AVS1996 Session SS1-MoM: Fermi Surfaces and Electronic Excitations

Monday, October 14, 1996 8:20 AM in Room 204C

Monday Morning

Start	Invited?	Item
8:20 AM	Invited	SS1-MoM-1 The Dissociative Adsorption of Hydrogen: From Simple Metals to Alloys A. Hanbicki, K. Pohl, E. Plummer (University of Tennessee & Oak Ridge National Laboratory) The interaction of hydrogen with metal surfaces has been a subject of interest to the scientific community for many years. The appeal of this subject originates from its presence in such diverse areas as hydrogen storage, methanation and hydrogenation type catalytic reactions and as a testing ground for theoretical concepts governing surface reactivity [1]. The conventional wisdom [2] is that metals which have a low density of d-like holes at the Fermi level, or metals which have no d-states at all will not dissociate molecular hydrogen, i.e. adsorption is an activated process. A general discussion of the status of our understanding of this interaction will be presented with particular emphasis put on experimental observables such as the dissociation barrier, the H-metal bond energy, saturation coverages, H induced reconstructions, H binding sites, etc. These observations will be compared to recent first principles calculations [1, 3-5]. Three general classes of metal substrates will be discussed: simple s-p bonded metals, e.g. Be [5]; the noble metals, which have filled d bands; and surfaces of the ordered bimetallic alloy NiAl [6]. 1. B. Hammer and J. K. Norskov, Surf. Sci. 343 (1995) 211. 2. J. Harris and S. Andersson, Phys. Rev. Lett. 55 (1985) 1583. 3. B. Hammer and J. K. Norskov, Nature 376 (1995) 238. 4. B. Hammer and M. Scheffler, Phys. Rev. Lett. 74 (1995) 3487. 5. R. Stumpf and P. J. Feibelman, Phys. Rev. B 51 (1995) 13748. 6. A. T. Hanbicki, A. P. Baddorf, E. W. Plummer, B. Hammer, and M. Scheffler, Surf. Sci. 331-333 (1995) 811.
9:00 AM		SS1-MoM-3 Vibrationally Resolved Core Level Photoemission of Chemisorbed Hydrocarbons J. Andersen, A. Beutler, R. Nyholm, S. Sorensen (Lund University, Sweden); D. Heskett, B. Setlik (University of Rhode Island) We have investigated by high resolution core level spectroscopy applied to the C 1s level the adsorption of several hydrocarbon molecules on the Rh(111) surface. The C 1s spectra are found to contain additional finestructure which we assign to vibrational excitations during the core level photoemission process. One example of this is the Rh(111)-(2x2)-C\sub 2\H\sub 3\ structure where the ethylidyne molecules are known to stand upright on the surface with the CH\sub 3\ group outwards. For this overlayer structure we find two main C 1s components corresponding to the two inequivalent C atoms. The component from the outer C atom displays additional finestructure. This finestructure is assigned to excitations of the C-H stretch and is thus not caused by the existence of C atoms that are inequivalent because they bond to different numbers of H atoms. The implications of these results for high resolution core level spectroscopy investigations of other chemisorption systems will be discussed
9:20 AM		SS1-MoM-4 Influence of Adsorbates on the Ultrafast Electron Dynamics in Image Potential States at a Metal Surface T. Hertel, E. Knoesel, A. Hotzel, M. Wolf, G. Ertl (Fritz-Haber-Institut der MPG, Germany) Image potential states provide a model system to study the ultrafast dynamics of energy and charge transfer between electronic states localized at a surface and the delocalized excitations of the bulk continuum. We have investigated the electron dynamics in the n=1 and n=2 image potential states at bare and rare gas-covered Cu(111) surfaces using femtosecond time-resolved two-photon photoemission. To study the influence of energy and phase relaxation on the two-photon photoemission (2PPE) process we have performed density matrix calculations which provide a basis for the analysis of time-resolved 2PPE - in particular, if the excited state lifetimes are comparable to the laser impulse duration. We find that the lifetime T\sub 1\ of the n=1 image state increases from 18\+-\5 fs for bare Cu(111) to 75\+-\15 fs for the Xe-covered surface. Our experiments also reveal that both energy and phase relaxation are predominantly determined by the overlap of the image state wave functions with the bulk. * T. Hertel - present address: IBM Research Division, T.J. Watson Research Center, Yorktown Heights, NY 10598
