AVS2001 Session SS3-TuP: Adsorption/Desorption Poster Session
Tuesday, October 30, 2001 5:30 PM in Room 134/135
Tuesday Afternoon
Time Period TuP Sessions | Topic SS Sessions | Time Periods | Topics | AVS2001 Schedule
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SS3-TuP-3 The Evolution of Surface Topography of Alkali Halides Crystals During Ion Stimulated Desorption
F. Krok, J.J. Kolodziej, B. Such, P. Czuba, P. Piatkowski, M. Szymonski (Jagiellonian University, Poland) Ion-stimulated-desorption (ISD) of alkali halide (RbI, KBr) surfaces under 5 keV He+ bombardment is investigated in UHV by means of non-contact Atomic Force Microscopy (nc-AFM) and Quadrupole Mass Spectroscopy (QMS). Irradiated surface topography and corresponding desorption fluxes are studied. Although, for the energy range of He+ ions used in the present work, desorption of alkali halides occurs predominantly via electronic processes, there are significant differences between ISD and the electron stimulated desorption (ESD). Recently, present authors1,2 have demonstrated that ESD of alkali halides proceeds in a layer-by-layer mode and oscillating desorption fluxes are observed. These ESD oscillations have been explained to be due to creation of Frenkel pairs in the bulk, their diffusion, and their recombination with surface modulated accordingly to changing surface topography. For ISD studied in the present work AFM images show that the surface erosion does not follow the layer-by-layer scheme and no oscillation in desorption fluxes are found. We propose that these (not anticipated) differences between electron- and ion- stimulated desorption are caused by ballistic processes at the surface. While most of the primary ion beam energy is deposit deep in the bulk leading to production of Frenkel pairs some projectiles colliding violently with top-layer ions may introduce additional nucleation centers for vacancies on surface. Moreover the shallow collision cascades, which extend to the top crystal layers are likely to induce mobility in alkali halide top crystal layer. These ballistic processes interfere with electronic processes and destroy the layer-by-layer desorption scheme.
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SS3-TuP-4 Nano-scale Modification of Ionic Surfaces by ESD
B. Such, J.J. Kolodziej, P. Czuba, P. Piatkowski, P. Struski, F. Krok, M. Szymonski (Jagiellonian University, Poland) Surface modification of alkali halide crystals by electron beam was investigated with the use of dynamic force microscope (DFM). It was found that randomly spread rectangular pits of monolayer depth, in the topmost layer of the crystals, were formed during irradiation. Growth and coalescence of the pits led to almost layer-by-layer desorption mode. Average size and density of the pits could be changed by varying parameters of an electron beam and surface temperature leading to controlled nano-modification of the surface. Moreover, by comparison of DFM and mass spectroscopy data we were able to establish that the surface stepdensity, changing in time, controlled the desorption process. The new model of electron-stimulated desorption (ESD) of alkali halides was proposed to explain observed behavior, based on the idea of existence of two kinds of F-centers in the crystal: ground state (immobile) and excited (mobile) ones. |
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SS3-TuP-5 The Simultaneous Observation of Adsorption Isobars of Xe/Ag(111) and Xe/Ag(100)
A. Tosaka, T. Mitake, T. Hirayama, I. Arakawa (Gakushuin University, Japan) The growth and the structure of a Xe film physisorbed on the surface of a silver single crystal, Ag(111) or Ag(100), have been investigated by means of an ellipsometry and an eXtremely-low-current LEED (XLEED). We obtained adsorption isobars of Xe/Ag(111) and Xe/Ag(100) simultaneously: one was observed by the ellipsometry and the other by the spot intensity measurement of XLEED in the temperature range between 60K and 90K and the equilibrium Xe pressure range between 5X10-6Pa and 2X10-4Pa. The first layer condensation of Xe/Ag(111) occurs at the temperature 0.3±0.1 K higher than that observed for Xe/Ag(100), while the second layer condensation occurs at the same temperature within the experimental accuracy . On the assumption (1) that this temperature difference is mainly caused by the difference of the induced dipole moment between the Xe atom on Ag(111) and that on Ag(100) and (2) that the system follows the two dimensional van der Waals equation, we calculated the difference of the induced dipole moment µ to reproduce the experimentally determined temperature difference. The result shows that µ on Ag(100) is 5% larger than that on Ag(111). *T. Hirayama Present Address: Department of Physics, Rikkyo University, Toshima, 171-8501 JAPAN. |
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SS3-TuP-6 Measurement and Quantitative Analysis of Photon or Electron Stimulated Desorption Yields of Solid Argon and Krypton
