AVS2001 Session SS2-ThP: Beam Interactions with Surfaces Poster Session

Thursday, November 1, 2001 5:30 PM in Room 134/135
Thursday Afternoon

Time Period ThP Sessions | Topic SS Sessions | Time Periods | Topics | AVS2001 Schedule

SS2-ThP-1 Evaluation of Surface Activity using a Molecular Probe
K. Iizumi, K. Ueno, K. Saiki, A. Koma (The University of Tokyo, Japan)
It is important to investigate activity of various surfaces, since surfaces provide stages where various kinds of phenomena occur. We have examined to evaluate the surface activity through the interaction between C60 and the surface. When the surface is active, the interaction between C60 and the surface is strong. This strong interaction breaks the high symmetry of C60 molecule and modifies its electronic structure. Then the electronic structure of adsorbed C60 molecule indicates activity of the surface, onto which C60 adsorb. In this sense, a C60 molecule can be a good probe to know the degree of activity of the surface. In this paper, we chose three types of surfaces and estimated their activity. These three type of surfaces are as follows; (i) Si(111)-7x7 surface, (ii) dangling bond terminated Si(111) surface: Si(111)-√3x√3-Ag, Si(111)-√3x√3-Ga and Si(111)-1x1-As, (iii) layered material: MoS2 cleaved surface. Monolayer C60 films were grown on these surfaces and investigated electronic structures of C60 by electron-energy-loss (EEL) spectroscopy in the reflection geometry. This is a powerful technique to study surface electronic structures of various materials. EEL spectra of C60/Si(111)-7x7 and C60/Si(111)-√3x√3-Ga are quite different from that of bulk C60. On the other hand, EEL spectra of C60/MoS2 and C60/Si(111)-1x1-As are the same as that of bulk C60. The EEL spectrum of C60/Si(111)-√3x√3-Ag resembles to that of bulk C60. However there is small difference in peak intensities. As a result, we conclude that degrees of activity of above five surface are as follows: Si(111)-7x7, C60/Si(111)-√3x√3-Ga >> Si(111)-√3x√3-Ag > C60/MoS2, C60/Si(111)-1x1-As.
SS2-ThP-2 Low-Energy Ion Scattering Study of Ag-Al Surface Composition1
R. Bastasz, J.A. Whaley (Sandia National Laboratories); W.P. Ellis (Los Alamos National Laboratory)
Diffusion and segregation effects have been examined for the Ag-Al system using the surface-specific technique of low-energy ion scattering (LEIS), which probes the outer atomic layer of materials.2,3 Thin (25 µm) foils of high-purity Ag and Al were placed in contact with each other and mounted on a heater assembly in a UHV chamber with the Al surface exposed. Initially LEIS signals from O and Al atoms were observed on the native oxide surface, but after cleaning by 1 keV He+ bombardment only He+ scattering from surface Al atoms was detected. The surface composition was then measured as a function of temperature up to 575 °C. No Ag was observed on the outer Al surface of Ag-Al foil samples heated to <500 °C. At 500 °C, prolonged heating resulted in the appearance of a LEIS signal from Ag at the Al surface. When the temperature of a fresh sample assembly was increased to near the eutectic (about 555 °C), the solids reacted rapidly to form a two-phase system4 and a LEIS signal from surface Ag immediately appeared. LEIS measurements of the surface composition of the clean binary system in the range 25-550 °C showed the Ag surface coverage to increase with temperature. Above the eutectic temperature, the Ag coverage decreased. Such LEIS measurements make it possible to determine the diffusivity of Ag in Al as well as the segregation enthalpy for Ag on Al surfaces.

1 This work was supported by the US Department of Energy under contract DE-AC04-94AL85000.
2 E. Taglauer, in: "Surface Analysis- The Principal Techniques", J. C. Vickerman, Ed. (Wiley, 1997) Chap. 6.
3 R. Bastasz, J. A. Whaley, and W. P. Ellis, Surf. Rev. Lett. 6 (1999) 605.
4 A. J. McAllister and J. J. Murray, Bull. Alloy Phase Diagr. 1 (1983) 36.

