on the growth temperature, TEG to ammonia ratio, and buffer layer growth conditions. The hydrogen and carbon surface composition on the surface of GaN films was monitored in-situ by MSRI. Our result show that the incorporation of carbon is both temperature and TEG/ammonia ratio dependent. A direct correlation between surface carbon composition and crystallinity of the GaN films will be demonstrated. The microscopic surface morphology of the films, as measured by AFM, was also found to strongly depend on growth conditions and present features that suggest that different nucleation processes might be at play. |
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SS-TuP-3 Single Bond Activation by Inelastic Tunneling Electrons
B.C. Stipe, M.A. Rezaei, W. Ho (Cornell University) Inelastic tunneling electrons from the tip of a scanning tunneling microscope were used to induce single bond vibrational excitations and bond breaking. In one set of experiments, three chemisorption sites for O2 on Pt(111) were observed and characterized at 40K to 150K1. Fcc site molecules were dissociated one at a time by inelastic electron tunneling. Through repetition of the experiment, the dissociation rate as a function of tunneling current and voltage was determined and indicate one, two, and three electron processes for tunneling voltages of 0.4, 0.3, and 0.2 volts, respectively, in agreement with a theoretical model. Inelastic tunneling experiments performed on the other two chemisorbed species could lead to either dissociation or desorption and required somewhat higher electron energy. In these experiments, or when O2 molecules were dissociated in situ by UV light irradiation or by heating to above 100K, two O atom products were found one to three lattice constants apart and within two lattice constants of the original molecule on both fcc and metastable hcp three-fold sites. In another set of experiments, inelastic tunneling electrons were used to reversibly transfer single center-site Si adatoms of the Si(111)-(7x7) surface to one of two equivalent metastable adjacent T4 sites at 30K to 175K. Studies of the Si-Si bond breaking rate as a function of current and sample bias voltage indicate the process is induced by inelastic scattering in a one electron process.
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SS-TuP-4 Vibrational Studies of Amorphous/Crystalline Ice Interactions on Ag(110)
D.S.W. Lim, E.M. Stuve (University of Washington) The structure of interfacial water ice is of fundamental interest to areas like electrochemistry, atmospheric chemistry, and interstellar ice chemistry. In this work, we examine the structure and growth of surface ice by infrared spectroscopy in both the mid- and far-infrared range. The experiments were performed at the U4IR facility of the National Synchrotron Light Source, Brookhaven National Laboratory. Upon adsorption at 98 K we observed typical broad features of amorphous ice and spectral changes during adsorption that suggest growth of two-dimensional cluster that eventually transform to three-dimensional structures. During crystalline ice growth at 145 K, we observed effects in librational and bending modes that indicate variations in substrate-water and water-water interactions as ice the layers grow. Compared with amorphous ice, we observed a factor of 5 increase in the librational mode intensity during crystalline ice adsorption which indicates enhancements of hydrogen bonding in the crystalline phase. In both phases of ice we observed a low frequency vibration feature, 127 cm-1 - 157 cm-1, which we assign to a lattice vibration of surface ice. The presence of this mode confirms formation of bilayer-like clusters on Ag(110) at low water coverage. |
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SS-TuP-5 The Occupied and Unoccupied Molecular Orbitals of Ferrocene
T. McAvoy, C. Waldfried, P.A. Dowben (University of Nebraska, Lincoln) The molecular adsorption of ferrocene on Mo(112) has been studied by both photoemission and inverse photoemission. Photoemission results indicate that the initially adsorbed surface species closely resemble that of molecular ferrocene. The shift in photoemission binding energies relative to the gas phase is largely independent of the molecular orbital. The lowest unoccupied molecular orbital (LUMO) is determined from inverse photoemission and the highest occupied molecular orbital (HOMO) by photoemission. The ultraviolet light does lead to partial fragmentation of the ferrocene, but fragmentation occurs only in the presence of incident radiation. |
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SS-TuP-6 Cesium Promoted Oxidation of Zinc and Copper-Zinc Surfaces: A Combined Experimental and Theoretical Study
S. Chaturvedi, J.A. Rodriguez, J. Hrbek (Brookhaven National LAboratory) The interaction of O2 with Zn, Cs/Zn and Cs/CuZn surfaces has been investigated using photoemission and ab initio self-consistent-field (SCF) calculations. On zinc films, the sticking probability of O2 is extremely low (10-3-10-2), and O2 exposures in the range of 103-104 langmuirs are necessary to produce a significant adsorption of oxygen and the transformation of metallic zinc into zinc oxide. The presence of sub monolayer coverages of cesium enhances the oxidation rate of zinc by 2-3 orders of magnitude. In the Cs/Zn system, the alkali atom donates electrons to zinc. This charge transfer facilitates the formation of Zn-O2 dative bonds and breaking of the O-O bond. For the coadsorption of Cs and O2 on Zn(001), the larger the electron transfer from Zn into the O2 (1 pig) orbitals, the bigger the adsorption energy of the molecule and the elongation of the O-O bond. In general, cesium does not promote the oxidation of copper. In the Cs/CuZn system, copper withdraws electrons from zinc. The presence of copper in the Cs/CuZn system inhibits the oxidation of the Zn component as compared to the Cs/Zn system by lowering the electron density on the Zn atoms. After exposing the Cs/CuZn system to O2, zinc is oxidized at a rate that is larger than that found for clean CuZn surfaces and smaller than seen in Cs/Zn surfaces. Molecular hydrogen is found to have no effect on oxidized Cu, Zn and CuZn films. However, atomic hydrogen reduces ZnO to metallic zinc and CuO to Cu2O. In the oxidized CuZn alloy, CuO is reduced first followed by the reduction of ZnO. A comparison of the behavior of O2/Cs/Zn and H2O/Cs/Zn systems shows that while O2 causes severe oxidation of Cs promoted Zn surfaces, H2O has little or no effect. |
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SS-TuP-7 Measurement of the Electronic Structure of Thin Solid Films and Interfaces By Electron Momentum Spectroscopy.