9:40 AM		SS1-MoM-5 Femtosecond Photoemission Study of Electron Relaxation in Two-dimensional Layered Electron System S. Xu, J. Cao, C. Miller (University of Rochester); D. Mantell (Xerox Wilson Center for Research & Technology); R. Miller (University of Toronto, Canada); Y. Gao (University of Rochester) The transient behavior of excited electrons at surface is the central subject in surface dynamics and photochemistry. The photoexcited electron can relax and transport inside materials, or inject between surface and adsorbate. In order to get deep understanding of electron relaxation processes, we performed femtosecond time-resolved photoemission study of electron relaxation on model systems including metals, semiconductors and layered compound. Our study shows the dominant relaxation process in 3-dimensional metals is mainly due to electron-electron scattering, which is predicted by the Fermi liquid theory. However, our recent study on graphite, manifests the relaxation process is non-Fermi liquid like. In this contribution, we will discuss about this issue.
10:00 AM		SS1-MoM-6 Measurements of Surface State Transitions on the Bare Diamond C(100) and C(110) Surfaces T. Mercer, P. Pehrsson (Naval Research Laboratory) The Diamond C(100) and C(110) surfaces have been investigated with High Resolution Electron Energy Loss Spectroscopy (HREELS). By examining electronic excitation spectrum in the 0 eV -15 eV range, two mid-gap states have been identified on the bare C(100) surface. Evidence of these states is not present in the spectra acquired on the H-terminated surface and thus it is concluded that these states are localized at the surface and are due either to pi-bonded dimers, or dangling bonds on the surface. There were no apparent mid-gap states on the C(110) surface and only a small surface band-gap was measured. From a single particle picture, the bulk terminated C(110) surface should exhibit a metallic nature. The fact that this surface was found to be semiconducting implies that either the surface reconstructs or electron-electron interactions render the surface semiconducting.
10:20 AM		SS1-MoM-7 Surface Fermi Contours and Phonon Anomalies on H/W(011) E. Rotenberg, S. Kevan (University of Oregon); J. Denlinger (University of Wisconsin, Milwaukee) Recent measurements, motivated in part by our early fermi-contour (FC) measurements,\super 1,2\ indicate anomalies in the surface phonon dispersion of Mo(011) and W(011)-(1x1)H surfaces.\super 3-5\ These anomalies are attributed to excitation of electron-hole pairs across the Fermi level (E\sub F\) and can be correlated with measured FC nesting vectors. While calculations for the clean Mo and W surfaces\super 6,7\ agree with our measurements, these same measurements produce FCs for the H-covered surfaces which are quantitatively different from our experimental FCs but which do predict the observed phonon anomalies well. Using a new undulator beamline and more systematic angle-resolved photoemission, we have repeated our measurements of the surface FCs for various coverages of atomic hydrogen adsorbed onto W(011). Our new results at saturation coverage are now much closer to the calculated contours and also predict critical wave vectors close to the observed anomalies. We find that the FCs and thus also the nesting vectors are significantly coverage-dependent. We will report the nesting vectors as a function of coverage in a few different azimuthal directions, and compare these to existing results for phonon anomalies where available. 1. R.H. Gaylord, et al, Phys. Rev. Lett. 62, 203 (1989). 2. K. Jeong, et al, Phys. Rev. B 39, 2973 (1989). 3. E. Hulpke and J. Ldecke, Phys. Rev. Lett. 68, 2846 (1992). 4. E. Hulpke and J. Ldecke, Surface Sci. 287/288, 837 (1993). 5. M. Balden et al, Surface Sci. 307-9, 1141 (1994). 6. B. Kohler et al, Phys. Rev. Lett. 74, 1387 (1995). 7. M.-Y. Chou, private communication (1996).
