Hydrogen adsorption on metal surfaces

Extensive calculations of the ground state properties of hydrogen chemisorbed on transition metal surfaces are presented. The calculations are performed using the effective medium theory. The results for the chemisorption energies on all the 3d, 4d and 5d metals presented are in good agreement with experiment. The trends along a particular row are shown to be dominated by the degree of filling of the d band. The full adiabatic potential energy surface is presented for a number of experimentally interesting systems, including H/Ni(111), H/Ni(110), H/W(100) and H/W(110). Equilibrium sites, bond lengths, vibrational frequencies and surface diffusion energies are deduced and compared with experiment. Again, agreement is good. The surface and adsorbate parameters determining those observables are discussed. It is shown that a simple canonical relationship exists between the perpendicular vibrational frequency and the metal-hydrogen bond length. This formulation, which is not based on pair potentials, should be useful as a first estimate of bond lengths from measured vibrational data.     

Hydrogen chemisorption on metal surfaces

The adsorption of H atoms on metal (jellium) surfaces has been investigated using linear response theory within the density functional formalism. The adsorbate is represented initially by a 1S orbital on the H atom, which perturbs the jellium surface and indirectly the adsorbate itself. The interaction energy curves, atomic binding energies, induced dipole moments, chemical shifts associated with the adsorbate, and vibrational excitation energies at the equilibrium internuclear separation have been calculated for a single H atom chemisorbed on metal surfaces. The sum of the atomic binding energy and the ionization potential of the H 1S level may be regarded as the initial state energy in the case of photoemission from the chemisorbed H. The rather satisfactory overall agreement between the theory and the experimental results for binding energies, vibrational excitation energies, and dipole moments suggests that this simple formalism could also have useful applications in more complicated chemisorbed systems.     

Enhanced Raman scattering on metal surfaces

We present a theory which describes enhanced Raman scattering from molecules adsorbed on metals. The enhancement is due to the screening fields induced by the optically polarized adsorbate in the presence of the substrate. These fields interact coherently with the applied optical field and the vibrational motion of the adsorbed molecule. The enhancement is shown to be coverage dependent and also rather sensitive to the optical dielectric function of the substrate. Predicted enhancements compare well with experiment.     

Adsorption of halogens on metal surfaces

This paper presents a review of the experimental and theoretical investigations of halogen interaction with metal surfaces. The emphasis was placed on the recent measurements performed with a scanning tunneling microscope in combination with density functional theory calculations. The surface structures formed on metal surface after halogen interaction are classified into three groups: chemisorbed monolayer, surface halide, bulk-like halide. Formation of monolayer structures is described in terms of surface phase transitions. Surface halide phases are considered to be intermediates between chemisorbed halogen and bulk halide. The modern theoretical approaches in studying the dynamics of metal halogenation reactions are also presented.     

Quantization of electronic states on metal surfaces

An electron in front of a metal surface experiences an attractive force due to the induced image charge. Band gaps in the band structure can prevent a penetration into the metal along certain directions. The Coulomb-like potential supports bound states in front of the surface which correspond to a hydrogen atom in one dimension. These image states can be measured with high resolution by two-photon photoemission. The adsorption of metals modifies the states. If the electrons can penetrate into the metal, quantum-well states can develop corresponding to standing waves in the overlayer. Image states on small islands show the quantization effects due to the lateral localization. The spectroscopy of image states by two-photon photoemission permits the investigation of growth and morphology of deposited metal layers, a well as the illustration of fundamental quantum-mechanical effects.     

Hydrogen on metal surfaces: Forever young

This paper provides a perspective on the article "Hydrogen adsorption, absorption and diffusion on and in transition metal surfaces: A DFT study" by Ferrin et al. in this issue.     

Soft fermi surfaces and breakdown of Fermi-liquid behavior

Electron-electron interactions can induce Fermi surface deformations which break the point-group symmetry of the lattice structure of the system. In the vicinity of such a "Pomeranchuk instability" the Fermi surface is easily deformed by anisotropic perturbations, and exhibits enhanced collective fluctuations. We show that critical Fermi surface fluctuations near a d-wave Pomeranchuk instability in two dimensions lead to large anisotropic decay rates for single-particle excitations, which destroy Fermi-liquid behavior over the whole surface except at the Brillouin zone diagonal.     

Alkali-metal-affected adsorption of molecules on metal surfaces

The study of coadsorption of alkali metals and simple molecules on transition metal surfaces has been a favored topic of research ever since the pioneering work by Langmuir in 1923. The main reasons for this continued interest are both of fundamental and applied nature. There are a number of interesting physical effects, such as work function changes, charge transfer, two-dimensional ordering, bond energy and molecule orientational changes, and altered surface reactive properties, which have been investigated by a large number of different surface analytical techniques. From an applied point of view, alkali metal covered surfaces are important in the areas of electron and ion emission and heterogenous catalysis, for example.In this paper we will give a short review of the adsorption of alkali metals on well-defined transition metal surfaces. The interaction of these adsorbed alkali metals with subsequently adsorbed atomic or molecular species will be treated more extensively. The emphasis will be on recent experiments dealing with well-characterized surfaces. In particular we will consider questions of adsorption energetics and kinetics, but also review in detail the vibrational, electronic, structural and reactive properties of the coadsorbed complex. Based on a wealth of experimental data, several models of the coadsorbed alkali metal-molecule complex will be introduced and discussed.     

Hydrogen dissociation on metal surfaces – a model system for reactions on surfaces

Received: 23 March 1998/Accepted: 21 August 1998     

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.     

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.     

Relaxation near transition metal surfaces

A simple calculation of the relaxation near transition metal surfaces is described. The d electrons are treated in a tight-binding approximation and Born-Mayer type potentials are used to simulate the repulsive part of the energy at short distances. The interplanar spacing is found to decrease at the center of the series and to increase on the wings. For almost empty or filled bands one obtains no relaxation.