Adsorbate-covered Metal Surfaces Research Articles

Theoretical study of resonant charge exchange between H−ion and adsorbate-covered metal surface Cu(111)/Cu(110) with adsorbates Li+/Na+

The influence of the surface type on resonant charge exchange between H−ion with a metal surfaces Cu(111) and Cu(110) covered by adsorbate Li+/Na+has been studied. A model static problem was considered. For modeling, athree-dimensional realization of the wave-packet propagation method was used. The Cu(110) andCu(111) surfaces were described by a pseudopotential, derived from a density functional theory (DFT).An occupation of H− ion, an electron density dynamics and an ion level width were analyzed. As aresult, we obtained that an electron oscillates more in case of the surface Cu(111). Furthermore, the resonant charge exchange is more efficient for Cu(111) surface.

Attosecond time-resolved streaked photoemission from Mg-covered W(110) surfaces

We formulate a quantum-mechanical model for infrared-streaked photoelectron (PE) emission by ultrashort extreme ultraviolet (XUV) pulses from an adsorbate-covered metal surface, exposing the influence of microscopic PE dispersion in substrate and adsorbate on the interpretation of streaked photoemission spectra and photoemission time delays. We validate this numerical model first by reproducing measured relative photoemission delays (a) between valence-band and $2p$-core-level (CL) PEs emitted from clean Mg(0001) surfaces and (b) between conduction-band (CB) and $4f$-CL PEs from clean W(110) surfaces at two XUV-pulse central photon energies. Next, applying this model to ultrathin Mg adsorbate layers on W(110) substrates, we reproduce (i) the measured nonmonotonic dependence of relative photoemission delays between CB and $\mathrm{Mg}(2p$) PEs and (ii) the monotonic dependence of relative delays between $\mathrm{W}(4f$) and $\mathrm{Mg}(2p$) PEs in a recent experiment [S. Neppl et al., Nature (London) 517, 342 (2015)].

Quantum-mechanical simulation of attosecond streaked photoemission from Mg-covered W(110) surfaces

SynopsisWe apply a quantum-mechanical model to simulate infrared-streaked photoelectron emission by an ultrashort extreme ultraviolet pulse from adsorbate-covered metal surfaces. Incorporating effects of energy- dependent electron mean-free paths, the properties of initial states, photoelectron energy dispersion, and the screening of the streaking field, this model is able to reproduce recently measured photoelectron spectrograms and adsorbate-thickness-dependent photoemission time delays.

Jürgen Behm: Building Bridges from Surface Science to Catalysis and Fuel Cell Research

Jürgen Behm, to whom this special issue of ChemPhysChem is dedicated on the occasion of his 60th birthday, 1 started his career at one of the best places imaginable for a young surface scientist in the late seventies—the laboratory of 2007 Nobel Laureate Gerhard Ertl at the Ludwig Maximilians University in Munich. Jürgen Behm′s early research focused on the structure of H and CO adsorbate layers on transition metals and remains a classic work in surface science. However, early on he also developed a major interest in surface reactions, which he pursued during his post-doc sabbatical with Dick Brundle at the IBM Almaden Research Center. Although having worked successfully with a wide variety of techniques, he is arguably best known for scanning tunneling microscopy, notably building up one of the two first instruments of this type in Germany after returning to the Ertl group in 1984. After Jürgen Behm had accepted a professor position in Munich in 1987, the STM technique became his major work horse. In the following five years, a stream of high-impact publications on atomic-resolution studies of clean and adsorbate-covered metal surfaces as well as on chemical processes at semiconductor surfaces and electrochemical interfaces emerged. These publications have been cumulatively cited more than 5000 times by now. In 1992 Jürgen Behm was ready for the next big leap. Moving to the newly created chair of surface science and catalysis at the University Ulm, he set out to build a group that covered all aspects of heterogeneous reactions, from atomic-scale studies of the surface reactivity of planar model systems up to real supported catalysts. In particular, he focused on electrochemical and gas-phase reactions relevant for low-temperature fuel cells—a topic for which he is nowadays as well known as for STM. Throughout his career, Jürgen Behm’s work has been characterized by utmost scientific rigor as well as by a great willingness to cross boundaries between different approaches, such as fundamental studies and applied research, and different disciplines, such as UHV surface science and electrochemistry, resulting in more than 300 publications. The contributions in this issue, coming from the large international community of his former and current collaborators, reflect the breadth of his interests.

