AVS1997 Session SS1-ThA: Electronic Structure: Surfaces and Adsorbates

Thursday, October 23, 1997 2:00 PM in Room A7/8

Thursday Afternoon

Start	Invited?	Item
2:00 PM		SS1-ThA-1 Electronic Structure vs. Coverage of Alkali Atoms on W(110) E. Rotenberg (Advanced Light Source); S.D. Kevan (University of Oregon) We present comprehensive measurements of band structure and Fermi level crossings for the entire family of alkalis (H, Li, Na, K, Rb, Cs) on W(110) as a function of coverage (from 0 to ≥ 1 ML) and as a function of momentum parallel to the surface. First, we discuss the nature of the bonding between adsorbate and substrate. We find that the bonding of hydrogen to the substrate differs dramatically from that of the rest of the family, since only the former saturates the hole orbits at N-bar, H-bar and S-bar at ~ 0.5 ML This can be directly related to interaction with particular W dangling bonds. These orbits are preserved in the rest of the family, except that their filling changes as discussed below. Second, we discuss the very important question of charge transfer between adsorbate and interface W atoms. As a function of coverage, charge transfer into W interface bands may occur directly from the adsorbate atoms (ionic bonding) or else as bulk screening charge! in response to the altered surface potential. We find that the net charge transfer into interface W bands is significant and may be observed as the approx. rigid motion of surface bands downward, with an associated shrinkage of surface Fermi hole orbits. By measuring the size of the 2-D fermi contours, we quantify the change in filling of these bands as a function of coverage and compare these results to previous measurements of the core-level lineshapes and workfunctions.¹ While many of the observed trends are systematic, we also find some interesting anomalies for Li and Na. ¹D. M. Riffe, G. K. Wertheim, and P. H. Citrin, Phys. Rev. Lett. v64 p571 (1990)
2:20 PM		SS1-ThA-2 Surface Photoelectric Effect in Thin Layers of Alkali Metals S.R. Barman, K. Horn (Fritz-Haber-Institut der MPG, Germany); P. Häberle (Universidad Técnica Federico Santa María, Valparaíso, Chile); H. Ishida (Nihon University, Japan); A. Liebsch (Forschungszentrum Jülich, Germany) The response of a free-electron-like metal to an external electromagnetic field is strongly influenced by single-particle and collective excitations in the surface region. The screening of the incident field via these excitations gives rise to rapid spatial variations of the effective local field. At clean simple metal surfaces, these processes lead to the well-known local-field enhancement of the photoemission intensity associated with the so-called multipole surface plasmon. Here, we report photoemission spectra of thin alkali metal overlayers on Al(111) over a wide range of coverage. At coverages less than one monolayer, the spectra of Na, K, and Li exhibit the threshold excitation caused by transitions from the Fermi energy to the vacuum level. Beyond one monolayer, the Na and K spectra reveal the multipole surface plasmon and a bulk-like excitation corresponding to a standing wave within the overlayer. The Li spectra, however, show the multipole peak, but the bulk-like plasmon is very broad and much weaker than in the case of Na and K. The Na and K spectra confirm theoretical predictions based on the time-dependent local density approximation (TDLDA) for the overlayer jellium model. Thus, lattice effects in these metals are negligible. The strong ionic pseudopotential of Li, on the other hand, gives rise to interband transitions within the overlayer which heavily broaden the Li bulk-like mode. The multipole surface plasmon is less affected by these transitions since it is localized near the overlayer-vacuum interface. These observations are in excellent agreement with new TDLDA calculations for realistic Li overlayers on Al.
2:40 PM		SS1-ThA-3 Predicting Scanning Tunneling Microscopy Images of Molecules Adsorbed on Metal Surfaces D.N. Futaba, S. Chiang (University of California, Davis) We present calculations based on extended Hückel molecular orbital theory predicting STM images of molecules adsorbed onto metals. Hückel theory is a semiempirical method whose assumptions may limit it to organic systems but allow very large systems to be soluble. Images are generated by evaluating the density of states from the Hückel routine at the Fermi level at a given height above the molecular plane. We have applied this method most recently to para-xylene and meta-xylene on Rh(111). For both isolated molecules and for molecules chemisorbed onto a metal cluster, we computed images for high symmetry binding sites and orientations. We found para-xylene has a predominantly ring-like feature with an additional features on each side. Meta-xylene was found to have a triangular shape regardless of the binding site. These images contain detailed structural information which compares well with experimental findings ¹ and with calculations for isolated molecules. Other systems for which we are computing images include thiophene on Pd(111), ethylene on Cu(110) and n-alkanethiols (n = 8,10) on Au(111). ¹P. Cernota, G.A. Somorjai, unpublished
