Real-time imaging of K atoms on graphite: interactions and diffusion

Scanning tunneling microscopy (STM) at liquid helium temperature is used to image potassium adsorbed on graphite at low coverage (≈0.02 monolayer). Single atoms appear as protrusions on STM topographs. A statistical analysis of the position of the atoms demonstrates repulsion between adsorbates, which is quantified by comparison with molecular dynamics simulations. This gives access to the dipole moment of a single adsorbate, found to be 10.5±1 D. Time-lapse imaging shows that long-range order is broken by thermally activated diffusion, with a 30 meV barrier to hopping between graphite lattice sites.     

Effect of various adsorbates in dephasing and population decay of the Cu(1 0 0) image potential states

A theoretical study of the electron dynamics in image potential states on Cu(1 0 0) surfaces with different types of adsorbates is presented. Scattering of the image state electron by an adsorbate induces inter-band and intra-band transitions leading respectively to the population decay and to the dephasing of the image state. We compare results obtained with low coverage (typically 1 adsorbate atom per 1000 surface atoms) Cs, Ar, and a model electronegative adsorbates. As follows from our results, Cs adsorbates lead to both appreciable dephasing and decay, while electronegative adsorbates mostly affect the dephasing rate. The effect of low coverage Ar adsorbates is small, consistent with their neutrality.     

Atoms interacting with a metal surface with adsorbates: local and non-local effects on the charge transfer

A model system representing a collisional Li atom interacting in the “on top” geometry with an Al surface partly covered with Li adsorbates is studied. The cases of a unique adsorbate and of a uniform adsorbate layer are considered and compared. The energies and widths of the atom levels are much modified in the vicinity of the adsorbate. This is interpreted in terms of molecularisation of the atomic and adsorbate levels. These results also determine the relative importance of the local and non-local effects of the adsorbates on the resonant charge transfer process in atom-surface collisions.     

Stark effect of adsorbate vibrations

The theory of the vibrational Stark effect of adsorbates is discussed. In particular, the reported linear variation of CO vibrational frequency with potential difference across the Pt-electrolyte interface is shown to be consistent with a recent measurement of the Stark tuning rate of CO on Ni [110] in UHV and the differential capacitance of the Pt-electrolyte interface. We also demonstrate that the Stark tuning rate of CO on Ni [110] is itself in agreement with theoretical prediction. Our vibrational Hamiltonian applies to atomic adsorbates and molecules. It relates frequency shifts of the adsorbate to the local electric field at an adsorbate free surface.     

Two-electron bond approach to chemisorption

A new approach to chemisorption is presented which considers both neutral and ionic configurations of the adsorbate on the same footing. The two-electron bond between the adsorbate and an atom of the metal surface is characterised by the Heitler-London method, the metal band structure being described by the cluster-Bethe lattice method. The only parameter of this theory is the hopping integral of the Anderson-Newns Hamiltonian. Quantities such as charge populations, one electron levels, and excitation energies which can be immediately compared with the spectroscopic data (ELS, UPS, ESD, etc.) can be easily deduced. An application is given to the H/W(100) system.     

Scanning tunneling microscopy and spectroscopy of individual C60 molecules on Si(100)-(2 × 1) surfaces

Individual C₆₀ molecules chemisorbed on Si(100)-(2 × 1) surfaces have been studied by scanning tunneling microscopy and spectroscopy. Chemisorption was realized by annealing the samples with room-temperature deposited adsorbates to 600°C. Spectroscopic results on individual adsorbates reveal a transition of their electronic structure from that of a near-free adsorbate to that of SiC, as the adsorbate-substrate interaction increases.     

Momentum-resolved inverse photoemission

Inverse photoemission including isochromat spectroscopy is shown to be a versatile technique to probe empty electronic states in solids and at clean and adsorbate covered surfaces. The complete set of quantum numbers of an electronic state can be determined and examples will be discussed for bulk and surface electronic states. For sufficiently low kinetic energy of the primary electrons, inverse photoemission is shown to be applicable to adsorbates also. This allows one to assess directly the unoccupied electronic states of the adsorbate which play an important role in the formation of the surface chemical bond. Examples are discussed for atomic and molecular chemisorption as well as adsorption on alkali promoted surfaces.     

Imaging and tunneling spectroscopy of individual iron adsorbates at room temperature

We have imaged for the first time individual atoms and small clusters of metal species on a metal substrate at room temperature by means of a scanning tunneling microscope (STM) operated under ultrahigh vacuum conditions. The system we have studied is Fe on W(110), for which a carbon-induced (15×3)-reconstruction of the W(110) substrate has been found to prevent surface diffusion of Fe atoms at 300 K. Upon positioning the STM-tip above individual Fe adsorbates, local tunneling spectra could be obtained. A pronounced empty-state peak at 0.5 eV above the Fermi level has been found, characteristic for individual Fe adsorbates. This peak can serve as a fingerprint for the identification of Fe adsorbate species.     

