Papers

AVS1996 Session SS2-TuM: Surface Dynamics

Tuesday, October 15, 1996 8:20 AM in Room 203B

Tuesday Morning

Start	Invited?	Item
8:20 AM		SS2-TuM-1 Real Time Scanning Tunneling Microscopy of Hydrogen Desorption Process on Si(111) Surface H. Tokumoto (National Institute for Advanced Interdisciplinary Research, Japan) Real-time STM observation of the structural change induced by the hydrogen desorption from the Si(111)-1x1:H surface was carried out at elevated temperatures (1). We started from a Si(111)-1x1:H surface with hydrogenated Si clusters at room temperature, which was prepared by exposing the atomic hydrogen atoms onto the clean Si surface at high temperature (above 400 C). When we elevated the sample temperature up to 485 C, the STM images of the 1x1:H phase remained basically unchanged and some Si clusters changed their shapes to two-dimensional islands. The drastic change in the STM images appeared at 495 C, where the hydrogen atoms started to desorb and we observed fuzzy images with no atomic arrangements. Later for certain period while the H-atoms were desorbed from the surface, we obtained a \sr\3x\sr\3R30 structure which was characterized by the Si adatom arrangement at T4 sites on the Si(111)-1x1 surface. This feature was clearly seen at higher temperatures such as 535 C. The appearance of \sr\3x\sr\3R30 structure can be explained by the Si adatom redistribution on Si(111)-1x1 surface during desorption process while all dangling bonds of both the Si clusters and Si(111) surfaces being terminated by the hydrogen atoms. Above 580 C where no H-atoms remained any more on the surface, the 7x7 structure appeared. By analyzing the decrement of the fuzzy area, we propose a model for the hydrogen desorption process which might become important below the hydrogen coverage of 0.1. This work was done in collaboration with Y. Morita, K. Miki, T. Sato, T. Sueyoshi and M. Iwatsuki, and supported by NEDO. (1) Y. Morita, K. Miki and H. Tokumoto, Surf. Sci. 325 (1995) 21.
9:00 AM		SS2-TuM-3 The Effect of Asymmetric Molecular Bonding on the Chemisorption of ICI on the Si(111)-7x7 Surface Y. Liu, D. Masson, A. Kummel (University of California, San Diego) The stereo-selective chemisorption of monoenergetic iodine chloride (ICI) molecules on the Si(111)7x7 surface has been comprehensively investigated using supersonic molecular beams, scanning tunnelling microscopy (STM), Auger electron spectroscopy (AES), and thermal desorption spectrometry (TDS). The sticking probability of ICI measured by using beam reflection technique was found to be constant (0.89\+-\0.07) for 0.2 and 1.2 eV incident translational energies and over the surface temperature range of 25\super o\C to 700\super o\C, suggesting direct rather than precursor-mediated adsorption. From the number of single versus paired reacted sites in empty-state STM topographic images, two competitive channels were observed: 85% abstractive and 15% dissociative adsorptions. In the abstractive adsorption, one of the two atoms in a diatomic molecule forms chemical bond with the surface and the other is ejected into the gas phase. In dissociative adsorption, the chemical bond of a halogen molecule breaks so that both halogen atoms separately form chemical bonds with the surface. Significantly, the abstractive probability of ICI is considerably higher than Ci\sub 2\ and Br\sub 2\ at similar incident translational energies. AES analysis demonstrated that the absolute ratio of chlorine to iodine chemisorbed on the Si(111)7x7 is approximately 0.2 at low coverages. This was verified by mass-resolved TDS experiments. Despite the much larger exoergicity for the formation of surface silicon monochloride (Si-Cl) than surface silicon monoiodide (Si-I), it was found that the chlorine atom in an ICI molecule is ejected from the surface and the iodine atom remains on the surface in a single abstractive adsorption event. The high sticking probability and the selective abstraction of iodine imply that the interaction between the asymmetric molecular orbitals of an ICI molecule and the silicon dangling bonds on the Si(111)7x7 orients the ICI molecule into Si-I-Cl conformation during abstraction.