V. Sashin, S.A. Canney, Z. Fang, X. Guo, M.J. Ford (The Flinders University of South Australia) Electron momentum spectroscopy (EMS), or (e,2e) spectroscopy, is an experimental technique which measures directly the band structure of solids 1, 2. In an (e,2e) event an incoming electron (of well-defined energy and momentum) ejects a target electron by a direct collision, the energies and momenta of the two outgoing electrons are detected in coincidence. The binding energy and momentum of the target electron immediately before the collision is then known from conservation laws, and at high impact energy and momentum transfer the (e,2e) cross section is proportional to the square of the momentum-space wavefunction of the target electrons. We can map directly the energy -momentum dispersion curves, or band-structure of the solid target. Our spectrometer 3 operates in an asymmetric transmission mode. The outgoing and incoming electrons are detected on opposite sides of the target, with energies for the incident, scattered and ejected electrons of nominally 20, 18.8, and 1.2 keV respectively . To reduce unwanted multiple scattering of the electrons, the targets are made as thin as practicable. The upper limit of film thickness we require is typically 10-20 nm (free-standing over 1 mm2). The low energy of the ejected electron makes the measurement relatively sensitive to the surface of the thin film, we sample the electronic structure within a layer approximately 1 nm in depth. The asymmetric geometry therefore makes our spectrometer ideally suited to studying interfaces, for example metal overlayers on a Si substrate. In this poster we will present the results of recent work to illustrate the detailed information that is obtained about the electronic structure of solids using EMS.
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SS-TuP-8 Study of Surface Behaviour and Gas Sensing Properties of WO3 Thin Films Deposited by High Vacuum Thermal Evaporation
S. Santucci, C. Cantalini, L. Lozzi, M. Pelino, S. Di Nardo, M. Passacantando, R. Phani (Universita' dell'Aquila, Italy) n-type semiconducting WO3 thin films have been deposited by high vacuum thermal evaporation method onto different substrates such as SiO2, Si3N4 and sapphire. The deposited films have been exposed to thermal treatment at 500 C for different times ranging from 1h to 200h in dry air ambient. The crystallinity of the annealed films has been investigated by grazing angle XRD showing principally <200> orientation in crystalline structure, which has not changed even after increasing the annealing time. Surface morphology measurements carried out by Atomic Force Microscopy have shown substrate dependence with a well defined crystalline structure only for the samples annealed for long time (192h). The surface chemical composition of the films have been investigated by XPS, by examining the W and O core level peaks from which we have observed oxygen deficiency on the surface, which decreased upon increasing the annealing time. High resolution XPS valence band spectra for all the samples showed a well defined W metallic peak near the Fermi edge indicating the occurrence of oxygen vacancies on the film surface. The chemoresistance of some films in synthetic air, with and without the presence of NO2 and the cross sensitivity towards interfering oxidizing and reducing gases have also been measured. Finally the aging of the film surfaces maintained in ambient atmosphere have also been investigated by XPS. |
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SS-TuP-9 Interaction of Al and Ti with the (0001) Surface of Highly Oriented Pyrolitic Graphite1
Q. Ma, R.A. Rosenberg (Argonne National Laboratory) Highly oriented pyrolytic graphite (HOPG) has been used as a model two-dimensional material for many studies of fundamental and practical importance due to the strong covalent sp2 bonding within each layer and the weak Van der Waals interaction between layers. Understanding of the reactions occurring between Al and Ti with graphite should have impact on our understanding of the interfacial chemistry in metal-matrix composites composed of these metals and carbon-coated SiC fibers. No clear consensus has emerged from theoretical studies as to whether the bonding is weak or strong between these metals with the HOPG surface. Most previous experimental studies have indicated that a chemical bond forms when Al is deposited on HOPG; however, these studies may have been compromised by the presence of oxygen. We have studied the interfacial reactions of Al and Ti, deposited in situ under ultra-high vacuum conditions, with the (0001) surface of HOPG using soft x-ray photoelectron spectroscopy. Aluminum was deposited at 300 K, 573 K, and 773 K. The most striking observation is that no chemical reaction occurs at any temperature. Reactions were only observed if the surface integrity was compromised. This may indicate that upon deposition of Al, there is little, if any, adatom-induced surface reconstruction on the HOPG surface. This result should resolve discrepancies that have arisen in previous experimental and theoretical studies. Results obtained for similar experiments involving Ti/HOPG will also be discussed. Work supported by U.S. Depart ment of Energy, Office of Basic Energy Sciences under Contract No. W-31-109-ENG-38. |
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SS-TuP-11 Reconstruction of a Ni(111) Surface Induced by Carbide Formation
J. Nakamura, T. Kobayashi, S. Kawakami, T. Uchijima (University of Tsukuba, Japan) We have studied the structure of Ni(111) with surface carbide formed by Boudouard reaction (2CO ---> CO2 + C) using UHV-STM. STM images of Ni(111) after exposure to 1000 Langmuir of CO at 500 K showed the reconstructed structure called "clock reconstruction", which has been reported for the decomposition of ethylene on Ni(111) by Klink et al. In this structure, carbon occupies four-fold hollow sites of Ni atoms, and the square units are rotated and incommensurate. However, at a low coverage of the carbide below 0.3 monolayer, (100)-like reconstructed islands were observed in STM images, in which square units were aligned without rotation, and one of the unit vectors corresponded to the unit vector of Ni(111). The results suggested that the growth of carbide and the concomitant reconstruction to the (100)-like structure take place along step edges. It is considered that further increase in the coverage of carbidic carbon causes the square unit cell to rotate, and the (100)-like structure changes to the "clock reconstruction" to lessen the stress induced by the reconstruction to the (100)-like structure. It was also observed that carbidic carbon stably existed at 700 K at step edges without decomposition to graphitic carbon. |