T. Adachi, T. Hirayama (Gakushuin University, Japan); M. Sakurai (Kobe University, Japan); I. Arakawa (Gakushuin University, Japan) Electron or photon irradiation of the surface of rare gas solids produces the electronic excitations which can be followed by the desorption of various kinds of particles. Investigation of the desorption characteristics such as desorption yields, kinetic energy distributions and angular distributions, will reveal the dynamics of the electronic excitations and relaxations in rare gas solids. We have reported the absolute photo-desorption yields of solid neon at the excitonic excitation energy range and shown that the yields were quantitatively explained by an internal sputtering mechanism.1 Here, we present the absolute desorption yields of the solid argon and krypton for the both case of photon impact and low energy electron impact. At the films thicker than 400 atomic layers, the absolute photo-desorption yields were about 1.5, 0.07, and 0.03 atoms/photon for neon, argon, and krypton, respectively, at the excitation energy of first order bulk exciton. In the case of 220 eV electron impact, the desorption yields were about 1 and 0.1 atoms/electron for argon and krypton, respectively. We will present the quantitative evaluation of the absolute desorption yields and these thickness dependencies for argon and krypton using the the classical molecular dynamics calculation results.2,3 *T. Hirayama Present address: Department of Physics, Rikkyo University, Toshima, Tokyo 171-8501 JAPAN. |
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SS3-TuP-7 The Adsorption and Thermal Decomposition of Trimethylamine on Si(100)
L.H. Zhang, A.J. Carman, J. Liswood, S.M. Casey (University of Nevada, Reno) Trimethylamine (TMA) adsorption on Si(100)-(2x1) has been investigated using low-energy electron diffraction, Auger electron spectroscopy (AES), and thermal desorption spectroscopy (TDS). TMA appears to undergo molecular adsorption on this surface at room temperature. By comparison to the AES results from the adsorption of methyl iodide on Si(100), it was concluded that the initial surface saturation coverage of TMA on Si(100) is 0.5 monolayers. TDS reveals a parent TMA desorption channel, as well as competing surface decomposition channels. In order to gain further physical insight into the available reaction pathways for this molecule on this surface, we have used density functional theory (DFT) to study the adsorption of TMA on simplified silicon clusters. By experimental and theoretical studies of the adsorption of TMA and its subsequent surface pyrolysis, we try to understand the bonding characteristics and possible deposition reaction pathways for TMA and similar amines on the surfaces of semiconductor wafers. |
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SS3-TuP-8 Resonances in Electron-Stimulated Desorption of Europium Atoms from Oxidized Tungsten: Effects of Temperature
V.N. Ageev, Yu.A. Kuznetsov, N.D. Potekhina (Academy of Sciences of Russia); T.E. Madey (Rutgers, The State University of New Jersey) The electron-stimulated desorption (ESD) yield for neutral europium (Eu) atoms from Eu layers adsorbed on oxidized tungsten surfaces has been measured as a function of electron energy, europium coverage and degree of oxidation of tungsten, with an emphasis on effects of substrate temperature. The measurements have been carried out using a time-of-flight method and surface ionization detector. We expand on an earlier report, and compare ESD of multivalent Eu with ESD of monovalent alkali atoms, studied previously. The Eu atom ESD is a complicated function of Eu coverage, electron energy and substrate temperature. In the coverage range 0.05 to 0.35 monolayer (ML), overlapping resonant-like Eu atom yield peaks are observed at electron energies of 36 and 41 eV that might be associated with Eu 5p and 5s level excitations. Additional resonant - like peaks are seen at electron energies of 54 and 84 eV that are associated with W 5p and 5s level excitations. The Eu atom yield peaks at 36 and 41 eV are seen only in the temperature range 220 to 300 K at an europium coverage of 0.07 ML, and the temperature range becomes narrower with increasing europium coverage. In contrast, the peaks at 54 and 84 eV persist above 500 K. The surface ionization current calculations show that the yield associated with the tungsten core level excitations may arise from the ESD of EuO molecules rather than Eu atoms. This is the only reported system for which the electron energy dependence of neutral ESD yield has a resonant character, in the absence of a non-resonant background. |
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SS3-TuP-9 Cs Adsorbed Structure and Change of Magnetism in fcc Co Thin Films Grown on Cu(001) Surfaces