9:20 AM		SS2-TuM-4 Molecularly Chemisorbed Intermediates and the Implications for Direct Dissociation and Direct Molecular Chemisorption in Gas-Surface Dynamics J. Davis, C. Mullins (University of Texas, Austin) The utilization of supersonic beams to probe gas-surface interactions has been instrumental in investigating the details of dissociative chemisorption at the molecular level. Beam measurements that show an increase in the reaction probability of a gas species with increasing kinetic energy have been interpreted via a direct dissociation mechanism, since increased translational energy is expected to assist in surmounting activation barriers to dissociation. Recently, evidence for another high kinetic energy dissociation mechanism, a two-step mechanism consisting of direct chemisorption as a molecule with subsequent thermally-induced dissociation, has been presented in studies of N\sub 2\ on Fe(111), O\sub 2\ on Pt(111), O\sub 2\ on Ag(110), and NO on Ir(111), with no evidence for direct dissociation. Also, stable molecularly chemisorbed precursors to dissociation have been identified for these gas-surface systems. Indeed, the absence of direct dissociation for incident energies < 2 eV appears to be a common characteristic of systems that exhibit intrinsic molecular intermediates. We will consider this trend in a review of dynamical studies of diatomics interacting with single-crystalline metal surfaces. In particular, we will discuss adsorption measurements that support direct dissociation and direct molecular chemisorption mechanisms, with emphasis on evidence concerning the existence of intrinsic molecular intermediates. Additionally, we will consider systems for which the distinction between these two mechanisms and the evidence for stable molecularly chemisorbed states is less clear.
9:40 AM		SS2-TuM-5 Collision Induced Desorption and Dissociation of O\sub 2\ on Ag(110) and Pt(111) Induced by Energetic Xe Atoms: Kinetics, Energetics and Branching Ratios C. \Ao\kerlund, I. Zoric, B. Kasemo (Chalmers University of Technology, Sweden); A. Cupolillo, F. Buatier de Mongeot, M. Rocca (University of Genoa, Italy) An energetic beam of Xe atoms impinging on a cold Ag(110) or Pt(111) surface, covered with molecular oxygen, causes both desorption and dissociation of adsorbed molecules. Using EELS, Mass Spectrometry and TDS we have measured the energy dependence of the collision induced desorption and dissociation kinetics and derived the relevant cross-sections for Xe beam energies up to 8 eV. The measured cross-sections exhibit surprisingly large energy thresholds (~ 3 eV) when compared with the corresponding thermal energy threshold (~ 0.5 eV). After the threshold, the cross sections rise rapidly with increasing Xe energy. A model for the dynamics of the collision induced desorption has been developed. It includes: low corrugation of the model PES, multiple collisions between incident Xe and adsorbed molecule and adsorbate - adsorbate interactions. In contrast the understanding of the collision induced dissociation channel and its coupling to the desorption channel is not so well understood at the atomic level. In view of that we have also examined the branching ratio between desorbed and dissociated adsorbates as a function of Xe beam energy, surface temperature and surface coverage, respectively. These branching ratios are compared with the corresponding ones, for the same system, when desorption and dissociation are induced by photons or thermally. Possible implications of these combined results for the general picture of O\sub 2\ dissociation and desorption mechanisms are discussed.