3:00 PM		SS1-ThA-4 Chemisorption on Metal Surfaces via a New Embedding Theory N. Govind, A.J.R da Silva, E.A. Carter (University of California, Los Angeles) First principles theories for chemisorption on metal surfaces tend to fall into two classes: cluster and slab models. The cluster models are limited in accuracy because metallic wavefunctions tend to be delocalized and edge effects unphysically constrain the wavefunction. The slab calculations on the other hand are limited by the nonsystematic predictive capability of current implementations of density functional theory (DFT). Here we present a new embedding theory that combines the salient features of density functional theory and traditional quantum chemistry methods. The scheme involves using DFT slab information within a cluster embedding method. We test the new procedure by calculating experimentally well known adsorbate binding energies on metal surfaces.
3:20 PM		SS1-ThA-5 An X-ray Absorption Study of Benzene Bonded to Clean and Chemically Modified Metal Surfaces and Metal Clusters K. Weiss (Ruhr-Universität Bochum, Germany); S. Gebert (Universität Heidelberg, Germany); A. Schertel (On leave from Universität Heidelberg, Germany); H. Wadepohl (Universität Heidelberg, Germany); Ch. Wöll (Ruhr-Universität Bochum, Germany) X-ray absorption spectroscopy (NEXAFS) has been used to study benzene adsorbed on a variety of clean (Au, Cu, Ru, Pt) and chemically modified (H/Ru, O/Ru, CO/Ru) metal surfaces. The NEXAFS-spectra reveal significant differences with regard to the shape and positions of resonances and the dichroism observed for the resonance intensities, which can be attributed to the different electronic coupling to the substrate and changes in the molecular structure of the benzene molecule ¹. In particular NEXAFS-spectra of benzene chemisorbed on clean Ru(0001) show pronounced differences to those for benzene adsorbed on H(1x1)/Ru(0001) and O(2x1)/Ru(0001) substrates, resulting from the different hybridization of molecular electronic states with those of the metal. Data were also obtained for the benzene-metal cluster complex C₆H₆Ru₃(CO)₉, where the benzene ring is bonded in a face-capping coordination to a triangular arrangement of ruthenium atoms. The structure of this metalorganic compound is similar to the one of benzene on Ru(0001) ² and a comparison can provide information on the so-called metal-cluster analogue. ³ ¹C. Mainka, P. S. Bagus, A. Schertel, T. Strunskus, M. Grunze, Ch. Wöll Surf. Sci. 341, L1055-L1060 (1995) ²C. Stellwag, G. Held, D. Menzel, Surf. Sci. 325, L379-L384 (1995) ³E. L. Muetterties, T. N. Rhodin, E. Band, C. F. Brucker, W. R. Pretzer, Chem. Rev. 79, 91 (1979)
3:40 PM		SS1-ThA-6 The Electronic Structures of Ir(100): A Photoemission Study. A.P.J. Stampfl, O. Schaff, Ph. Hofmann, M. Polcik, A.M. Bradshaw (Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany) Angle-resolved photoemission spectra were measured along the main symmetry directions for the (hex) and (1X1) phases of Ir(100). Transitions from the bulk and surface state related bands were identified using three different final state models: free electron final states, non-free electron final states and indirect transitions. Surface related features were further identified from the results of dosing small quantities of CO on both phases and by direct comparison with the calculated surface band structure. Surface bands observed on the (1x1) phase agree well to calculated ones. Surface related transitions were found to dominate the spectra from the Fermi level to approximately three electron volts below. In this region the band dispersions of both phases are different, notably the surface related bands passing through the Fermi level of the (1X1) phase are observed to disappear upon reconstruction in a similar fashion to previous work reported for Pt(100) ¹. Several models will be discussed relating this observed behaviour to the possible mechanism of reconstruction. ¹A.P.J. Stampfl, R. Martin, P. Gardner and A.M. Bradshaw, Phys. Rev. B51, 10197 (1995).