F. Komori, M. Xu, T. Iimori, K. Lee, M. Yamada, K. Nakatsuji (ISSP, University of Tokyo, Japan) Magnetic properties of ferromagnetic thin films are often largely modified by adding foreign atoms on the surface. Some of the adsorbed non magnetic atoms or molecules make the magnetization small and change the magnetic anisotropy. To study the effect of Cs adsorption on magnetism as a simple metal, we have deposited Cs on a fcc Co(001) thin film grown on a clean Cu(001) surface, and measured correlation between the ordered structure of Cs adsorbed surface and the ferromagnetic hysteresis loops of the thin film. A fcc Co thin film (about 5 mono-atomic layers thick) was prepared on a clean Cu(001) surface in an ultra-high vacuum, and the surface structure was monitored by LEED with increasing the amount of adsorbed Cs both at 300 K and at 100 K. In both cases, a ring pattern appeared at the first stage of the deposition, and then quasihexagonal LEED spots with two domains were observed. These features are similar to the Cs adsorption on a clean Cu(001) surface, and the distance between the adjacent Cs atoms on the Co surface is the same as that on the clean Cu surface. The relation between the ferromagnetic hysteresis loops and the amount of the adsorbed Cs was in situ studied using magneto-optical Kerr effect. Cs adsorption does not change the magnitude of the magnetization. This is quite in contrast with hydrogen adsorption on the same surface. With increasing the amount of Cs on the surface, the coercive force once decreases and then increases at 100 K while it is almost constant after the initial decrease at 300 K. These are interpreted the increase of the uni-axis magnetic anisotropy due to the steps on the surface at 100 K.. |
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SS3-TuP-10 Real-Time Monitoring of Desorption and Restoration on Cl/Si(111) Surface with Second Harmonic Generation
K. Shudo, T. Sasaki, M. Tanaka (Yokohama National University, Japan) Surface etching with halogen is most elementary process in semiconductor fabrication. To elucidate thermal process on Cl-covered Si(111)-1x1 surface, second harmonic (SH) generation was measured in real time with 1.17 eV photons from a pulsed laser (8 nsec). From isothermal change in the SH signal, energetics on related reactions can be discussed. On clean Si(111)-7x7 dimer-adatom-stackingfault (DAS) surfaces, SH intensity was calibrated in Cl-adsorption compared with temperature programmed desorption. The weakened SH signal by disappearing dangling bonds was interpreted as chlorine coverage on the surface. Initial sticking probability of Cl on the surface was determined to be 0.58, that is actually the same as obtained with SDR.1 Under the isothermal treatment of the Cl/Si(111)-1x1 surface from 843 to 963 K, time-evolution of SH recovery showed that fast and slow steps are involved in the process. The fast one turned out to be of a first order process. Temperature dependence of the rate in the fast component indicates a barrier of 2.1 eV. It corresponds to an energy against emerging of dangling bonds at desorption of silicon chlorides. The following slow component has 2.4 eV of barrier, being activation energy to reform the surface into 7x7 DAS structure. To reduce the dangling bond, 5x5 or 9x9 structures are known to appear temporarily before 7x7 DAS appears. The barrier is considered as the energy required for the slow structural restoration to form and/or move stacking faults.
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SS3-TuP-11 Adsorption and Decomposition of Dimethylisopropylsilane on Si(111) Surface
T.S. Yang, S.J. Cho, J.-H. Boo, J.-W. Lee, S.-B. Lee (Sungkyunkwan University, Korea); Y. Kim (Korea Research Institute of Chemical Technology) The adsorption and decomposition of dimethylisopropylsilane,(CH3)2CHSiH(CH3)2 on Si(111) surface have been studied in the temperature range of 100 - 1200 K in ultrahigh vacuum by X-ray photoelectron spectroscopy, thermal desorption spectroscopy, and low-energy Cs ion reactive scattering. Dimethylisopropylsilane adsorbs molecularly on the surface at 115 K and its thermal desorption spectrum shows a board peak centered at about 250 K. Even at this low temperature, the results of the low-energy Cs ion reactive scattering indicate that the adsorbed dimethylisopropylsilane was found to partially decompose to adsorb as the Si(CH3)2 and (CH3)2CHSiH species. The former decomposes to form CH4Si and C2SiHx species with increasing temperature up to about 600 K and the latter may be converted C3Hy on the surface. Above 900 K the intermediate species completely decomposes to form SiC. The possible decomposition mechanism of dimethylisopropylsilane will be proposed. |
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SS3-TuP-12 Negative Ion Formation in Electron-Stimulated Desorption of CF2Cl2 Co-adsorbed with Polar and Non-polar Gases on Ru(0001)