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SS-TuP-12 New Phenomena in Heteroepitaxial Growth: STM Studies of Copper Films on Pt(111) and Pt(100)
B. Schaefer, M. Nohlen, M. Schmidt, K. Wandelt (Universität Bonn, Germany) The structure of Cu deposits on Pt(111) and Pt(100) was studied in UHV using STM and work function measurements. While the work function changes could be measured during the Cu deposition by means of a newly designed Kelvin probe characteristic surface structures were examined after deposition with atomic resolution. -In contrast to the common finding that increased substrate temperatures promote the formation of more compact islands Cu forms fractal structures on Pt(111) at 650K. This surprising observation is explained in terms of the onset of intermixing of Cu and Pt as suggested by the work function results. Variation of the deposition rate results in the formation of different types of dendritic Cu islands with different degrees of anisotropy. Deposition of Cu on Pt(100)-hex leads to a strictly local lifting of the Pt(100) reconstruction and to an ordered incorporation of those platinum excess atoms into the first layer of growing copper films, which are displaced from the transforming Pt-hex layer. At 340K parallel, monoatomically wide platinum chains are formed within the first copper layer, whereas at 650K broader line structures appear, in which the platinum atoms are again hexagonally arranged. Annealing of a 10ML copper film grown at 340K results in the formation of nearly regular, rectangular "nanogrids". |
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SS-TuP-13 AFM and STM/STS Investigation of Transition Metal Ditellurides
A. Saidi, A. Hasbach, W. Raberg, K. Wandelt (Universität Bonn, Germany) The van der Waals surfaces of the layered materials α-MoTe2, ß-MoTe2 (a high-temperature modification), TaTe2 and Mo0.8Ta0.2Te2 were investigated by means of atomic force microscopy(AFM) and scanning tunneling microscopy (STM). On all samples atomically resolved images of the surface structures were obtained. As one part of this survey measurements of the surface of ß-MoTe2 are presented. Cristallographically the sandwich layers of a ß-MoTe2 crystal consist of Mo-atoms surrounded by Te-atoms in a distorted octaeder. The Mo-atoms in the subsurface layer form paired zig-zag-chains along the b-axis of the crystal. The Te-atoms on top of these chains are lifted above the originally hexagonal surface, resulting in the formation of a surface structure consisting of rows along the b-axis at two different heights. In the present investigation of the ß-MoTe2 surface an interesting deviation from this "normal" structure was observed. At various positions of the sample one finds an anisotropic corrugation of the surface with alternating ascending and descending atomic rows. This modification was obtained both with the scanning tunneling microscope and the atomic force microscope. In this work the results of the AFM and STM/STS investigations are described and similarities and differences are worked out. The possible causes of the modification (such as charge density waves, subsurface imaging or reconstruction effects) are discussed. Furthermore, the effect of the AFM parameters on the imaging process is examined and for the TaTe2-surface a "phase diagram" (loading force, scan angle) can be devised. |
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SS-TuP-14 Resolving the Atomic Surface Structure of Amorphous Bariumsilicate with the AFM
W. Raberg, K. Wandelt (Universität Bonn, Germany) In this work the investigation of an amorphous bariumsilicate with the atomic force microscope (AFM) under ultra high vacuum conditions is described. For the first time atomically resolved images of a disordered surface were obtained. Using the atomic positions it is possible to calculate radial distribution functions which can be compared with data from diffraction and electron energy loss spectroscopy measurements. The analysis of the interatomic distances is divided in two parts: a) The investigation of the range up to 5 Å proves that the AFM measurements are indeed atomically resolved. The glass seems to consist of a network of SiO4-tetrahedra (analogous to the classical glass structure theories 1), and terminates with oxygen atoms at the surface. The characteristic distances (0.27 nm, 0.41 nm) can be clearly identified. In the intermediate range (0.3 nm - 1 nm) the analysis of distances and angles gives several characteristic features clearly suggesting the existence of some medium range order. Such direct atomically resolved imaging of an amorphous surface will make a large contribution to the understanding of the structure of glasses, since the real space data of the scanning probe methods is in principle not limited to the first few coordination spheres. By combination with diffraction techniques it offers a way to a deeper understanding of the interactions being present in amorphous solids, which are of great technological importance.
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SS-TuP-15 Novel UHV Facility for Low-Temperature Surface Science Research
M. Sander, T. Becker, D. Funnemann (OMICRON Vakuumphysik GmbH, Germany); B. Grimm, H. Hoevel, C. Quitmann, B. Reihl (University of Dortmund, Germany) A novel two-chamber UHV facility has been developed which combines a low-temperature STM with a high resolution electron energy analyzer for photoemission spectroscopy connected by a LHe cooled sample transfer rod. Such a combination provides the optimum way to obtain both structural and electronic information from specimen surfaces, which exhibit particular low-temperature effects associated with, e.g., superconductivity, phase transitions, adsorption, surface states, charge density waves, etc. or where phenomena like corrosion and diffusion can be slowed down to be measured in a time-resolved manner. First measurements on a metallic Au(111)23xsqrt3 surface show atomically resolved STM topographs at 4.9 K. High resolution STS has been performed at T = 6 K on Pt clusters deposited on HOPG1. A width of 18 meV of the Fermi level (16-84%) was measured in UV- photoemission on polycrystalline silver at 10 K. The details of the low-T UHV facility will be presented together with measurements on adsorption phenomena.