10:40 AM		SS1-MoM-8 Surface Electronic Structure of GaN(0001) S. Dhesi, C. Stagarescu, M. Bunea, K. Smith, R. Singh, T. Moustakas (Boston University) We report the results of the first angle-resolved photoemission (ARP) spectroscopy study of the surface electronic structure of GaN(0001). The properties of nominally stoichiometric surfaces have been determined, as well as those of N-deficient and Ga-rich surfaces. The GaN(0001) films were grown on sapphire substrates at Boston University using electron-cyclotron resonance molecular-beam epitaxy and were auto-doped n-type. Nominally stoichiometric clean surfaces were prepared in situ by evaporating a thick Ga film onto the surface, flashing it off, and then annealing in an N\sub 2\ partial pressure. This method of surface preparation results in a sharp (1x1) LEED pattern on a low background, with minimal carbon or oxygen contamination, as measured by Auger electron spectroscopy. No evidence of facetting or surface reconstruction was detected using LEED. The position of the Fermi level above the valence band maximum for the nominally stoichiometric surface implies an upward band bending at the surface of GaN(0001). However, we found that this band bending and the valence band electronic structure vary radically as a function of the density of N defects or the presence of excess Ga at the surface (N defects were created in situ by Ne\super +\ ion bombardment). The measured valence band dispersion along the high-symmetry directions of the surface Brillouin zone is much smaller than predicted by band-structure calculations. These spectra also reveal the existence of a feature near the top of the valence band that could possibly be due to emission from a surface state. The modification of the Ga 3d core-level following the deposition of sub-monolayer quantities of Cs implies that metallic Ga is produced at the Cs/GaN interface. This results will be discussed with respect to other metal deposition studies on the surface of GaN(0001). Work supported in part by NSF grant DMR 95-04948. ARP experiments were performed on the Boston University/National Synchrotron Light Source beamline U4A.
11:00 AM		SS1-MoM-9 Photoemission Study of As/InP(110) Y. Khazmi, Z. He, P. Nilsson, J. Kanski (Chalmers University of Technology, Sweden) An ordered As/InP(110)1x1 surface was prepared by exposing cleaved InP(110) to an As\sub 4\ beam at RT. The surface was investigated by means of angle-resolved photoelectron spectroscopy. The core level spectra revealed two As components with equal intensities, reflecting coordination to the substrate anions and cations. In the valence band spectra As-related dispersive surface states were identified. Empirical LCAO calculations were carried out and used for interpretation of the experimental data. The results will be discussed in terms of surface structural models and compared with corresponding data on similar, isoelectronic systems.
11:20 AM		SS1-MoM-10 Strain-induced Depopulation of the Surface State in Epitaxial Ag Films on Si(111) G. Neuhold, K. Horn (Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany) Surface states are a sensitive probe of surface geometric and electronic structure. Here we present a photoemission study of the energy and intensity of the Ag(111) surface state near E\sub F\ at the center of the Brillouin zone for silver films on Si(111) surfaces. Because of its narrow line width and high cross section, small changes in this occupied surface state can be sensitively probed. It is found that the surface state intensity varies considerably depending on preparation and annealing conditions. This observation is traced to a binding energy shift of the surface state towards the Fermi energy which leads to a reduced occupancy. This can be demonstrated experimentally by canceling the shift responsible for the depopulation through a lowering the work function by alkali metal adsorption. Since the energy of the surface state according to the phase accumulation model depends strongly on the energy of the L\sub 2\\super '\ edge of the Ag s-p band, we have analyzed the Ag band structure by means of empirical pseudopotential calculations. It is found that the energy of L\sub 2\\super '\ is markedly influenced by lattice deformations in the epitaxial silver film. On the basis of the thermal expansion coefficient the calculations describe the temperature-dependent surface state energies found by Paniago et al.[1]. The results for volume-conserving deformations suggest that the silver films on Si(111) are strained, and that the amount of strain induced by different preparation and annealing procedures can be derived.1. R. Paniago, R. Matzdorf, G. Meister, A.Goldmann, Surf. Sci.336,113 (1995).