S.M Solovev (A. F. Ioffe Physico-Technical Inst., Russia); D. Kusmierek, T.E. Madey (Rutgers University) Photon-induced dissociation of CF2Cl2 (freon-12) in the stratosphere contributes substantially to atmospheric ozone depletion. We report recent results on dissociation and negative ion formation in electron-stimulated desorption of CF2Cl2 on Ru(0001), when CF2Cl2 is coadsorbed with a polar gas ( NH3) and non polar Xe, for electron energies from 50 eV to 300 eV. Two different time-of-flight methods were used in this investigation : (a) an ESD ion angular distribution (ESDIAD) detector with wide collection angle, and (b) a quadrupole mass spectrometer with narrow collection angle and high mass resolution. Many negative ESD fragments are seen (F-, Cl-, FCl-, CF-, F2-, Cl2-, CFCl2-, whose intensities depend on the surface preparation. Using both detectors we observe a giant enhancement of Cl- and F- yields for ESD of CF2Cl2 coadsorbed with ~1ML of NH3; this enhancement ( > 103 for Cl-) is specific to certain ions, and is attributed to an increased probability of dissociative electron attachment due to "trapped" low-energy secondary electrons.1 The magnitude of Cl- yield enhancement as a function of electron energy scales with the secondary electron yield, demonstrating the role of low-energy secondaries. In further studies, the influence of polar NH3 and non-polar Xe space layers (1-10 ML) on ESD of top-layer CF2Cl2 is determined, and compared with thick films of condensed CF2Cl2. The magnitudes and energy-dependences of the Cl- yields are different in these cases, due to several contributing factors. |
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SS3-TuP-13 Collision Induced Process of Adsorbates on Ni(100)
T. Takaoka, M. Inamura, S. Yanagimachi, I. Kusunoki (IMRAM, Tohoku University, Japan) Surface reactions are usually described as reactions between adsorbates on surfaces. However, effect of collision of molecules in gas phase to surfaces is drawing attention at present. In this work, the collision effect on a N2 monolayer, a benzene multilayer, and CO and H coadsorbates on Ni(100) surfaces were investigated. The experiment was carried out in the homemade molecular beam apparatus, which was equipped with a FTIR spectrometer. A supersonic molecular beam technique was used for the control of translational energy of Xe atoms. (1) N2 on the Ni(100) surface In FTIR spectra of a Ni(100) c(2x2)-N2 surface, a peak was observed at 2205 cm-1. From the analysis of FTIR spectra recorded after the surface began to be irradiated with Xe beam, it was found that the desorption of N2 molecules is induced by collision with the Xe atoms. The desorption is induced when Xe energy is above threshold energy of 0.8 eV. The threshold energy can be explained with a classical collisional model. (2) benzene multulayer on the Ni(100) surface The effect of the collision onto a multilayer of molecules has been studied. No desorption was observed when a benzene multilayer on the Ni(100) surface was irradiated with Xe atoms with energy of 1.2 eV. The energy transferred from the Xe atom was probably dissipated in condensed molecules and no desorption was observed. (3) CO and H coadsorbates on the Ni(100) surface After the Ni(100) surface was pre-exposed to H2 and subsequently exposed to CO, a peak at 2100 cm-1 was observed. In FTIR spectra recorded after the surface began to be irradiated with the Xe beam, decrease of the intensity of the 2100 cm-1 peak and increase of the intensity of a 1950 cm-1 peak with increasing Xe irradiation were observed. It was found that the adsorption site of CO is changed owing to the collision of Xe atoms and a cross section for the change is dependent on the Xe energy. |
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SS3-TuP-14 Desorption of Silicon Chloride and Formation of Dimer-Adatom-Stacking Fault Structure on Si(111) Studied by Surface Differential Reflectivity Spectroscopy
M. Tanaka, S. Minami, K. Shudo (Yokohama National University, Japan) The processes of isothermal desorption of silicon chloride from the chlorine-saturated Si(111)1x1 surface has been investigated by means of in-situ real-time surface differential reflectivity (SDR) spectroscopy. Temperature range was 873-933K. SDR spectra at each temperature were obtained by the normalization with the reflectance spectrum of the clean surface having 7x7 dimer-adatom-stacking fault (DAS) structure. The spectral features observed at 1.8 and 2.5eV come from missing of adatom dangling bonds and adatom back bonds of DAS structure, respectively. The feature at 1.8eV is found to be removed faster than that at 2.5eV due to the desorption of silicon chloride, which means that the feature at 1.8eV originates from not only adatom dangling bonds but also dangling bonds on the 1x1 surface. The restoration of the dangling bonds therefore represents the development of the desorption. The desorption process is found to follow first order kinetics, suggesting the mechanism that SiCl supplied from clusters and/or steps on the chlorine-saturated Si(111)1x1 surface recombines with Cl on the surface and SiCl2 species is desorbed. The activation energy of this process is obtained as 2.2eV from the temperature dependence of the rate constant. On the other hand, the restoration of the back bonds represents the formation of DAS structure after the desorption. The restoration process is found to follow also first order kinetics, and the activation energy is obtained as 2.8eV. The mechanism of the desorption of silicon chloride and the formation of DAS structure is discussed from these activation energies in terms of potential barriers. |