10:00 AM		SS2-TuM-6 Collision Induced Desorption and Dissociation of Chemisorbed O\sub 2\ on Ag(001) L. Vattuone, P. Gambardella, U. Burghaus, A. Cupolillo, U. Valbusa, M. Rocca (University of Genoa, Italy) We studied the processes of dissociation and desorption of O\sub 2 \ chemisorbed on Ag(001) induced by bombardment with non thermal Xe atoms, dosed by a supersonic beam, with high resolution electron energy loss spectroscopy. For normal incidence a threshold at an impact energy of about 2.5 eV is observed both for desorption and dissociation. At an impact energy of 3.2 eV and an initial dioxygen coverage of 0.2 ML the branching ratio between dissociation and desorption reads 1:3. Such value is much larger than the ratio of 1:80 obtained thermally, indicating that a different reaction pathway is followed. For non-normal incidence the branching ratio approaches again the thermal value. Temptatively we suggest therefore that the dissociation mechanism is connected to the squeezing of the molecule between the Xe atom and the surface. Measurements are currently in progress to determine the possible dependence of the cross section for desorption on oxygen coverage and crystal temperature. The results are compared to previous findings on O\sub 2 \ Ag(110), where a similar threshold for dissociation and desorption upon bombardment with Xe atoms has been found, and on Pt(111) by the group of Goteborg.
10:20 AM		SS2-TuM-7 Diffusion of Single Pd Atoms on W(110): Second-Neighbor Jumps S. Koh, G. Ehrlich (University of Illinois, Urbana-Champaign) The standard picture of metal atom motion on bcc(110) surfaces is that atom jumps along the close-packed [1-11] direction dominate diffusion. On this model, the mean-square displacement projected along the orthogonal directions [1-10] and [001], in units of the lattice spacings in the two directions, should always be equal to each other. To test the validity of this view of diffusion, we have made extensive observations of the migration of palladium on a W(110) surface, using the field ion microscope to reveal individual Pd atoms and their location. The mean-square displacements along [1-10] and [001] are found to differ significantly, with the disparity between them increasing at higher temperatures. Observations are presented showing that at higher temperatures, atomic jumps occur directly along [001]; the activation energy for these jumps is > 0.3 eV larger than for jumps along the close-packed [1-11] direction. Jumps along [1-10] are not important.(SJK has been supported by a grant from the UIUC Campus Research Board)
10:40 AM		SS2-TuM-8 Mechanistic Behavior of N-alkane Chains on Metal Surfaces J. Raut, K. Fichthorn (The Pennsylvania State University) Detailed investigations of dynamical processes of adsorbed molecules are necessary to improve understanding of many important surface phenomena. One very significant aspect of the diffusion of adsorbed molecules is their mechanistic behavior. We have used molecular dynamics and transition-state theory to study the diffusion dynamics of adsorbed alkane chains on metal surfaces using a realistic model for the n-alkanes.One set of studies has focused on the mechanisms by which n-alkanes of different lengths (C2-C20) diffuse on Pt(111). Since many degrees of freedom can be involved in molecular adsorption, there is a possibility for complex and interesting diffusion mechanisms. By determining the reaction coordinate using energy minimization we have elucidated the mechanisms by which these molecules diffuse. The barriers and rates associated with these mechanisms has also been determined. Unlike atoms, chain molecules exhibit unique non-nearest neighbor jumps. We will discuss the dependence of diffusion barriers and rates on the chain length of the molecule for different jumps across the surface. The contributions of the internal degrees of freedom of the molecule to its mobility will also be discussed. Molecular dynamics studies have shown that the diffusion barrier reaches a constant asymptotic value for sufficiently long n-alkane chains. Our calculations provide insight into this trend. Another interesting aspect of the surface mobility of these chain molecules is the mismatch between their geometry and that of the substrate. Mismatch plays an important role in the mechanistic behavior as well as in the barriers and rates. As a consequence the diffusion mechanisms depend on both the surface geometry and the lattice spacing. We will discuss diffusion and mechanistic behavior for varying degrees of mismatch and different surface geometries. Results will be compared to existing information from molecular dynamics as well as experiments.
