Relaxation at clean metal surfaces

An electrostatic model of the energy of the surface layers of a metal is shown to describe in detail complex phenomena of surface relaxation in clean metals. The model accounts for relaxation effects that go many layers deep, that have both parallel and perpendicular components and that show large variations from surface to surface of the same metal. The model adds a new physically plausible assumption to the simple electrostatic model previously proposed by Finnis and Heine, which increases the force binding each ion to its bulk position by an amount fixed empirically for each metal. The equilibrium configuration of surface layers is found by minimizing the energy with respect to rigid translations of ion nets in a fixed electronic background density. The many surface structure parameters thus determined fit low-energy electron diffraction data on six surfaces of bcc Fe and six of fcc Al well in almost all cases.     

Transport phenomena at metal surfaces

Previous theoretical treatments of the scattering of conduction electrons at surfaces are reviewed and it is found that a more complete theory is necessary if the effects of surface scattering in transport phenomena are to be adequately understood. The boundary condition for the distribution function is derived and analysed in terms of the angle-dependent scattering probability. The skin effect, cyclotron resonance and the resistance of thin films and wires are considered in detail.     

Optical rectification at metal surfaces

The emission of freely propagating terahertz (THz) radiation coming from optical rectification at metallic surfaces has been detected and characterized for the first time to the authors' knowledge. The observed THz transients are induced through nonlinear electronic processes at gold and silver surfaces on intense pulsed optical photoexcitation and exhibit a peak electric field of as much as 200 V/cm. This finding opens a qualitatively new way to investigate nonlinear phenomena at metal surfaces and also can be exploited for the development of new THz emitters.     

Phonons and electrons at metal surfaces

Some aspects of the coupling between phonons and electrons and the interaction between electrons at metal surfaces are reviewed. Surface science techniques as diverse as electron energy loss spectroscopy, angle-resolved photoelectron spectroscopy, and scanning tunnelling microscopy are employed to study these interactions. Electron–phonon and electron–electron coupling are discussed in terms of renormalized phonon dispersion relations, and increased decay rates of electronic excitations. PACS 63.20.Kr; 68.35.Ja; 68.37.Ef; 68.43.Pq; 72.10.Fk; 72.15.Qm; 73.20.At     

Hydrogen vibrations at transition metal surfaces

Chemisorbed hydrogen has been studied on a variety of simple and composite transition metal surfaces using local density functional calculations. The goal of these studies is to elucidate trends and general principles in bond energies, atomic geometries, vibrational frequencies, and dynamic effective charges. The theoretical approach, based on the linear augmented plane wave method, will be reviewed. Results will be discussed for H in 2-, 3-, and 4-fold sites on W(001), Ru(0001), Cu(111), Pt(111), Rh(001), and the metal overlayer systems Cu-Ru(0001) and Ni-W(001). In most cases, all the harmonic vibrational modes were studied. Some anharmonic effects were examined. While good agreement has been obtained with much of the available experimental data, some outstanding disagreements will be discussed. Our results will be compared with the nearest-neighbor harmonic potential model and effective medium approach.     

Second harmonic generation at metal surfaces

The nonlinear response of a metal to optical fields is derived for a hydrodynamic model of electron dynamics. The results are used to analyse recent experiments on harmonic generation from a prism-silver film-α quartz sample.     

Lattice relaxation at alkali metal surfaces

The surface relaxation of the first interlayer spacing for both the (110) and (100) faces of alkali metals has been calculated selfconsistently using the density functional formalism and a new density matrix method which obviates the use of wavefunctions. For the (110) surfaces of all alkali metals we find no or only very small relaxation. The (100) surfaces show bigger relaxation effects and these vary from 3.5% inward for Li to 1% outward for Cs. These findings are consistent with general trends found in LEED or Ion scattering studies. Force constants for the movement of the last atomic layer are also determined.     

Identification of adsorbed species at metal surfaces by electron energy loss spectroscopy (EELS)

Electron energy loss spectroscopy (EELS) is a surface analysis method for measuring vibrational spectra of adsorbed species on metal surfaces. This paper summarizes recent work on the study of bonding of simple adsorbates on metal surfaces, and the identification of new chemical intermediates in reactions between two or more species in the adsorbed monolayer. The spectra of atomic oxygen, di-oxygen, water and ammonia adsorbed on platinum, copper and silver are discussed with emphasis on identification of the adsorbed species and their orientations relative to the surface plane. Surface reactions between atomic oxygen and water, methanol and formic acid yield the new surface intermediates hydroxyl (OH), methoxy (CH₃O) and formate (HCOO), respectively, on copper and silver surfaces. Each species was identified by comparison of surface spectra with known infrared spectra and through the use of deuterium isotopic shifts. The ability to identify and distinguish between chemical species at surfaces with high sensitivity will allow direct correlation of low pressure UHV surface experiments with high pressure surface reactions on catalysts and liquid-solid interfaces.     