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SS3-TuP-15 Spatially Resolved Thermodynamic and Kinetics of Adsorption on BaTiO3 Surface by Variable Temperature Scanning Probe Microscopy
S.V. Kalinin, D.A. Bonnell, S. Gupta (University of Pennsylvania) Variable temperature scanning surface potential microscopy is used to determine thermodynamic and kinetic parameters associated with polarization and charge dynamics on BaTiO3 (100) surface. Potential retention above the ferroelectric phase transition, observation of domain wall motion, and local piezoresponse indicate that the temperature dependence of surface potential results from the interplay between the fast dynamics of atomic polarization and slower dynamics of screening charge. At room temperature surface potential has the sign of the screening charges and is reverse to that expected from polarization orientation. Increasing the temperature results in a decrease of polarization charge leaving the screening charges uncompensated, thus increasing the effective surface potential. On decreasing the temperature spontaneous polarization increases and for a short period of time sign of domain potential is determined by the polarization charge. This phenomenon is referred to as temperature induced potential inversion (TIPI). Under isothermal conditions, polarization and screening charges equilibrate and the potential returns to an equilibrium value. The relaxation kinetics are found to be weakly dependent on temperature with activation energy Ea ~ 4 kJ/mole. The equilibrium domain potential difference was found to be linearly dependent on temperature. A thermodynamic description of ferroelectric screening based on the Ginzburg-Devonshire theory was developed and enthalpy and entropy were obtained as ΔHads = 164.6 kJ/mole, ΔSads = -126.6 J/mole K for BaTiO3 (100) surface in air. These values are within the range expected for adsorption from the gas phase. Thus, in the case where the charge compensation mechanism is surface adsorption, scanning probe microscopy allows kinetics and thermodynamics of adsorption to be studied with the advantage of spatial localization over techniques such as temperature programmed desorbtion. |
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SS3-TuP-16 COsmic COcktails: CO-ice Chemistry under Interstellar Conditions
H.J. Fraser, W.A. Schutte, E.F. van Dishoeck (Leiden University, Netherlands) Ices are observed throughout the universe: on planetary bodies, comets, in the Interstellar Medium and in protoplanetary disks. In the laboratory it is possible to study the chemistry and physics of molecular ices under pseudo interstellar conditions. Our experiment combines UHV surface science techniques with an atomic beam to study chemical reactions occurring on interstellar ice grain mimics. The experiment is aimed at identifying the key barrierless reactions and desorption pathways on ices of H2O and CO that generate simple molecular species in the gas phase. In this poster we will present results illustrating the desorption rate of CO ice when H2 is formed from H recombination reactions on the ice surface, and identify the reaction products formed during thermal warm-up, e.g. HCO, CO2, H2O, HCOOH. We will also present results on the reaction between CO and O, before, during and after ice processing by UV irradiation. The chemical and astronomical implications of these results will be discussed. |
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SS3-TuP-17 Phosphorus Behaviors upon the Annealing of the Heavily Phosphorus Doped Silicon with Thin Native Oxide Film Evaluated by XPS and AFM