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SS-TuP-16 Mass Spectroscopy of Recoiled Ions From Oxide Surfaces
G.S. Herman, S. Thevuthasan, C.H.F. Peden (Pacific Northwest National Laboratory); J.A. Schultz, K. Waters (Ionwerks); B.J. Morris, T.T. Tran (Los Alamos Technical Associates Inc.) The interaction of adsorbates with oxide surfaces is of considerable importance. The role of defects and the resulting adsorption mechanisms is of much interest. Many techniques require the formation of defect densities in the range of 5-10% of the surface atomic concentration. Mass spectroscopy of recoiled ions (MSRI) allows the study of defect densities that are orders of magnitude lower. We have performed MSRI from several different oxide surfaces. The single crystals studied include TiO2(110), MgO(100), ZnO(11-20), and Al2O3(0001). Details of the instrumentation and experimental procedures will be described. Adsorption experiments with isotopic labeling were performed to determine the extent of H2O dissociation and to characterize adsorption sites. Work supported by DOE/ER/OHER |
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SS-TuP-17 The Adsorption of Sb on Cu(111) Study by LEED and AES.
L. Morales de la Garza (IFUNAM-Lab. de Ensenada) We have study the adsorption of antimony on the Cu(111) surface by LEED, AES. The evaporation of the antimony was performed by heating a reservoir of Sb at 700K, at this temperature, Sb sublimes onto the Cu(111) clean surface kept at room temperature. The deposition rate at this conditions are very low and difficult to calibrate with a quartz balance, we estimated the deposition rate to be below 1 monolayer per hour. During evaporation, we monitor the LEED pattern and it did not change appreciably from (1X1) pattern up to saturation of the surface were the pattern was very diffuse and still (1X1), Auger spectra was obtain during evaporation using the RFA optics as an electron analizer and we monitored the content of Sb on the Cu(111) surface. After evaporation, we heated the Sb/Cu(111) system at 650K were we observed a clear (√3X√3)R30o, at this point, the Sb Auger signal did not change but an increase of the Cu Auger signal was noticeable, heating at higher temperatures showed no change in the LEED pattern and a gradual decrease in the Sb Auger signal, we did not heat higher than 900K due to the contamination of Sulphur on the surface which diffuse from the bulk and we needed to Argon bombard the surface to clean off the surface. Finally, our results give an evidence that the Sb do not form a superstructure at room temperature, and at heating it at 650K formed a (√3X√3)R30° with an Auger evidence that Sb could go under the first Cu layer due to the increase in the Cu signal. |
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SS-TuP-18 Electronic Structure Calculations of Small Molecule Absorbates on (110) and (100) TiO2 Surfaces
K.F. Ferris, L.-Q. Wang (Pacific Northwest National Laboratory) The chemical reactivity of metal-oxide surfaces are often described in terms of the acid-base properties of isolated cation and anion sites, and the introduction of surface defect sites. In order to probe the interaction of these effects, ab initio Hartree-Fock calculations have been performed on model structures for ideal and defect-containing (110) and (100) TiO2 surfaces. Our results indicate that H2O binds strongly to all four test cases in agreement with previous experimental results. Further, these results affirm earlier work in which we observed similar binding energies for water onto isolated Ti(4) sites and Ti(3) defect sites. From the acid-base properties of absorbed water for both surfaces, we have noted the importance of the geometric arrangement for the acidic and basic sites of TiO2. For example, the bridging two coordinate O2- sites of TiO2 offer potential strong interactions for the protons of absorbed water species. However, the closer Ti(4)-O2- bridging distance for the (100) surface of TiO2 results in more facile water dissociation than the (110) surface. The reactivity of Ti(3) sites in defective TiO2 are affected by similar geometric constraints. As such, water dissociation characteristics are controlled by the combination of the acid characteristics of absorbed species at the Ti(4) sites and the availability of the O2- basic site. Calculations on small molecule absorbates (water, formate, methanol) reinforce the need to include these geometric constraints in discussions of surface reactivity. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences Division under contract DE-AC06-76RLO 1830. |
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SS-TuP-19 Degrease of Rolled Aluminum Foil by Ozone Treatment with Heating
T. Momose (Miyagi National College of Technology, Japan); S. Abe (Showa Aluminum Corporation, Japan) Ozone treatment has been tested on chambers and equipments for UHV. 1The treatment was applied to degrease surface of a rolled aluminum foil (N30-H18) cylinder with outer diameter of145 mm, inner diameter of 75 mm, and length of 250 mm, where foil length is 600 m and foil thickness is 20 microns. The foil was exposed for one hour to oxygen gas of one atmosphere with flowing rate of 0.5 l/min including ozone of 630 ppm at160 C in average (max temp 200 C for a half hours). Another foil was exposed for one hour to oxygen gas of one atmosphere with flowing rate of 0.5 l/min with no ozone at same temperature condition. Three foil surface including non treated one were analyzed using XPS and contact angle of a water droplet. XPS showed: Atomic concentration of carbon (C), a measure of quantity of oil, is 24 % for the nontreated foil, 19-13 % for the heated one, and 9-7 % for the ozone treated one. The scattering data for the two foils showed the position dependence, edge and center in the axial direction. The data showed that ozone can penetrate into the gap 32 microns between aluminum foils. Required industrial level of the concentration is 10 %. The ozone treated one cleared the level. Instead, the traditional heating method needs 50 to 100 hour heating to obtain the level. The ozone method contributes much for saving energy.
