Papers

AVS2001 Session SS2-WeA: Adsorption on Metal Surfaces

Wednesday, October 31, 2001 2:00 PM in Room 121

Wednesday Afternoon

Start	Invited?	Item
2:00 PM		SS2-WeA-1 Chemical Bonding of Alkanes on Metal Surfaces: Adsorption of n-octane on Cu(110) L. Triguero, K. Weiss, H. Öström, D. Nordlund, H. Ogasawara (Uppsala University, Sweden); L.G.M. Pettersson (Stockholm University, Sweden); A. Nilsson (Uppsala University; Sweden and Stanford University) Using Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy and X-ray emission spectroscopy (XES), we have investigated the electronic structure and chemical bonding of n-octane adsorbed on Cu(110). The high degree of NEXAFS dichroism reveals that the molecule is well oriented on the surface. The NEXAFS spectra also reveal large changes in the unoccupied electronic structure of the adsorbed octane relative to the free molecule. XES, which shows the occupied density of states, reveals new states showing up near the Fermi level. In order to understand these changes in the electronic structure, we have performed cluster model calculations in the framework of Density Functional Theory. The calculations indicate significant charge transfer and formation of a chemical bond between the molecule and the substrate. Our study also gives new experimental evidence of chemical interaction and insights into the bonding mechanism of saturated hydrocarbons adsorbed on metal surfaces, which is of importance for the understanding of the C-H bond activation mechanism.
2:20 PM		SS2-WeA-2 Two Dimensional Crystallisation of Nucleic Acid Bases on Cu{110} N.V. Richardson, D.J. Frankel, Q. Chen (University of St Andrews, UK) There is considerable interest in the characterisation and control of biomaterial surfaces. An important step towards this goal is a better, molecular level understanding of model systems based on two dimensional arrays of biorelevant molecules. In this presentation, the adsorption of the nucleic acid bases, uracil (U), thymine (T), cytosine (C), guanine (G) and adenine (A), on Cu{110} has been studied in depth by scanning tunneling microscopy, electron energy loss spectroscopy and low energy electron diffraction. In general, all the molecules form well-ordered 2D structures when vacuum deposited on a room temperature Cu{110} crystal, or at least after moderate annealing. A variety of ordered structures are found for each molecule dependent on coverage and temperature. The pyrimidine bases U, C and T form related structures involving upright molecules and, following loss of hydrogen, adsorbed species in well ordered structures are stable to over 700K. Further heating, results in major re-faceting of the uppermost (ca. ten) layers of the copper substrate giving rise to major new one or two dimensional features. In contrast the purine bases, A and G, form large, two dimensionally ordered domains based on flat-lying molecules without faceting. Hydrogen bonding is a key feature of all the networks. Models of the surface structures will be presented. We also note that although these planar molecules are optically inactive, the single mirror plane is destroyed by adsorption and the resulting surface species is then chiral. This is important in the 2D crystal structures and can result in chiral domains on the surface. .
2:40 PM		SS2-WeA-3 Electronic State of DNA Molecules on Surface: Cytosine on Cu(110) M. Furukawa (Osaka University, Japan); T. Komeda, M. Kawai (RIKEN, Japan); T. Kawai (Osaka University, Japan); H. Ogasawara (RIKEN, Japan); A. Nilsson (Uppsala University, Sweden) Electric property of DNA strands and/or molecules being attractive as they are considered to be a candidate for a piece of molecular system in nano-scale technology. The purpose of the present study is to experimentally define the local electronic state of the molecule of the molecular systems by use of x-ray spectroscopies. Here, X-ray photo-electron spectroscopy (XPS) gives the energy level of core sates against the Fermi level, X-ray absorption spectroscopy (XAS) gives the atom specific energy difference between the core state and the state above the Fermi level. Combining all the information then the atom specific view of the energy levels across the Fermi level be understood. Here we give an example for Cytosine adsorbed on Cu(110) surface, where we have observed the core level for C1s, O1s and N1s and XAS from these core levels to the unoccupied states. As for the orientation of the molecule, electric field direction dependence in the XAS experiment for all C, O and N K edge exhibited that the molecular plane of cytosine sits perpendicular to the surface and also parallel to the [1-10] row of the surface. On of the most interesting feature of the electronic states is the fact that the LUMO state of the molecule is found to sit very close to the Fermi level, indicating that the system has a natural tendency to be electron doped conductor is the level is connected in space.