Laser-assisted photoemission from adsorbate-covered metal surfaces: Time-resolved core-hole relaxation dynamics from sideband profiles

Illumination of an adsorbate-covered metal surface with an xuv and a delayed ir laser pulse can result in sidebands in the photoelectron (PE) spectra. We present a theoretical model for the delay-dependent PE spectra and show how the relaxation dynamics of xuv-induced core-level holes in adsorbate atoms can be deduced from the temporal shift between sideband peaks in the spectra of secondary adsorbate (Auger) electrons and conduction-band PEs from the substrate. Furthermore, in comparison with gaseous targets, we find a characteristic sideband-intensity enhancement in the laser-assisted photoemission from the substrate core-level bands. This sideband enhancement effect can be tested in experiments with tunable xuv wavelength. Our calculated PE spectra support time-resolved experiments for Xe-covered Pt(111) surfaces, promoting the direct analysis in the time domain of surface dynamical processes.

Evolution of the surface science of catalysis from single crystals to metal nanoparticles under pressure

Vacuum studies of metal single crystal surfaces using electron and molecular beam scattering revealed that the surface atoms relocate when the surface is clean (reconstruction) and when it is covered by adsorbates (adsorbate-induced restructuring). It was also discovered that atomic steps and other low coordination surface sites are active for breaking chemical bonds (H-H, O=O, C-H, C=O, and C-C) with high reaction probability. Investigations at high reactant pressures using sum frequency generation-vibrational spectroscopy and high pressure scanning tunneling microscopy revealed bond breaking at low reaction probability sites on the adsorbate-covered metal surface and the need for adsorbate mobility for continued turnover. Since most catalysts (heterogeneous, enzyme, and homogeneous) are nanoparticles, colloid synthesis methods were developed to produce monodispersed metal nanoparticles in the 1-10 nm range and controlled shapes to use them as new model catalyst systems in two-dimensional monolayer film or deposited in mesoporous three-dimensional oxides. Studies of reaction selectivity in multipath reactions (hydrogenation of benzene, cyclohexene, and crotonaldehyde) showed that the reaction selectivity depends on both nanoparticle size and shape. The oxide-metal nanoparticle interface was found to be an important catalytic site that is associated with the hot electron flow induced by exothermic reactions such as carbon monoxide oxidation.

Dynamics of O 2 photodesorption from metal clusters: A significant difference from bulk behaviour

Photodesorption of O 2 from size-selected Ag n O 2 - cluster anions with n = 2, 3, 4 and 8 was studied using time-resolved photoelectron spectroscopy (TR-PES). The spectra indicate that relaxations of photo-excited Ag n O 2 - clusters with n = even numbers accompany ultrafast direct O 2 photodesorption. For the odd-numbered cluster Ag 3 O 2 - , in contrast, a long-lived excited state is observed, since O 2 might be dissociatively chemisorbed, suppressing direct photodesorption of oxygen. Both, direct desorption and long-lived excited states, have not been observed from adsorbate covered metal surfaces, suggesting unique photochemical properties of such small clusters.

High resolution core level photoemission of clean and adsorbate covered metal surfaces

A brief review is given of some fundamental aspects of high-resolution core-level photoemission and of its application to studies of pure and adsorbate covered metal surfaces. We first give a short description of the basic theory and then turn to a number of examples. The first group of examples is aimed at demonstrating the use of core-level binding energy shifts for deriving the surface and interface geometrical structure whereas the second group deals with the core-level lineshape and, in particular, the influence of vibrational and other excitations.

Remote projection and quantum mirages in STS and STM

The phenomena of remote projection and quantum mirages are investigated using standard quantum mechanics. The information inherent in delocalized wave functions in the vicinity of the Fermi level, including contributions from localized states, is available wherever the waves propagate coherently and have a non-vanishing amplitude and therefore can be probed remotely. This can explain the observation of a “quantum mirage” by Manoharan et al.: Nature 403, 512 (2000), i.e., the Kondo antiresonance due to a single adsorbed Co atom on Cu (111) far from the location of the cobalt atom. Similar quantum effects can give rise to “mirage” features in the scanning tunnelling spectrum (STS) both on clean and adsorbate-covered metal surfaces, features which are not resolved in other surface-sensitive spectroscopies (IPES, 2PPE). Within a theory based on a many-particle treatment of the tunnelling phenomena in STS and in scanning tunnelling microscopy (STM), the unexpected features in the scanning tunnelling spectra are associated with the spectral weight of transient ion-resonance states generated in the process of electron injection. They transport in a coherent way the information from the tip towards the sample and vice versa over distances of the order of 10 A or more, generating spectroscopic structures. These “mirage” states are important for the tunnelling current and the imaging properties.