SS2-ThP-4 New Approaches to Diffusion and Electronic Properties of Surfaces: Spin-Echo Quasielastic Helium Scattering (SE-QHAS) and Metastable Helium Atom Scattering (MHAS)
P. Fouquet (Cavendish Laboratory, UK and MPI für Strömungsforschung, Germany); A.P. Jardine, J. Ellis, W. Allison (Cavendish Laboratory, UK); G. Witte (Ruhr-Universität, Germany)
We present current progress of two major new developments in atomic beam surface scattering: QHAS is a uniquely powerful technique for studying diffusion on atomic length and time-scales. Since helium atoms scatter from the diffusing species as they move, QHAS gives a detailed reciprocal space map of the paths the atoms take as they traverse the surface. We demonstrate the way that QHAS data can be used to study potential energy surfaces of diffusing atoms and adatom-adatom potential energies, as illustrated by analyses of experimental data on the CO/Cu(001) and Na/Cu(001) systems.1 Till now, QHAS has been limited to systems showing very high mobilities (D > 5 x 10-6 cm2 s-1), we show how measurements can be made over a much wider range of mobilities by using spin echo techniques and give details of a new ultra high resolution scattering machine that is being developed in the Cavendish. In contrast to that, metastable, excited He-atoms are highly sensitive to the surface electron density. We have shown experimentally that MHAS is capable of measuring the valence state occupation of alkali metals during growth on Cu(001).2 Our data interpretation has since found support by theoretical work which proved that, for the investigated systems, MHAS exclusively probes the electronic density of states projected onto the He 1s state.3 We have extended our measurements to alkaline earth metals (Ba), semiconductor surfaces (GaAs(100)) and oxygen coadsorption. The data supply novel information about the metallisation transition of ultrathin metal films as well as the catalytic oxidation of semiconductors.

1 J. Ellis, A.P. Graham, F. Hofmann, J. P. Toennies, Phys. Rev. B 63, 195408 (2001).
2 P. Fouquet, G. Witte, Phys. Rev. Lett. 83, 360 (1999).
3 L.N. Kantorovich et al., Surf. Sci. 444, 31 (2000).

SS2-ThP-5 A Molecular Beam Study of Helium Absorption and Diffusion in Amorphous and Crystalline Ice
J.L. Daschbach, P. Ayotte, G.A. Kimmel, Z. Dohnalek, R.S. Smith, B.D. Kay (Pacific Northwest National Laboratory)
Molecular beam and thermal desorption techniques are employed to study the absorption and diffusion of He atoms in thin amorphous and crystalline ice films at low temperatures (20-100 K). The He absorption probability increases strongly with increasing translational energy (0.065 - 0.23 eV) and decreases dramatically as the incident angle is moved away from the surface normal (0-60°). These findings are indicative of a large activation barrier and a strong steric effect for the insertion of the He atoms into the bulk. Comparison between crystalline and amorphous ice suggests that absorption into the bulk occurs through hexagonal ring structures existing at the ice surface. Diffusion of He in ice is studied by growing ice films of varying thickness on top of a He impregnated ice substrate, and subsequently measuring the He permeation through the overlayer using temperature programmed desorption (TPD). The resultant He TPD data is analyzed using a coupled diffusion/desorption model to extract the temperature dependent diffusivity. The diffusion exhibits Arrhenius behavior with a diffusion barrier of 0.14 eV. The details of the experimental findings and their implications will be presented.

Pacific Northwest National Laboratory is a multiprogram National Laboratory operated for the U.S. Department of Energy by Battelle Memorial Institute under contract DE-AC06-76RLO 1830.

Time Period ThP Sessions | Topic SS Sessions | Time Periods | Topics | AVS2001 Schedule