3:00 PM		SS2-WeA-4 Understanding the Pressure and Structure Gaps in Catalysis: Role of Steps and Terraces in the O₂ and C₂H₄ Interaction with Ag(410) L. Savio, L. Vattuone, M. Rocca (Universita' di Genova, Italy) The effort to unravel the dynamics of real catalysts by ultra high vacuum investigations of gas-surface interaction suffers of two major limitations, known as pressure gap (10 order of magnitudes difference) and structure gap (the surface of a real catalyst has a high density of steps and defects in contrast to the almost perfect low Miller index surface of a single crystal). A large effort was devoted to bridge the pressure gap; in order to face the structure gap, attention was focused on defected surfaces, obtained either by ion bombardment or by cutting a single crystal along vicinal planes. We report on a combined supersonic molecular beam and High Resolution Electron Energy Loss Spectroscopy investigation of the angle and energy dependence of the sticking probability, S, of O2 and C2H4 on Ag(410), a vicinal surface with (100) teraces and (110) step edges. In agreement with our previous results for the ion bombarded Ag(100) surface, we find for O2 adsorption that dissociation and molecular chemisorption coexist already at temperatures at which only molecular adsorption is stable on flat Ag(100). The angular dependence of S shows that when the molecules impinge against the step edges the activation barrier for molecular adsorption is strongly reduced or even eliminated. The reactivity of Ag atoms at terrace sites is, on the contrary, reduced with respect to the flat surface. Dissociation takes place preferentially at the upper side of the steps as proven by the temperature and angular dependence of S. For C2H4 we observe nearly unitary sticking probability at the step edge and stable adsorption in the pi-bonded state. S scales approximatively with total energy, but it is slightly larger for molecules impinging grazing on the step edges and decreases when the step edges are in shadow. The interaction is mediated by an extrinsic precursor.
3:20 PM		SS2-WeA-5 Chemical Bonding in Structurally Complex Chemisorption Systems D.A. King (University of Cambridge, UK) The combination of (i) quantitative LEED, infrared and STM structural analyses of complex adsorbate structures with (ii) SCAC (single crystal adsorption calorimetry) measurements of energetics, on the one hand, and state-of-the-art first principles DFT slab calculations of these systems on the other has provided a unique experimentally benchmarked approach to chemical bonding in structurally complex chemisorption systems.¹ Of particular importance to reactivity in molecular systems at surfaces, this has given rise to the new concept of surface and promoter-induced molecular polarisation, and its relationship to stereo-electronically driven chemical reactivity in catalysis.² The presentation will draw particularly on our recent results for CO + K coadsorption on Co{1010},^2-4 C₆H₆ adsorption on Ni{111}⁵ and on Ir{100}⁶ and coadsorption with O, CO and NO on Ni{111}⁷ and anisole adsorption on Pt{110}.⁸ ¹"Adsorption Energetics and Bonding from Femtomole Calorimetry and from First Principles Theory", Advances in Catalysis, 43 (2000) 207-259. ²Q. Ge, R. Kose and D. A. King. "A Role for Induced Molecular Polarisation in Catalytic Promotion: CO Coadsorbed with K on Co{1010}", J. Am. Chem. Soc., 122 (2000) 10610-10614. ³S. J. Jenkins and D. A. King. "Full Structure Determination of an Alkali Metal/CO Co-Adsorption Phase, for Co{1010} (2x2)-(K + CO)", Phys. Rev. B, 51 (1995) 17063-17067. ⁴P. Kaukasoina, M. Lindroos, P. Hu, D. A. King and C. J. Barnes. "The Coadsorption of CO and K on Co{1010}", Surf. Sci., 349 (1996) 19-42. ⁵R. L. Toomes and D. A. King "Realistic Molecular Distortions and Strong Substrate Buckling Induced by the Chemisorption of Benzene on Ni{111}", J. Chem. Phys, 105 (1996) 11305-11312. ⁶G. Held, M. P. Bessent, S. Titmuss and D. A. King. "Benzene Adsorption on Ir{100} Studied by LEED I-V Analysis: Evidence for Formation of Tilted Benzyne", J. Chem. Phys., in press. ⁷K. Johnson, B. Sauerhammer, S. Titmuss and D. A. King. S. Titmuss, D. A. King, to be published. ⁸Y. P. Tan, S. Khatua, S. Jenkins and D. A. King, to be published.
4:00 PM		SS2-WeA-7 The Dynamics of CO and H co-adsorption on Ru(0001) B. Riedmüller (FOM Institute for Atomic and Molecular Physics, The Netherlands); I.M. Ciobica, R.A. van Santen (Eindhoven University of Technology, The Netherlands); A.W. Kleyn (Leiden University, The Netherlands) Chemical reactions at surfaces form an exciting example of co-adsorbate systems on surfaces. The dynamics of such systems is still poorly understood. In this paper we will consider the dynamics of CO and H co-adsorption on Ru(0001). In earlier experiments we have demonstrated that H-adsorption turns the Ru surface in an almost perfect inert mirror for CO molecules. However, the sticking probability does not go to zero. There are adsorbing spots on the mirror. We carried out DFT calculations that demonstrate the nature of the activated adsorption of CO on H-Ru(0001). There is one specific site (atop) exclusively binding CO. In contrast, at the clean surface the entire unit cell strongly binds CO, and the molecule will reside at the most strongly bound site. This means that the reactivity of the H-covered surface strongly varies over distances of much less than an Angstrom. The nature of the bonding will be elucidated. Although we have computed a stable CO-H-Ru(0001) configuration this does not mean that this is the most stable. In fact the system shows phase separation. We studied its formation by Thermal Energy Atom Scattering. We find that on adsorption of CO on a H-Ru(0001) surface the CO nucleates in islands. The island size depends on the CO flux. In contrast to what has been seen for epitaxy of metals on metals at low CO flux small 'magic' CO-7 clusters are formed, at high flux large islands nucleate.