Imaging and tunneling spectroscopy of individual iron adsorbates at room temperature

We have imaged for the first time individual atoms and small clusters of metal species on a metal substrate at room temperature by means of a scanning tunneling microscope (STM) operated under ultrahigh vacuum conditions. The system we have studied is Fe on W(110), for which a carbon-induced (15×3)-reconstruction of the W(110) substrate has been found to prevent surface diffusion of Fe atoms at 300 K. Upon positioning the STM-tip above individual Fe adsorbates, local tunneling spectra could be obtained. A pronounced empty-state peak at 0.5 eV above the Fermi level has been found, characteristic for individual Fe adsorbates. This peak can serve as a fingerprint for the identification of Fe adsorbate species.     

Quantum theory of electron stimulated desorption

The process of electron stimulated desorption of adsorbates from metal surfaces is investigated within the framework of quantum mechanical scattering theory. The Born-Oppenheimer adiabatic approximation is assumed to be valid for the adsorbate motion. The transition amplitude for desorption via the resonant excitation of excited states of the adsorbate then can be factorized into an electronic excitation amplitude and a Franck-Condon factor. The Franck-Condon factor is more complicated than in molecules. The continuum of substrate excitations coupling to the adsorbate gives rise to an absorptive part of the Born-Oppenheimer potential governing the motion of the adsorbate in the excited state. This absorptive part leads to a considerable reduction of the desorption cross section. Explicit quantum mechanical expressions for the corresponding reduction factor are given.The desorption of neutrals is considered in some detail. It turns out that within the adiabatic approximation this process requires the existence of neutral excited states of the adsorbate. The reneutralization of ionic excited states by electron capture from the substrate back into the ground state of the adsorbate, while possible on purely energetical grounds, occurs with zero probability in the adiabatic approximation and thus cannot be responsible for the large abundance of neutral desorbing particles. Neutral excited states of the adsorbate in principle should show up in inelastic electron scattering. The relation between electron stimulated desorption cross sections and inelastic electron scattering cross sections is discussed briefly.On leave of absence from (present address) Physikdepartment der TUM, München-Garching and Max-Planck-Institut für Festkörperforschung, Stuttgart     

STM simulation of molecules on ultrathin insulating overlayers using tight-binding: Au-pentacene on NaCl bilayer on Cu

We present fast and efficient tight-binding (TB) methods for simulating scanning tunneling microscopy (STM) imaging of adsorbate molecules on ultrathin insulating films. Due to the electronic decoupling of the molecule from the metal surface caused by the presence of the insulating overlayer, STM can be used to image the frontier molecular orbitals of the adsorbate. These images can be simulated with a very efficient scheme based on hopping integrals which also enables the analysis of phase shifts in the STM current. Au-pentacene complex adsorbed on a NaCl bilayer on Cu substrate provides an intricate model system which has been previously studied both experimentally and theoretically. Our calculations indicate that the complicated shape of the molecular orbitals may cause multivalued constant current surfaces - leading to ambiguity of the STM image. The results obtained using the TB methods are found to be consistent with both DFT calculations and experimental data.     

Quantum theory of electron stimulated desorption

The process of electron stimulated desorption of adsorbates from metal surfaces is investigated within the framework of quantum mechanical scattering theory. The Born-Oppenheimer adiabatic approximation is assumed to be valid for the adsorbate motion. The transition amplitude for desorption via the resonant excitation of excited states of the adsorbate then can be factorized into an electronic excitation amplitude and a Franck-Condon factor. The Franck-Condon factor is more complicated than in molecules. The continuum of substrate excitations coupling to the adsorbate gives rise to an absorptive part of the Born-Oppenheimer potential governing the motion of the adsorbate in the excited state. This absorptive part leads to a considerable reduction of the desorption cross section. Explicit quantum mechanical expressions for the corresponding reduction factor are given.The desorption of neutrals is considered in some detail. It turns out that within the adiabatic approximation this process requires the existence of neutral excited states of the adsorbate. The reneutralization of ionic excited states by electron capture from the substrate back into the ground state of the adsorbate, while possible on purely energetical grounds, occurs with zero probability in the adiabatic approximation and thus cannot be responsible for the large abundance of neutral desorbing particles. Neutral excited states of the adsorbate in principle should show up in inelastic electron scattering. The relation between electron stimulated desorption cross sections and inelastic electron scattering cross sections is discussed briefly.     