Y. Mizokawa, W.B. Ying, H. Iguchi, Y. Kamiura (Osaka Prefecture University, Japan); K. Kawamoto (Denso Co. Ltd., Japan) Phosphorus redistribution and its chemical structure of the heavily phosphorus doped Si(100) upon the annealing were investigated using x-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). The samples were prepared by predeposition of POCl3, and annealing was performed on the HF-treated samples with about 2nm-thick native oxide film in nitrogen atmosphere at 450°C, 660°C and 750°C for 1 hour. The depth profiling was carried out by the chemical etching using 0.1-1% HF solutions. The true in-depth profiles of P obtained after correcting the mean free path effect showed that the segregated-P was piled up at the interface, and its concentration decayed exponentially toward both directions of oxide film and substrate. Although the dominant chemical structure of P was unoxidized states throughout the oxide/Si, the peak position of the P2p photoelectron shifted toward higher binding energy side with approaching the interface. The results suggest that upon annealing a part of the segregated P atoms located in the top surface region of silicon lattice protrude into the oxide film as a form of P-cluster with P-P bonding. The amount of protruded-P in the oxide film increased with annealing temperature, where the activation energy was about 1 eV. The amount was estimated to be about 1x1015 P-atoms/cm2 after 750°C annealing. The AFM image showed an unique pattern of various-sized plateaus of about 2nm height, and after 750°C annealing, some of the plateaus grew in height keeping their lateral shape. The total increments were in rough accord with the estimated volume of protruded-P in the oxide film. |
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SS3-TuP-18 Vibrational Excitation of Methane Physisorbed on Ag(111) using HREELS
M. Sakurai, S. Nishida (Kobe University, Japan) We measured the incident energy dependence of the relative cross section of both elastic scattering and vibrational excitation of methane molecule physisorbed on the surface of Ag(111) using high-resolution electron energy loss spectroscopy (HREELS). The substrate was mounted on a closed-cycle refrigerator attached on a manipulator for XYZ translation and rotation, and is cooled down to 15K. It has been measured that the elastic cross section shows double dips suggesting the existence of surface resonance.1 At the present experiment with improved resolution, the variation of resonance structure was measured depending on the ambient pressure of CH4, which possibly corresponds to the variety of the structure of adsorbed CH4 . In addition, the shape of the loss peaks in EEL spectra has a tail which might indicate the rotation of CH4 molecule on the surface. |
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SS3-TuP-19 Adsorbate Induced Electronic Relaxation at Transition Metal > Surfaces
G. Fahsold, A. Priebe, M. Sinther, A. Pucci (Universität Heidelberg, Germany) We investigate the relaxation of electrons at transition metal surfaces by means of IR-spectroscopy. The adsorbate induced relaxation of electrons is responsible for the DC-resistivity of metal thin films, it is connected to the damping of adsorbate motion, and it contributes to enhancement effects observed in adsorbate vibrational spectroscopy. We measured the contribution of adsorbates (CO, O) to the relaxation of electrons at iron and copper surfaces by observing adsorbate induced broadband changes, firstly, in the IR-reflectivity from thick (opague) epitaxial films and, secondly, in the IR-transmission through ultrathin (< 5 nm) smooth epitaxial films. The higher (compared to IRAS measurements) sensitivity for adsorbate resistivity in IR-transmission is well demonstrated by our experimental results. We understand our results on the basis of a proper description of the optical properties of bulk metals and metal thin films. The talk will focus on the importance of the inclusion of non free-electron-like properties for calculating both the magnitude, the sign, and the spectral shape of adsorbate induced broadband effects. |
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SS3-TuP-20 Electron-hole Pair Generation in Adsorption of Gas-phase H(D) Atom on Pt(111) and Cu(111)
J.H. Kim, S.J. Lee, J.S. Choi, J. Lee (Seoul National University, Korea) Adsorption of gas species at solid surface proceeds via trapping of incident atom(or molecule) in the surface potential well, which requires an energy transfer to the surface. It is generally believed that multi-phonon creation is the dominant mechanism for the energy transfer. However, for the gas-phase H(D) atom, the lightest of all, phonon creation is expected to be negligible because of an extremely poor mass matching, and therefore other loss mechanism such as electron-hole pair creation may be the dominant mechanism.1. We have investigated adsorption on Pt(111) and Cu(111) surfaces at 100K of the gas-phase H(D) atom generated in a hot tungsten capillary tube at 1900K. The hot electrons and holes generated upon adsorption of H(D) atom were detected as an external short-circuit current using metal/n(p)-type Si(100) Schottky diodes. We will compare the results for the two surfaces and interpret in terms of the different electron density of states near the Fermi level. Based on these results, we will also discuss about the widely different sticking probability, saturation coverage, and abstraction reaction probability of H(D) atom on the two surfaces.