Calculation of the field-emission current from a surface using the Bardeen transfer Hamiltonian method

We have developed a method for calculating the field-emission current from a clean or adsorbate-covered metal surface using the transfer Hamiltonian method of Bardeen. The present formalism can be incorporated in accurate atomistic electronic structure methods, and so is capable of addressing system specific band structure effects, adsorbate-induced resonances, and is amenable to accurate treatments of the exchange-correlation potential close to as well as far from the metal surface. It therefore goes beyond the conventional Fowler-Nordheim treatment of field emission from a metal surface. We illustrate the utility of our method by calculating the field-emission current from a model jellium surface, using a local-density approximation exchange-correlation potential, modified to include the correct $\ensuremath{\sim}1/4x$ asymptotic behavior in the vacuum region. We find that the Fowler-Nordheim behavior can be recovered in the limit of low fields; in the limit of high fields, where the details of the self-consistent effective potential in the neighborhood of the surface become important, meaningful deviations from the Fowler-Nordheim current result.

Reactions at surfaces studied by ab initio dynamics calculations

Owing to the development of efficient algorithms and the improvement of computer power it is now possible to map out potential energy surfaces (PESs) of reactions at surfaces in great detail. This achievement has been accompanied by an increased effort in the dynamical simulation of processes on surfaces. The paradigm for simple reactions at surfaces — the dissociation of hydrogen on metal surfaces — can now be treated fully quantum dynamically in the molecular degrees of freedom from first principles, i.e., without invoking any adjustable parameters. This relatively new field of ab initio dynamics simulations of reactions at surfaces will be reviewed. Mainly the dissociation of hydrogen on clean and adsorbate covered metal surfaces and on semiconductor surfaces will be discussed. In addition, the ab initio molecular dynamics treatment of reactions of hydrogen atoms with hydrogen-passivated semiconductor surfaces and recent achievements in the ab initio description of laser-induced desorption and further developments will be addressed.

Neutralization of a proton at adsorbate-covered metal surfaces

Charge exchange between a proton and adatoms on the metal substrates has been studied theoretically. The neutral fraction may increase or decrease, depending on the electronic environments of the adatom. The neutral yield of a proton depends significantly on the interaction between the adatom and the substrate metal. One remarkable aspect is the creative or destructive interference between two charge-exchange processes: one is the neutralization between the proton and the adatom, and the other is the neutralization between the proton and the substrate metal. Using the parameter values derived from molecular orbital calculations for cluster atoms, the remarkable interference effect is demonstrated.

Surface structure characterization by photoelectron holography

The applicability of photoelectron holography to determine the structure of clean and adsorbate-covered metal surfaces is investigated. The real space structure of various systems is reconstructed from photoelectron diffraction patterns at various kinetic energies using (1) the single wavenumber and (2) the multiple wavenumber reconstruction algorithm. It is demonstrated that adsorption sites can be unequivocally identified using the multiple wavenumber approach.

Reconstruction of Clean and Adsorbate-Covered Metal Surfaces.

Reconstruction of Clean and Adsorbate-Covered Metal Surfaces.

Quantum theory of infrared-reflection spectroscopy from adsorbate-covered metal surfaces in the anomalous-skin-effect frequency region.

A fully quantum-mechanical theory of infrared-reflection spectroscopy from adsorbate-covered metal surfaces is presented. In contrast to our earlier semiclassical theory [B. N. J. Persson and A. I. Volokitin, Surf. Sci. 310, 314 (1994)], which is valid only for parallel vibrational modes of atomic adsorbates, the present theory is applicable for arbitrary dipole-forbidden modes. It is shown that for a metal with a broad conduction band, the asymmetry of the line shape of dipole-forbidden adsorbate vibrational modes is determined essentially by nonlocal optics effects, with negligible contribution from nonadiabaticity. The theory is in good agreement with experimental data.