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SS-TuP-20 Atomic Structure and Growth Mode of Ag on Si(001)
Y.W. Kim, N.G. Park, C.N. Whang (Yonsei University, Korea); D.S. Choi (Kangwon National University, Korea); S.S. Kim (Mokwon University, Korea) The atomic structure and growth mode of Ag on Si(001) surface has been studied by coaxial impact collision ion scattering spectroscopy(CAICISS) at various temperature. At room temperature, Ag grows in SK mode and the two dimensional Ag layer is consisted of Ag dimers, which show Ag(2x2) structure and are located in the hallow site between the four neighboring Si dimers. The Ag dimers are perpendicular to the Si dimer and its bond length is 2.89 Å. The three dimensional islands have a structures of Ag(011)/Si(001) and its [110] or [100] direction is parallel with the [110] direction of Si(001). The interlayer spacing between 1st and 3rd layer, d13 is 2.80 Å. After annealing this sample at 600°C the 2D Ag layer is disappeared and the 3D Ag islands are reconstructed to the structure of Ag(001)/Si(001). The [110] direction of Ag islands is parallel to the [100] direction and the interlayer spacing d13 in the 3D Ag island is 4.09 Å. By the way, for the deposition at a substrate temperature of 600°C, Ag grows in SK mode. In this case, the Ag atoms in the 2D layer is located at the bridge site in the height of 0.7 and 1.5 Å from the Si dimer. The 3D Ag islands have a structure of Ag(001)/Si(001) and the [100] direction of Ag islands is parallel to the [100] direction of Si(001). This work was supported in part by the Korea Science and Engineering Foundation(KOSEF) through the Atomic-scale Surface Science Research Center(ASSRC) at Yonsei University and in part by the Ministry of Education (Project No. BSRI-97-2426). |
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SS-TuP-21 Atomic-Force-Microscopy Study of the Faceting on MgO(110) Surface
G. Chern, J.J. Huang, T.C. Leung (National Chung-Cheng University, Taiwan); C.L. Chang (Tamkang University, Taiwan) Using atomic force microscopy, we have studied the morphologies of the MgO(110) surface. After annealing at 600 C, this surface facets into elongated triangular mesoscale islands along the [001] direction. A statistical counting of both the widths and the heights of these islands can be well fitted by a Gaussian normal distribution. The most popular island width and height are ~1200 Å and 150 Å, respectively, indicating that these faces are roughly (410) surfaces. This higher-Miller-index direction of the faces is believed due to a kinetic reason. Besides, both Gaussian distributions are much narrower than predicted for the microscopic terrace-step-kink model. This result may suggest that an island-island interaction is fundamentally different from a step-step interaction. This work is supported by the National Science Coundil of ROC under grant No. NSC 86-2112-M-194-008 |
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SS-TuP-22 Controlled Atom by Atom Restructuring of a Metal Surface with the Scanning Tunneling Microscope
G. Meyer, L. Bartels, S. Zöphel, E. Henze, K.H. Rieder (Freie Universität Berlin, Germany) We report the ability to completely restructure a metal surface by precision manipulation of individual atoms with the scanning tunneling microscope: Besides extracting atoms from kink sites on Cu(211) we are also able to 'dig out' atoms from the even more strongly bound intrinsic step sites and thus to create adatom-vacancy pairs. Together with the processes of moving adatoms along and across intrinsic step edges and the possibility of healing out adatom-vacancy pairs, we have a complete set of lateral manipulation processes at hand1. The reliability of all these processes opens up exciting 'engineering' possibilities for structuring of extended surface areas using foreign as well as native atoms with the prospect to include even lower lying substrate layers.
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SS-TuP-23 Nanostructure Investigation and Characterization of Crystalline SiC Layers Fabricated by Ion Beam Synthesis
W. Wu, D.H. Chen, J.B. Xu, S.P. Wong, W.Y. Cheung, I.H. Wilson (The Chinese University of Hong Kong) Thin silicon carbide layers have been formed by high dose carbon ion beam implantation into (100) silicon wafers with two different ion implantation energies, 40KeV and 65KeV. Subsequently, furnace annealing has been carried out in N2 at temperatures ranging from 600 to 1400 centigrade for two hours. Rutherford backscattering spectrometry analysis reveals carbon redistribution after the annealing procedure, and the formation of a SiC layer. Infrared spectra exhibit a sharp absorption peak produced by the Si-C bond at 795 cm-1, with FWHM of about 35cm-1. After annealing at 1000 centigrade for 2 hours, crystalline SiC layers are formed. The layer thickness is about 200nm measured by cross-sectional TEM, and it reveals a clear interface between the SiC and the Si substrate. Investigations of the surface morphology and dynamics of the crystallization procedures, are made by atomic force microscopy (AFM). The transition from an amorphous to a crystalline phase is clearly shown. Study of grain size and roughness shows that the surface morphology of the SiC layer is largely dependent on annealing temperature. At 1000 centigrade, a nanocrystalline SiC layer is formed, containing columnar grains with a mean width of tens of nanometers and a height of twenty nanometers. Further investigations will focus both on the macro optical properties of this crystalline nanostructure, such as photoluminescence, and micro properties, using the near field optical microscopy(NSOM). Potential applications to optoelectronics devices will also be discussed. |
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SS-TuP-24 UHV-STM Studies of Bi-Metallic Systems: Cobalt on Oxygen-Covered Molybdenum (110)
P.G. Clark, Jr., D. Jentz, D.A. Chen, C.M. Friend (Harvard University) We have designed and constructed a variable temperature, ultra-high vacuum, scanning tunneling microscope (STM) to study the structure and reactivity of cobalt overlayers on molybdenum (110). This system serves as a model for commercial Co-promoted molybdenum catalysts used in deoxygenation and desulfurization reactions. Previous results in our lab using XPS suggested that when oxygen is deposited on cobalt overlayers on Mo(110) and annealed to 965 K, the cobalt forms clusters on the surface. Specifically, the oxygen adsorbs onto the cobalt at low temperatures (100 K) and as the surface is heated to above 600 K, the cobalt forms clusters and the oxygen migrates to the Mo(110) surface.1 The STM has confirmed cobalt cluster formation with an average diameter of 40 to 50 nanometers covering approximately 20% of the molybdenum surface. The reaction of methanol on this surface is strongly dependent on the Co morphology. On the uniform Co monolayer supported on clean Mo(110), CO and H2 are the major products evolving at 430 K and 375 K, respectively. In contrast, on the cobalt clusters supported on the oxidized Mo(110), only CH3OH evolution is observed at 210 K. Since there are differences in the reactivity of methanol on the uniform and clustered cobalt, our project focuses on taking advantage of these differences and controlling reactivity via Co morphology. Specifically, we intend to vary both the Co coverage on O-covered surfaces and subsequent annealing temperatures to vary the height and diameter of these clusters. STM images of these different cobalt structures will be presented including atomically resolved images of oxygen on Mo(110). The alcohol reactivity of these structures will also be presented in order to provide possible explanations between the reactions on the uniform Co overlayers versus the Co clusters. In addition, preliminary structure and reactivity results on various cluster sizes will be presented.