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SS3-TuP-21 Thermal Accommodation Coefficients Measurement of Inert Gas on Surface of Stainless Steel Sphere
B.S. Jun, T.K Ghosh, R.V. Tompson, S.K Loyalka (University of Missouri-Columbia) Heat transfer in the fuel-clad gap in a nuclear reactor impacts the overall temperature distribution, stored energy and the mechanical properties of a nuclear fuel rod. Therefore, an accurate estimation of the gap conductance between the UO2 fuel and the clad is critically important for reactor design and operations. To obtain the requisite accuracy in the gap conductance estimation, it is necessary to take into account the thermal accommodation coefficients of the various gases that are involved. This paper summarizes some recent efforts to obtain stainless steel accommodation coefficients experimentally. To get these values, a high-vacuum system was constructed incorporated in tandem a mechanical vacuum pump and a turbo molecular pump. Thermal accommodation coefficients for helium on stainless steel have been obtained by measuring the cooling rates of a stainless steel sphere suspended in the vacuum chamber. The cooling rate in vacuum is measured and subtracted from the cooling rate in helium at various pressures to yield the approximate net cooling rate of the sphere due to molecular impacts. Knowing the heat capacity of the sphere, its net cooling rate, the temperature difference between the sphere and the ambient gas far from the sphere, the pressure, and the rate of impingement of the gas molecules on the surface of the sphere, one can calculate the efficiency of the energy transfer to the impinging molecules. These measurements have been made on so-called engineering surfaces where no special attempt other than standard baking under high vacuum to remove volatile surface contaminants was used to clean the surfaces. It has been found for helium and argon gas that the thermal accommodation coefficient values with stainless steel are quite constant over the range of pressures studied. |
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SS3-TuP-22 Hyperthermal Product Velocity Detected as a Signature of Electron Harpooning in Gas-Surface Reactions
G.C. Poon, J.-C. Gumy, K.A. Pettus, A.C. Kummel (University of California, San Diego) Multiphoton Ionization (MPI) and Time-of-Flight Mass Spectroscopy (TOF-MS) have been used to demonstrate that the reaction of ICl on the low work function Al(111) surface proceeds by a nonadiabatic electron harpooning process. In the nonadiabatic model of this reaction, as ICl approaches the Al(111) surface, an electron harpoons from the surface, suddenly converting ICl to ICl-. This places the molecule high on the repulsive portion of the ICl- potential curve leading to rapid dissociation of ICl- into I- and Cl fragments. Following this remote dissociation of ICl above the surface, I- proceeds towards the surface and sticks while Cl is ejected into the gas phase. This is referred to as chemically selective abstraction and is consistent with the observed excess iodine on the surface by Auger Electron Spectroscopy and the detection of ejected Cl atoms. The experimentally observed signature of this harpooning process is the hyperthermal translational energy of the ejected fragment. A harpoon process should give fast Cl whose energy is determined by the vertical transition between ICl and ICl- and be independent of incident translational energy. Conversely, a conventional adiabatic abstraction reaction should provide only a small translational energy for the ejected Cl and should increase with increased incident translational energy. Hyperthermal Cl (0.36 eV ± 0.14 eV) ejected from the surface has been observed using MPI/TOF-MS, which is evidence for the aforementioned nonadiabatic process. Furthermore, DFT calculations of the vertical transition energy from ICl to ICl- are in good agreement with experiment. Comparison of the translational energy of ejected Cl from the abstractive chemisorption of Cl2 and ICl will also be presented. Cl2 should lead to even faster Cl ejected from the surface than for ICl, since Cl2- is formed even higher on the repulsive portion of the potential curve. |
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SS3-TuP-23 Growth Process of Self-organized Ge Quantum Dots on Si(111)-(7x7) Surface Studied by STM
Y. Zhang, L. Yan, S. Xie, S. Pang, H. Gao (Chinese Academy of Sciences, P.R. China) The influence of substrate temperature on the nucleation and self-organized growth of submonolayer Ge on Si(111)-(7x7) surfaces grown by solid phase epitaxy (SPE) has been studied using scanning tunneling microscopy (STM). Ordered Ge quantum dots on the surface are formed through controlling the annealing temperature after submonolayer Ge deposition at room temperature. The formation of ordered Ge quantum dots is due to the preferential adsorption sites of Ge on Si(111)-(7x7). The formed ordered nanostructures may have a potential in the application of nanodevices. |
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SS3-TuP-24 Halogen Adsorption on Pt(110): A Coverage-dependent Charge Density Wave Transition