A simple theory of two-photon photoemission from clean and adsorbate covered metal surfaces

A simple theory is presented to study the elementary processes of two-photon photoemission (2PPE) from clean metals and adsorbates on metal surfaces. The model can be applied to metals having the occupied surface state and unoccupied image-potential state and also to adsorbates whose unoccupied states are populated from the occupied valence levels or the substrate by photoexcitation. The 2PPE spectrum is calculated on the basis of a simple energy diagram. It is demonstrated that one-photon photoemission from an initially unoccupied state (a fixed intermediate state which is populated by photon-driven electron excitation from an occupied metal state), is accompanied by two-photon photoemission from the occupied initial state via the unoccupied state as intermediate. When an electron is nonresonantly excited from narrow metal states such as d-bands or surface states, the lineshape of the corresponding 2PPE spectrum exhibits a characteristic asymmetry due to the underlying direct 2PPE from the occupied states. At resonant excitation from the narrow initial state to the intermediate state the 2PPE spectra exhibit a narrowing of the linewidth. It is shown that the lifetime of the unoccupied state cannot be deduced from the linewidth of the 2PPE spectra if a width of the initial state is comparable or narrower than the unoccupied state. No resonance enhancement and narrowing effect in the 2PPE spectra appear when a broad bulk continuum serves as an initial state of the 2PPE process.

Lifetime of image-potential states on metal surfaces

High-resolution bichromatic two-photon photoemission spectroscopy was used for an investigation of intrinsic linewidths and, thus, lifetimes of image-potential states on clean and adsorbate-covered metal surfaces. The linewidths of the first image state are 16\ifmmode\pm\else\textpm\fi{}4 and 70\ifmmode\pm\else\textpm\fi{}8 meV for Cu(111) and Ni(100), respectively. Together with the results for Ag(100) and Ni(111) published previously, a systematic comparison with theoretical predictions is presented. The discrepancies between experiment and theory can be understood qualitatively if the actual band structure is taken into account. A high density of unoccupied bulk or surface states offers many channels for the decay of the image state via electron-hole pair production and results in the large linewidth observed on the nickel surfaces. The opposite is found for Cu(111) where the band gap extends below ${\mathit{E}}_{\mathit{F}}$. For oxygen adsorption on Ni(100), an increase of the linewidth is observed, which can be explained by the lateral confinement of the image state and inelastic scattering by the adsorbate atoms.

Direct evidence for charge-transfer photodissociation at a metal surface: CCl_{4}/Ag(111)

Observation of negative ions desorbing from an adsorbate-covered metal surface following UV laser irradiation gives direct evidence that the mechanism responsible is charge-transfer photodissociation (CT-PDIS). We have studied photoemission of ions for ${\mathrm{CCl}}_{4}$/Ag(111)+h\ensuremath{\nu}\ensuremath{\rightarrow}Cl-(g)+${\mathrm{CCl}}_{3}$/Ag(111). The CT-PDIS mechanism is supported by comparison of the yield of desorbing ${\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ and photoemitted electrons with the variation in UV photon energy and surface coverage. Charge transfer leading to negative-ion emission is also shown to occur in the absence of photoelectron emission (i.e., h\ensuremath{\nu}) for ${\mathrm{CCl}}_{4}$/Ag(111).

Scanning tunneling microscopy on clean and adsorbate covered metal surfaces

The use of scanning tunneling microscopy (STM) for atomic scale characterization of clean and adsorbate covered (single-crystalline) metal surfaces is discussed. Topographic images reveal details on their periodic structure and on the atomic arrangement in the surface layer, and in particular on surface defects. The observation and characterization of individual adsorbate species gives access to the local electronic structure of the adsorption complex and to details of the chemical bond between substrate and adsorbate. Atomic resolution imaging opens new perspectives for the investigation of various surface processes such as surface diffusion, thin film growth or surface reactions.

Surface Phonons on Clean and Adsorbate Covered Metal Surfaces

Recent progress in surface phonon spectroscopy using inelastic electron scattering is reviewed. For a number of adsorbate systems the dispersion curves of the adsorbate modes have been measured. It is demonstrated that important structure parameters can be deduced from the local symmetry of the adsorbate revealed by the absence of a disperison branch under particular scattering conditions. Among the adsorbates hydrogen is a particularly interesting case, since there the classical picture of vibrational motion must be replaced by a quantum mechanical description. For some adsorbate covered or metastable clean surfaces the dispersion of the Rayleigh wave indicates a compressive or tensile surface stress. Finally the significance of an experimental determination of the dispersion of surface phonons with odd symmetry will be pointed out.