Calibration in Auger electron spectroscopy by means of coadsorption

The combined adsorption of two elements on a single crystal substrate can be readily examined by LEED and AES. By careful monitoring of the AES signals during adsorption it is possible to detect the formation of a complete, binary, monolayer. The method requires that the second absorbate (typically, an easily deposited metal) forms first on the uncovered substrate and then on top of the first adsorbate by a layer-by-layer growth mechanism. By obtaining binary monolayers in different compositions the spectrometer may be calibrated for both adsorbates. The basis of the method and its application to sulphur and oxygen on copper are described.     

SERS microRaman spectral probing of adsorbate‐containing,liquid‐overlayed nanosponge Ag aggregates assembled from fractal aggregates

Adsorbate‐containing, nanosponge Ag aggregates overlayed by a thin (~1.5 mm) liquid layer are reported as a new type of sample for Surface‐enhanced Raman scattering (SERS) microRaman spectral measurements and adsorbate (analyte) detection. Macroscopic Ag aggregates (of about 1.5 × 1.0 × 0.025 mm size) with the nanosponge internal morphology (revealed by Scanning electron microscopy (SEM)) were prepared by 3D assembling of fused fractal aggregates (D = 1.84 ± 0.04) formed in Ag nanoparticle hydrosol/HCl/adsorbate systems with 2,2’‐bipyridine (bpy) and/or a cationic free‐base tetrakis(2‐methyl‐4‐pyridiniumyl) porphine (H₂TMPyP) as the testing adsorbates. For SERS microRaman measurements, the macroscopic aggregate was overlayed by a thin (~1.5 mm) layer of the residual liquid. Preparation procedure, nanoscale imaging, and SERS spectral probing including the determination of the detection limits of the adsorbates revealed the following advantages of the adsorbate‐containing, liquid‐overlayed 3D nanosponge aggregate as a sample for SERS microRaman spectral measurements: (1) localization of adsorbate (analyte) into hot spots and, simultaneously, prevention of the analyte decomposition during the spectral measurement (carried out without an immersion objective), (2) fast and simple sample preparation, and (3) minimization of sample volume and an efficient concentration of hot spots into the focus of the laser beam. The advantages of the nanosponge Ag aggregates are further demonstrated by the 40 fmol limit of detection of bpy as Ag(0)‐bpy surface complex, as well as by preservation of the native structure of the cationic free‐base porphyrin H₂TMPyP. Copyright © 2015 John Wiley & Sons, Ltd.     

Stochastic resonance in a realistic model for surface adsorption

We study a model for a monolayer single adsorbate system used to describe pattern formation on adsorbates with lateral interactions, when it is submitted to pressure oscillations. Through numerical and analytical (based on a two-state approximation) methods to analyze the existence of stochastic resonance in such a bistable system. This is a first step toward the study of resonant phenomena in adsorbate systems with moving fronts and/or with presence of micro-reactors or spots.     

Young-type interference for scattering of fast helium atoms from an oxygen covered Mo(112) surface

In studies on two structures of oxygen adsorbates on Mo(112), we demonstrate the potential of fast atom diffraction to derive the surface unit cell size and its symmetry. Helium atoms with energies of 1-2 keV are scattered from an adsorbate covered Mo(112) surface along low indexed surface directions under grazing angles of incidence. From the observed diffraction patterns, the lateral periodicity of the surface structures is derived. In addition to the periodic lattice, information on the structure within the unit cell can be obtained from double slit type of interference.     

Vibrational states of the Cu(100) surfaces with nickel adlayers

The vibrational spectra of the Cu(100) surface covered with one or two Ni monolayers are calculated using the embedded-atom method. The surface relaxation, the dispersion of surface phonons, and the polarization of vibrational modes of the adsorbate and the substrate are discussed in detail. The theoretical results are in good agreement with experimental data and can be used for their interpretation. The changes observed in the interatomic interactions upon application of nickel adsorbates to the copper substrate are considered.     

The effect of chain reactions on the diffusion of absorbates

A chain reaction mechanism is developed that can dramatically increase the predicted diffusion coefficient as a function of coverage for thermally activated submonolayer coverages of adsorbates weakly chemisorbed on surfaces. The key idea is that when a diffusing adatom encounters another adatom it will tend to forward scatter it (due to the channeling effect of the periodic potential of the substrate lattice) thus increasing the effective mean free path. The enhancement effect can be quite dramatic at large adsorbate coverages. The formalism is essentially that of a Markoffian random walk on a lattice which allows for “billiard ball” type collisions. A general formalism for calculating the chemical diffusion tensor for an unspecified lattice is developed. The diffusion tensor is then related to the effective diffusion coefficient measured by the field emission fluctuation method. The special case of the two-dimensional rhombic lattice with two adsorbate species is worked out and shown to agree qualitatively with theoretical predictions of the coverage dependence of the diffusion coefficient with the experimental data for hydrogen and deuterium on W(110)[1].