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SS-TuP-25 Resonance Effects in Diffuse Elastic Atom Scattering from Surfaces
J.G. Skofronick (Florida State University); A. Glebov, J.P. Toennies (MPI f. Stroemungsforschung, Goettingen, Germany); J.R. Manson (Clemson University) We show that resonance selective adsorption followed by defect scattering, together with the inverse process, leads to significant contributions to the diffuse elastic scattering intensity which is routinely observed in He-atom-scattering experiments. A kinematic model predicts sharp peaks in the diffuse elastic intensity which should be symmetric in parallel momentum transfer about the specular direction. The effect is experimentally verified over a large range of incident energies through high resolution He-atom surface scattering experiments carried out on an in situ cleaved NaCl(100) crystal surface in the <100> and <110> high symmetry directions at temperatures in the neighborhood of 100 K. The effect can lead to large changes in intensity which can be several times as large as the diffuse elastic background signal. This work was performed at the MPI and supported in part by DOE Grant DE-FG05-85ER45208, NSF Grant DMR9419427 and NATO Grant 891059. |
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SS-TuP-26 Morphology of TiO2(100) Surface Under Electron Beam Irradiation at Elevated Temperature
P. Xiong-Skiba (Austin Peay State University); L.-Q. Wang, D.R. Baer, M.H. Engelhard (Pacific Northwest National Laboratory); L.A. Finley (Austin Peay State University) There has been a great interest in understanding the properties of transition metal oxide surfaces in the past two decades. Rutile TiO2 has been subjected to many such studies and many types of sample preparation methods. Some methods of preparation have been found to produce very rough surfaces. In this study, Atomic Force Microscopy (AFM) has been used to observe directly the morphology of TiO2(100) surfaces irradiated by 1 keV electron beam at different sample temperatures. Comparision between the irradiated and non-irradiated areas shows that electron beam irradiation at temperatures above 600°C can cause pitting. Images taken on the electron beam irradiated areas show randomly distributed pits. The sizes of the pits depends strongly on the temprerature of the sample, the electron beam dosage and irradiation time, and the partial oxygen pressure. At 800°C, pits formed at the damaged areas are about 20 nm deep and show a favorable spacial orientation. Much shallower row structures are observed from those undamaged, smooth areas on the same sample. The row structures corresponds to the well-known reconstruction of TiO2(100) surfaces at temperatures above 700°C. The deeper pits shown at the damaged areas are possibly related to the defects and local heating introduced by the electron beam irradiation. Pits as big as several micrometers were found on a sample which was repeatedly heated by 1 keV electron beam to 700°C in 1.0x10-6 Torr oxygen and then cooled in the same oxygen environment. In this severely electron beam damaged case, all the pits are elongated in one direction (close to rectangular shape) and have the same spacial orientation. The elongated sides of a rectangular pit form 45° angles relative to the TiO2(100) surface suggesting that these newly formed surfaces are the (110) surfaces. It seems that electron beam irradiation introduces defects which enhance diffusion toward what appears to be the most stable surface at elevated temperature. It is also found that many rectangular shaped pits were developed along mechanical scratches shown on untreated TiO2(100) samples when the samples were reduced in vacuum at temperatures above 500°C. |
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SS-TuP-27 Influence of Electric Field on Water Chemistry at Platinum and Silver Interfaces
T.D. Pinkerton, D.L. Scovell, E.M. Stuve (University of Washington) Interactions between water and metal surfaces pervade many systems, mostly electrochemical in nature, including corrosion, batteries and fuel cells. The double layer present in these systems sustains electric fields as large as 1 V/Å. So far, classical surface science and electrochemistry have been unable to directly probe the effect of this electric field on surface chemistry. Field ion microscopes (FIM) allow the independent control of the applied electric field, while maintaining the ability to determine the chemical identity of adsorbed species. In RFD, the applied electric field is increased linearly with time, while the temperature is held constant; the total ion intensity is then measured as a function of applied electric field. In some recent novel experiments, started by Stintz and Panitz, ramped field desorption (RFD) is used to examine water under high electric fields. In similar experiments, amorphous ice is irreversibly adsorbed onto platinum and silver field emitters held at cryogenic temperatures, while the onset of field desorption is studied as a function of water thickness, temperature, and ramp rate. It has been found that the electric field required for ionization is logarithmically dependent on ramp rate, while being linear with temperature. At temperatures above 100 K, in the presence of high electric fields (0.3 V/Å), the ice appears to be crystalline, while at temperatures below 100 K it appears to remain amorphous, even under fields as large as 1 V/Å. |
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SS-TuP-28 Sulfur Segregation on Pd(110) Studied with STM, AES AND LEED.