4:20 PM		SS2-WeA-8 Ordered Structures of CO on Pd(111) Studied by STM M.K. Rose (Lawrence Berkeley National Laboratory and University of California at Berkeley); J.C. Dunphy, T. Mitsui (Lawrence Berkeley National Laboratory); A. Borg (Norwegian University of Science and Technology, Norway); D.F. Ogletree, M. Salmeron (Lawrence Berkeley National Laboratory) The √3x√3-R30°, c(4x2), and (2x2)-3CO structures of CO on Pd(111) have been studied by STM. Shifts of the CO binding site with increasing coverage are observed. At coverages of 1/3 ML and below, CO occupies three-fold hollow sites. Near 1/2 ML, regions of c(4x2) CO with both FCC and HCP three-fold hollow site occupation coexist with bridge bonded c(4x2) CO. At high coverage, a partially disordered phase with no top site occupation directly precedes formation of the 2x2 structure. With additional adsorption of CO, bright maxima appear with 2x2 periodicity. The maxima at the edge of 2x2 domains exhibit a quasi-continuous range of corrugation, implying a gradual shift of CO to top sites. The high corrugation of top site CO obscures the other two CO molecules per unit cell, resulting in a symmetric 2x2 periodicity. In addition to this symmetric 2x2 structure, previously unreported domains of lower symmetry 2x2 are observed.
4:40 PM		SS2-WeA-9 Bridging the Pressure Gap at the Atomic Level P. Thostrup, L Österlund, I. Stensgaard, E. Laegsgaard, F. Besenbacher (University of Aarhus, Denmark) Surface science studies conducted under ultrahigh vacuum (UHV) conditions have contributed immensely to our current knowledge about catalytic processes. A fundamental question is however still, whether UHV data are in general applicable at technologically relevant pressures magnitudes higher than those obtainable in UHV studies. We have developed a novel high-pressure scanning tunneling microscope (HP-STM)¹ in order to compare eg. the UHV and HP response of the H/Cu(110) and CO/Pt(110) systems. The H/Cu(110) system is a classic model system for activated dissociation. We find that hydrogen reconstructs the surface both at UHV and high pressures. Through a detailed comparison we even find quantitative agreement between HP and UHV data,² thus providing support for the surface science approach to heterogeneous catalysis. The CO/Pt(110) system has attracted widespread attention since CO displays an unusually strong dependence upon the coordination number of the Pt atom to which it binds. This property makes the open Pt(110) surface ideal for HP experiments since high pressures of CO are expected to induce massive roughening of the surface. Notwithstanding, already in UHV this interesting property reveals itself:³ We find the equilibrium structure at intermediate coverages to be extremely rough to an extent where almost all top-layer Pt atoms are at steps. Interestingly, we have been able to reproduce this behavior quantitatively in Monte Carlo simulations where all energies involved are taken from ab initio density functional calculations. Preliminary experiments indicate that the roughening behavior described above is also present at high pressures, but in this case to an even more extensive degree. ¹ E. Laegsgaard, P. Thostup et al., submitted to Rev. Sci. Instr. ² L. Österlund, P. Thostrup et al., Phys. Rev. Lett. 86, 460 (2001). ³ P. Thostrup et al., submitted to Phys. Rev. Lett.
5:00 PM		SS2-WeA-10 A Single-crystal Adsorption Calorimeter for Low Vapor Pressure Molecules H. Ihm, H.M. Ajo, C.T. Campbell (University of Washington) We report a new microcalorimeter for measuring heats of adsorption of low vapor pressure molecules on clean single crystals. While temperature programmed desorption (TPD) and isosteric measurements can provide information on heats of adsorption, they are limited to reversible adsorption-desorption processes. In many catalytically interesting cases, adsorbates undergo irreversible chemical changes upon adsorption or heating. Our microcalorimeter enables direct measurement of these adsorption energies. The principle is similar to that pioneered by King's group:¹ a pulse of gas from a molecular beam impinges on a 1 µm-thick single crystal surface. The incident molecules adsorb either physically or chemically, causing a transient temperature rise. This heat input is detected by a 9 µm thick pyroelectric polymer ribbon, which is mechanically driven to make gentle contact with the back of the single crystal sample during a calorimetry measurement.² This process allows the determination of heats of adsorption as a function of coverage with femtomole resolution. While King's group has performed beautiful adsorption calorimetry, their measurements have been limited to high vapor pressure gases. We have added a specially designed molecular beam source for low vapor pressure molecules, thus significantly extending the capabilities of this technique. Sticking probabilities and absolute coverages are measured by quadrupole mass spectrometry (QMS), low energy ion scattering (LEIS), x-ray photoelectron spectroscopy (XPS), and TPD. As a first experiment, we present a microcalorimetric measurement of benzene adsorption on Pt(111). ¹ W.A. Brown, R. Kose, and D.A. King, Chem. Rev. 98, 797 (1998). ² J.T. Stuckless, N.A. Frei, and C.T. Campbell, Rev. Sci. Instruments, 69, 2427 (1998).