K. Swamy, C. Deisl, R. Beer, A. Menzel, E. Bertel (University of Innsbruck, Austria) STM and LEED results of halogen (Cl, Br) adsorption on Pt(110) show a sharp, coverage-dependent phase transition into a charge density wave ground state for a coverage of Θ = 0.5 ML. At this coverage Br orders in a c(2x2) and Cl in a (2x1) structure. The (1x2) missing-row reconstruction of clean Pt(110) is lifted. Contrary to our previously published mode,1 LEED-IV data and ab-initio calculations reveal that the halogens are not substitutionally adsorbed, but in every second short-bridge site.2 Both, the Br-c(2x2) and the Cl-(2x1) phase can be transformed into a (3x1) phase by minute amounts of excess halogens (Θ = (0.5 + δ) ML), but also of molecular species (CO and NO). The Br-c(2x2) derived (3x1) phase is stable up to T > 500 K, while the Cl-(2x1) derived (3x1) phase is long-ranged ordered only at T < 200 K. Low-temperature ARUPS spectra prove that the Br-(3x1) and the Cl-(3x1) phases have an almost identical electronic structure. We interpret the (3x1) structure as a charge density wave. This is compatible with the ARUPS data, i. e. we find a corresponding nesting vector and a Peierls gap. |
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SS3-TuP-25 A LEED Investigation of Xe Adsorbed on Pd(111)
J. Zhu, H. Ellmer, H. Malissa, D. Semrad, P. Zeppenfeld (Johannes Kepler Universität Linz, Austria) Xe adsorption on the Pd(111) surface was investigated by means of low energy electron diffraction (LEED) in a UHV chamber with 1.2x10-11 mbar base pressure. Depending on the Xe coverage and substrate temperature, a large number of different phases were identified, including a 2D gas phase and a 2D liquid phase, a commensurate (√3x√3)R30° solid phase (C), a compressed striped incommensurate phase (SI), ordered multilayer structures, and a clear Xe(111) crystal with sharp spots rotated 30° with respect to the substrate. In addition, a hexagonal incommensurate solid phase (HI) and a hexagonal incommensurate rotated phase (HIR) were observed as well as a C -> SI -> HI -> HIR phase transition. The (√7x√7)R19.2° and (√19x√19)R23.5° commensurate phases reported by Hilgers et al.1 were not obtained after Xe adsorption on the clean surface in the entire temperature and coverage range. However, small amounts of contaminants (CO and/or H2) on the Pd(111) surface have a striking influence on the Xe post-adsorption. With ~0.1 L of CO pre-adsorbed at 55 K, a clear (√7x√7)R19.2° commensurate phase is obtained. After pre-adsorption of ~0.1 L of H2 at 55 K, a commensurate phase (√19x√19)R23.5° appears in the high Xe coverage region. A combination of 0.1 L H2 and 0.1 L CO pre-adsorbed at 55 K leads to a phase transition sequence from (√3x√3)R30° -> (√19x√19)R23.5° -> (√7x√7)R19.2° similar to the one reported for Xe on Pd(111) in Ref. 1. From these observations, therefore, we conclude that the high-order commensurate (√7x√7)R19.2° and (√19x√19)R23.5° phases of Xe are related to CO and H2 impurities, respectively. |
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SS3-TuP-26 Adsorption and Absorption of Hydrogen by Ti(0001): A Study Combining Surface Characterization and Non-destructive H-Depth Profiling
M. Wilde, M. Matsumoto, K. Fukutani, T. Okano (University of Tokyo, Japan); Y. Mizuno, T. Homma (Chiba Institute of Technology, Japan) In the present work the interaction of the Ti(0001) single crystal surface with molecular H2 and atomic H is studied at temperatures of 100-300 K. We combine standard UHV techniques of clean surface preparation and characterization with hydrogen depth-profiling by nuclear reaction analysis (NRA, via the 1H(15N,αγ)12C reaction), which allows straightforward assigning of the features in H2 thermal desorption spectra (TDS). Molecular H2 admitted at T<120 K gives rise to three desorption features in TDS at 160 K, 240 K and 320 K. Absorption of hydrogen by the single crystal surface is only observed after exposure to atomic H. NRA depth profiles indicate that the amount and depth distribution of absorbed H atoms is strongly dependent on the exposure temperature Te. Below 240 K, the uptake of atomic H saturates and thin (≤5 nm) hydrogen-rich overlayers are formed that contain up to a few ML of H atoms and passivate the metal against further H absorption. This near-surface H accumulation results from the efficient surface penetration by H-atoms and the restricted diffusional transport of H in the metal at the given Te. Above 240 K facile diffusion readily distributes the absorbed H into the Ti bulk. Despite rather different H-depth distributions after H absorption between 100 K and 300 K, only a single H2 desorption peak at 480-500 K is observed in TDS in all cases. An ultra-thin oxide overlayer on Ti(0001) is found to completely de-activate the surface against hydrogen sorption, whereas the outgassing of H traces dissolved in the deeper metal bulk occurring at T>650 K is unaffected. The outgassing rate is characterized by an activation energy in excellent agreement with the heat of H solution in α-Ti of 21.6 kcal/mole. |