C. Achete (PEMM/COPPE/UFRJ, Brazil); H. Niehus (Humboldt Universitaet, Germany) The development of equilibrium surface structures of Pd(110) due to the sulfur segregation after elevated annealing temperatures, 950K, has been investigated with scanning tunneling microscopy(STM). The coverages as measured by Auger electron spectroscopy (AES) are in the range of 0.1 to 0.4 monolayers. LEED pattern represents a clear c(2x2) with extra spots indicating the formation of periodic antiphase boudaries. In fact large scan STM images and respective 2D Fourier transformation show that the surface is completely covered by a c(2x2) domains. This structure is mesoscopically ordered forming long domains with a width of 5 to 6 atoms separated by single or double spaced boundaries in Pd units. Based on our atomic resolution STM images we assigned the position of the adsorbed S atoms as being the fcc four fold hollow site. |
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SS-TuP-29 Negative Ion Formation in Electron Stimulated Desorption of CF2Cl2 on Ru(0001)
Q.B. Lu, Z. Ma, T.E. Madey (Rutgers, The State University of New Jersey) As part of a program to characterize DIET (desorption induced by electronic transitions) of negative ions, we have studied electron stimulated desorption (ESD) of F- and Cl- from CF2Cl2 adsorbed on Ru(0001) at 25 K over a large range of coverages (0~50 ML). The experimental techniques used include electron stimulated desorption ion angular distribution (ESDIAD) with time-of-flight (TOF) capability, thermal desorption spectroscopy (TDS) and Auger electron spectroscopy (AES). The TDS spectra of molecular CF2Cl2 (based on detection of CF2Cl+, the dominant fragment in the CF2Cl2 mass spectrum) show two monolayer desorption peaks at 155 K and 185 K and a multilayer peak at 140 K. The total TDS peak area increases linearly with increasing CF2Cl2 exposure; in addition, the AES spectra show negligible Cl signal on the surface after TDS up to 220 K. Together, these data suggest that CF2Cl2 is mainly molecularly adsorbed on the surface. Strikingly, for coverages up to several monolayers of CF2Cl2, a strong ESD yield of F- ions emitted along the surface normal is observed (~5E-7 ions/electron), while no Cl- ions are detected. At higher coverages, the Cl- yield along the surface normal appears, and there is a corresponding decrease in the F- yield. These results are different from the formation of negative ions for gas-phase CF2Cl2, where Cl- is the dominant product. The preferential quenching of Cl- ions on or near the metallic substrate is discussed in terms of image potential effects, charge transfer, and molecular orientation effects. |
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SS-TuP-30 Silicon Migration and Re-Crystallization on the Surface of Very Thin Copper Films Deposited on Si(111)-7x7
R.A. Simao, C. Achete (PEMM/COPPE/UFRJ, Brazil); H. Niehus (Humboldt Universitaet, Germany) Silicon is observed to migrate and re-crystallize on the surface of very thin copper films in two different situations: (i) Films of more than 10 monolayers (ML) of copper deposited at room temperature on Si(111)-7x7 and submitted to electron bombardment for some hours. In this case silicon re-crystallizes on the surface of the silicide film in a dendritic pattern. No significant carbon or oxygen contamination induced by electron bombardment was found as probed with Auger Electron Spectroscopy (AES). (ii) Films of 1 to 2 ML of copper on Si(111)-7x7 annealed to more than 800K. In this case the surface is covered by the incommensurate quasi-5x5 structure and migration of silicon is induced by thermal treatment. Scanning Tunneling Microscopy (STM) images show that only small amounts of silicon is present on the surface (less than 0.5ML) located preferentially nearby the step edges and the 2D crystals of the quasi-5x5 structure. Crystallization is enhanced by scanning the surface with the STM tungsten tip. The comparison of this two situations will be discussed in terms of the stability and structure of the copper/silicide layer. |
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SS-TuP-31 Ordering of Confined Fluids Between Molecularly Smooth Conductive Surfaces Within the Surface Force Apparatus
L.D. Broekman, A. Artsyukhovich, M. Salmeron (Lawrence Berkeley National Laboratory) Traditionally the surface force apparatus (SFA) utilizes molecularly smooth surfaces of muscovite mica to form a well defined interface in which the properties of confined liquids and interfacial forces may be studied. The novel use of capacitance to measure separation and contact area in the SFA has been recently demonstrated as a high speed method with very high precision compared with traditional Interferometry techniques. The capacitance method allows opaque interface materials such as gold or silicon to be used thus extending the capabilities of SFA to the study of technologically more relevant systems. This method also allows the potential for dielectric spectroscopy for insitu analysis of confined materials. Results obtained from lubricating liquid thin films confined between molecularly smooth gold and silicon interfaces will be presented that exhibit oscillatory solvation forces as previously observed using mica surfaces. |
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SS-TuP-32 Interfacial Reactions Between GaSe and Sapphire: Formation of a Template for the Subsequent Growth of Layered GaSe Thin Films.