Trapping and energy accomodation of atoms on metal surfaces

Expressions for the trapping and sticking probability and for the energy accomodation coefficient of atoms on metal surfaces are derived by using a Gaussian form for the distribution of the energy transfer. The results are in good agreement with recent numerical simulation calculations and experiments.     

Divacancy in 4H-SiC

Electron paramagnetic resonance and ab initio supercell calculations suggest that the P6/P7 centers, which were previously assigned to the photoexcited triplet states of the carbon vacancy-antisite pairs in the double positive charge state, are related to the triplet ground states of the neutral divacancy. The spin density is found to be located mainly on three nearest C neighbors of the silicon vacancy, whereas it is negligible on the nearest Si neighbors of the carbon vacancy.     

Binding energies for positrons at metal surfaces

The binding energies of surface states of positrons in the adiabatic approximation are estimated. The positron is allowed to penetrate the metal which is viewed as a dispersive plasma. Some other effects are also taken into account by introducing the positron work function as a phenomenological parameter. However, our results do not agree with experiment, which suggests that there are still effects unaccounted for by the theory.     

Laser generated ultrasound at modified metal surfaces

A study has been made of the ultrasonic pulses generated at modified metal surfaces by laser irradiation. Both longitudinal and shear acoustic modes have been investigated, generation at modified and free surfaces being compared. The use of a Q-switched Nd: YAG laser enabled experiments to be carried out at optical power densities above and below the threshold for plasma formation. Qualitative models of the acoustic sources existing in each case are proposed, which explain the experimental effects of surface modification; these include the observed changes in generation efficiency and longitudinal directivity.     

Dispersion of surface polaritons at metal surfaces

The dispersion relation for surface polaritons at a vacuum-metal interface has been obtained using a wave-vector-dependent electrical conductivity. Surface polariton attenuation coefficients calculated in the limits of local and extreme nonlocal conductivity vary with frequency ω as ω² and ω^{$\text{7}\text{3}$} respectively.     

Supramolecular architectures and nanostructures at metal surfaces

The controlled formation of non-covalent bonds (H-bonding, metal–ligand interactions) is the key ingredient for the fabrication of supramolecular architectures and nanostructures. Upon deposition of molecular building blocks at well-defined surfaces, this issue can be directly addressed. Scanning tunneling microscopy observations are presented, which provide insight into the interaction of functional groups on metal substrates at the molecular level. In particular, carboxylic acids were employed: (4-[(pyrid-4-yl-ethynyl)]-benzoic acid (PEBA), 4-[trans-2-(pyrid-4-yl-vinyl)]-benzoic acid (PVBA) and trimesic acid (1,3,5-benzenetricarboxylic acid, TMA), which could be stabilized in a flat geometry at the surface. By choosing the appropriate substrate material and symmetry, the sensitive balance of intermolecular and molecule–substrate interactions can be tuned to obtain well-defined supramolecular architectures and nanostructures. The head-to-tail hydrogen bonding of the related rod-like species PEBA and PVBA stabilizes molecular rows on Ag(111). The subtle difference in the molecular geometries is reflected in the lateral ordering: While 2-D islanding is encountered with PEBA, 1-D nanogratings of supramolecular chiral H-bonded twin chains evolve for PVBA. The threefold symmetry of TMA in conjunction with the self-complementarity of its exodentate groups accounts for the formation of H-bonded honeycomb networks on Cu(100) at low temperatures. Metal–ligand interactions were probed with PVBA and TMA at Cu surfaces at ambient temperature. Deprotonation of the carboxyl moiety takes place, which readily interacts with Cu adatoms evaporated from step edges. This leads to a head-to-head pairing of PVBA on Cu(111) and cloverleaf-shaped Cu–TMA coordination compounds on Cu(001). Received: 4 June 2002 / Accepted: 2 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +41-21/693-3604, E-mail: johannes.barth@epfl.ch     

Large negative Goos-Hanchen shift at metal surfaces

It has been previously established that for p-polarized light incident onto a semi-infinite absorbing medium, large negative Goos-Hanchen (GH) shifts can be expected in the case of weak absorption at incidence close to the Brewster angle. The effect has been demonstrated for certain semiconducting media at optical frequencies. Here we point out that similar phenomenon can take place for strongly reflecting and attenuating medium such as metal at IR frequencies, with large incident angles close to grazing incidence. Moreover, unlike the previously-studied case with semiconductors, the Brewster angle in the present case with metals plays an insignificant role in the possible hindrance of the observation of such large negative shifts.     

Intrinsic surface binding energy shifts in Yb metal

We report on the four-peak structure observed in the region of 4f photoemission in Yb metal, using synchrotron radiation in the energy range 70–200 eV. We conclude, contrary to previous reports, that the doublet associated with surface emissions results from an intrinsic surface shift on clean regions of the surface. We also demonstrate that the observed structure is consistent with earlier XPS measurements, and we set an upper limit on the width of the bulk peaks.