11:00 AM		SS2-TuM-9 Anisotropy in the Lateral Momentum of CO Chemisorbed on Cu(110) Studied by Time-of-Flight ESDIAD J. Ahner, D. Mocuta, R. Ramsier, J. Yates, Jr. (University of Pittsburgh) While it is well appreciated that ESDIAD measurements contain information about chemical bond directions at surfaces, it is also known that the lateral momentum of the adsorbate must influence the ESDIAD patterns. We present, for the first time, energy resolved ESDIAD patterns which allow one to understand the anisotropy in the lateral momentum of the chemisorbed species. Using a new method, ESDIAD patterns are acquired in narrow (~0.1eV) kinetic energy windows by time-of-flight selection. For CO chemisorbed on Cu(110), the excited neutral CO* species exhibits an elliptical pattern at low kinetic energies with the major axis of the ellipse in the [110] azimuthal direction. For increasing CO* kinetic energies, the ellipticity decreases, and at very high kinetic energies, the ellipticity changes to the [100] azimuth. These observations, together with the temperature dependence of the pattern shape over the range 32 K - 180 K, yield information about the dynamics of the adsorbed molecule, and permit a qualitative assessment of the shape of the potential energy well governing the adsorbate's librational motion.
11:20 AM		SS2-TuM-10 Understanding Gas/Surface Interactions from Direct Force Measurements using a Specialized Torsion Balance S. Cook, M. Hoffbauer (Los Alamos National Laboratory) The first comprehensive measurements of the magnitude and direction of the forces exerted on surfaces by molecular beams are discussed and used to obtain information about the microscopic properties of the gas/surface interactions. This unique approach is not based on microscopic measurements of the scattered molecules. The reduced force coefficients are introduced as a new set of parameters that completely describe the macroscopic average momentum transfer to a surface by an incident molecular beam. By using a specialized torsion balance and molecular beams of N2, CO, CO2, and H2, the reduced force coefficients are determined from direct measurements of the force components exerted on surfaces of a solar panel array material, Kapton, SiO2-coated Kapton, and Z-93 as a function of the angle of incidence ranging from 0=B0 to 85=B0. The absolute flux densities = of the molecular beams were measured using a different torsion balance with a beam-stop that nullified the force of the scattered molecules. Standard time-of-flight techniques were used to determine the flux-weighted average velocities of the various molecular beams ranging from to . The reduced force coefficients can be used to directly obtain macroscopic average properties of the scattered molecules such as the flux-weighted average velocity and translational energy, that can then be used to determine microscopic details concerning gas/surface interactions without the complications associated with averaging microscopic measurements.
11:40 AM		SS2-TuM-11 Adsorption of Alkali Metals on Cu(100): The Transition from Isolated Atoms to Multilayers G. Witte (Max-Planck-Institut f\um u\r Str\um o\mungsforschung, Germany); P. Senet, J. Toennies (Max-Planck-Insitut f\um u\r Str\um o\mungsforschung, Germany) The structure and dynamics of thin alkali metal films (Na,K,Cs) on Cu(100) have been studied using He Atom Scattering and SPA-LEED. For K and Cs the formation of well ordered and thermaly stable uniformly spaced structures are observed for low coverages down to 0.05 ML. Subsequently various quasi-hexagonal adsorbate structures were observed up to a full monolayer. On the other hand Na reveals only a weak ordering at low coverages but forms various commensurate structures presumably because of the significantly smaller dipole moment of the isolated Na atoms compared to K and Cs. The energies of the frustrated translational modes of isolated alkali atoms parallel to the surface have been measured by means of inelastic He scattering. To determine the effective force constants for the interaction with the substrate atoms a new model Hamiltonian which includes the substrate phonons and the charge transfer between the adsorbate and the substrate has been developed. The phonon dispersion curves of the Rayleigh mode and the quasi-2D longitudinal mode have been measured for a Cs monolayer over the entire Brillouin zone. This provides direct information on the intrafilm Cs-Cs interaction. For alkali metal layers thicker than 1ML an entirely different vibrational behavior is found. Due to the large impedance mismatch with respect to the substrate longitudinal standing waves normal to the surface ('organ pipe modes') appear whose energy scales with the number of the layers [1]. [1] G. Benedek et al., Phys. Rev. Lett. 69, 2951 (1992).