S.R. Chegwidden, Z.R. Dai, F.S. Ohuchi (University of Washington) A surface chemical exchange reaction is observed at the interface of GaSe and sapphire. Thin films of layered semiconductor GaSe have been grown on selenium terminated sapphire (0001) substrates overcoming a lattice mismatch of 21%. GaSe is an optical material with a direct band gap of 2.1 eV, which has potential applications in photovoltaics and non-linear optics. The optical properties of thin films are compared to bulk materials with these applications in mind. GaSe films are deposited in a molecular beam epitaxy (MBE) chamber with a compound GaSe Knudsen cell source and a separate Se source. GaSe films were grown at temperatures as high as 620 C, and annealed to temperatures as high as 1100 C. X-ray photoelectron spectroscopy (XPS) studies reveal a strong interface reaction which occurs between the film and substrate. Reflection high energy electron diffraction (RHEED) and XPS results indicate that the films deposited on clean sapphire are selenium rich Ga2Se3 with a Se:Ga stoichiometry of 1.65. When the initially deposited films are thermally removed, a selenium rich residue is left behind which terminates the sapphire surface. This reaction is initiated by the intermixing of substrate and film atoms at the interface. Angle resolved XPS (ARXPS) and cross-sectional transmission electron microscopy (TEM) are used to study the composition and depth of the termination layer. This interface layer acts as a growth template, which makes it possible to grow stoichiometric layered GaSe in subsequent depositions. A mechanism is proposed which describes the formation of this growth template, and the effect it has on the subsequent growth of GaSe. |
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SS-TuP-33 Ellipsometric and LEED Study of the Layer Growth of Xenon Physisorbed on Ag(111) Surface
S. Igarashi, Y. Abe, T. Hirayama, I. Arakawa (Gakushuin University, Japan) The layer growth of a physisorption system on a metal single crystal from a submonolayer range to a thick layer is investigated by in-situ observation of an ellipsometry and an electron diffraction. In order to observe a physisorption system and thick insulator film, we have developed an eXtremely-low-current Low Energy Electron Diffraction (XLEED) apparatus equipped with a Micro-Channel Plate (MCP) and a Position-Sensitive Detector (PSD) in place of a phosphor screen in an ordinary LEED optics. A data accumulation for one minute at the incident electron beam current of about 10-13A provides an diffraction pattern composed of 105~106 electron counts over the whole detector, which is sufficient for an quantitative analysis. Our experimental apparatus makes it possible to observe a layer growth and an adsorbate structure simultaneously with a minimum effect of defect formation and charging on the surface. The ellipsometric adsorption isotherms of Xe/Ag(111) at substrate temperatures from 50K to 80K were obtained in the wide pressure range between 10-7Pa and 1 Pa. We observed the surface structure by XLEED while monitoring the layer growth by the ellipsometry from a submonolayer to a thick layer where the equilibrium pressure was nearly equal to the bulk saturation vapor pressure. The surface structure of the Xe film deposited on the Ag(111) substrate strongly depends on the conditions of a substrate temperature and a Xe pressure to result in a variety of LEED patterns: a diffuse, a ring, or a six-fold one. In the case of isobaric layer growth, at the constant pressure of 5*10-4 Pa while the temperature was varied from 138K to 35K, the six-fold LEED pattern of Xe overlayer clearly developed, keeping its relative orientation to the Ag(111) substrate, up to a sufficiently thick, almost a bulk, film. The relation between the lattice parameter and the film thickness is under investigation. |
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SS-TuP-34 Desorption of Metastable Particles Induced by Electronic Excitation at the Surface of Rare Gas Solid with Physisorbed Hydrogen
A. Hayama, T. Kuninobu, T. Hirayama, I. Arakawa (Gakushuin University, Japan) The effect of physisorption of hydrogen on the desorption of metastable particles induced by electronic excitation from rare gas solid (RGS) was investigated. In our previous study1, it was found that the total desorption yield of metastable particles from the surface of solid Kr was enhanced by an order of magnitude or more by the physisorption of hydrogen. Two desorption mechanisms, excimer dissociation (ED) and cavity ejection (CE), are known for the desorption of metastable particles from the surface of RGS. The ED process is a dissociation of an excimer in the vicinity of a surface. The CE process is due to the repulsive interaction between the excited atom with an inflated electron orbital and surrounding ground state atoms. Desorption of metastable particles through the CE process is observed for solid Ne and Ar, whose electron affinities (EA) are negative, but not for solid Kr and Xe, whose EAs are positive. Physisorbed hydrogen reduces the effective EA of the surface of solid Kr to an negative value and can actuate CE process of Kr* and Kr2*. The ejected excimer, Kr2*, dissociates in vacua to yield Kr*. The desorption of H* or H2* is also observed. We report the effect of physisorption of hydrogen on the electron and photon stimulated desorption of metastables from the surface of solid Ne, Ar, Kr and Xe. After the surface of RGS at a temperature of about 30K was exposed to hydrogen at 10-4Pa-10-5Pa for 30min., the remarkable increase of the desorption yield of hydrogen metastables was observed for Kr and Xe, moreover new desorption species of Kr* via CE appeared in TOF spectra for Kr. For Ar, although no significant increase of metastable yield was found, the kinetic energy of Ar* desorbed through CE process was increased by hydrogen adsorption. For Ne, no change was noticed even at the lower surface temperature of 6K. These results should be discussed in terms of the change of EA and the interaction between hydrogen and rare gas molecules in excited states.
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SS-TuP-35 Adsorption of CO2 on the K-modified Ir(111) Surface.
M. Sushchikh (University of California, Santa Barbara); J. Lauterbach (Purdue University); M. Schick, W.H. Weinberg (University of California, Santa Barbara) Alkali metals are well known as promoters for catalytic reactions on transition metals. We have studied the adsorption and reaction of CO2 on the Ir(111) surface modified with various coverages of K atoms. Isotopic experiments with 12C18O2 using Fourier Transform Infrared Reflection-Absorption Spectroscopy (FT-IRAS) and Temperature Programmed Desorption (TPD) were performed in order to clarify the local binding geometry. |
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SS-TuP-37 Film/Substrate Interfacial Interactions by Mass-Selected Cluster Ion Deposition
K.J. Boyd, A. Lapicki, M. Aizawa, S.L. Anderson (University of Utah) We report studies of cluster-substrate reactions using a new instrument equipped with a novel phase-space-compressed cluster ion beam source. Metal and carbon cluster ions are prepared in one of several sources, formed into a highly collimated, monoenergetic ion beam using a phase-space compression technique developed in our lab. Clusters are deposited on a single crystal substrate in UHV. Morphology of the deposited material is examined by in situ STM, and reactions betwen the substrate and cluster atoms are studied by XPS. Data for carbon clusters on Ni substrates and Al clusters on HOPG and Ni will be presented. |