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Since the concept of a surface collective excitation was first introduced by Ritchie, surface plasmons have played a significant role in a variety of areas of fundamental and applied research, from surface dynamics to surface-plasmon microscopy, surface-plasmon resonance technology, and a wide range of photonic applications. Here we review the basic concepts underlying the existence of surface plasmons in metallic structures, and introduce a new low-energy surface collective excitation that has been recently predicted to exist.

Energy loss per unit path in grazing collisions with metal surfaces is studied by using the collisional and dielectric formalisms. Within both theories we make use of the band-structure-based (BSB) model to represent the surface interaction. The BSB approach is based on a model potential and provides a precise description of the one-electron states and the surface-induced potential. The method is applied to evaluate the energy lost by 100 keV protons impinging on aluminum surfaces at glancing angles. We found that when the realistic BSB description of the surface is used, the energy loss obtained from the collisional formalism agrees with the dielectric one, which includes not only binary but also plasmon excitations. The distance-dependent stopping power derived from the BSB model is in good agreement with available experimental data. We have also investigated the influence of the surface band structure in collisions with the Al(100) surface. Surface-state contributions to the energy loss and electron emission probability are analyzed.

We have performed self-consistent calculations of the dynamical response of the (111) surface of the noble metals Cu, Ag, and Au. Our results indicate that the partially occupied surface-state band in these materials yields the existence of acoustic surface plasmons with linear dispersion at small wave vectors. Here we demonstrate that the sound velocity of these low-energy collective excitations, which had already been predicted to exist in the case of Be(0001), is dictated not only by the Fermi velocity of the two-dimensional surface-state band but also by the nature of the decay and penetration of the surface-state orbitals into the solid. Our linewidth calculations indicate that acoustic surface plasmons should be well defined in the energy range from zero to 400 ~ meV.

Recently, a low-energy collective excitation has been predicted to exist at metal surfaces where a quasi two-dimensional (2D) surface-state band coexists with the underlying three-dimensional (3D) continuum. Here we present a model in which the screening of a semiinfinite 3D metal is incorporated into the description of electronic excitations in a 2D electron gas through the introduction of an effective 2D dielectric function. Our self-consistent calculations of the dynamical response of the 3D substrate indicate that an acoustic surface plasmon exists for all possible locations of the 2D sheet relative to the metal surface. This low-energy excitation, which exhibits linear dispersion at low wave vectors, is dictated by the nonlocality of the 3D dynamical response providing incomplete screening of the 2D electron-density oscillations.

A novel collective excitation is predicted to exist at metal surfaces where a two-dimensional surface state band coexists with the underlying three-dimensional continuum. This is a low-energy acoustic plasmon with linear dispersion at small wave vectors. Since new modern spectroscopies are especially sensitive to surface dynamics near the Fermi level, the existence of surface-state–induced acoustic plasmons is expected to play a key role in a large variety of new phenomena and to create situations with potentially new physics.

A first-principles parameter-free calculation that includes full three-dimensional band structure and dynamical exchange correlations is reported for the dynamical surface response and surface plasmon (SP) on a simple metal prototype surface Mg(0001). We demonstrate that band structure effects have a more profound impact on the SP characteristics than dynamical exchange correlations. A comparison with jellium and one-dimensional potential evaluations shows that the band structure is of paramount importance for the correct description of the SP linewidth and also leads to a better description of the SP energy dispersion. The inclusion of the exchange-correlation kernel results in a better agreement with experimental data. We show that lateral crystal local field effects have a negligible impact on the SP properties. Significant anisotropy is predicted for the SP linewidth.

Surface chemical reactions are influenced by inherent dissipative processes which involve energy transfer between the conduction electrons and the ionic motion. We will discuss how it is possible to model this electron–phonon coupling in order to estimate its importance. A relevant quantity for this investigation is the lifetime of surface localized electron states. A surface state, quantum-well state or surface image state is located in a surface projected band gap and becomes relatively sharp in energy. This makes a comparison between calculations and experimental data most attractive, with a possibility to resolve the origin of the lifetime broadening. We point out the importance of taking into account the phonon spectrum, electron surface state wave functions and the screening of the electron–ion potential.

The Be electron inelastic mean free path (IMFP) is calculated by using the GW approximation of many-body theory. It is demonstrated that the inclusion of band structure effects significantly improves the agreement between the calculated and measured IMFP in the energy range up to ∼30eV. We show that the main effect results from the electron group velocity obtained from ab initio band structure calculations, whereas the evaluated linewidth averaged over momenta is not significantly affected with respect to the prediction of a free electron gas model. The comparison of the IMFP computed within two models, namely, the free electron gas model and the full band structure model, supports the idea of the importance of band structure effects for the description of electron transport in this metal for energies below 30 eV, and a nearly free-electron-like behavior for excitation energies above 30 eV. The calculated plasmon dispersion is in excellent agreement with available experimental data.

We study band-structure effects on the energy loss of protons scattered off the Cu (111) surface. The distance dependent stopping power for a projectile traveling parallel to the surface is calculated within the linear response theory. The self-consistent electronic response of the system is evaluated within the random-phase approximation. In order to characterize the surface band structure, the electronic single-particle wave functions and energies are obtained by solving the Schrödinger equation with a realistic one-dimensional model potential. This potential reproduces the main features of the Cu (111) surface: the energy band gap for electron motion along the surface normal, as well as the binding energy of the occupied surface state and the first image state. Comparison of our results with those obtained within the jellium model allows us to characterize the band-structure effects in the energy loss of protons interacting with the Cu (111) surface.

Ab initio calculation of the density response function within the random-phase approximation is presented for lithium, berillium, and boron freestanding metal monolayers (ML’s). Although the monolayers manifest common features related to their reduced dimensionality, their different band structures lead to significant modifications in the density response function. Besides the common intraband and interband collective excitations, the beryllium ML also presents characteristic features of acoustic plasmons associated with the presence of two types of carriers at the Fermi level. Contrary to the bulk, long-wavelength intraband plasmons in lithium and beryllium ML’s cannot decay via umklapp processes associated with the band structure. At the same time, these processes lead to a significant reduction of the momentum threshold for decay of these plasmons comparing with a free-electron two-dimensional gas model. It is also shown how a change of the symmetry of the boron ML modifies completely the character of the intraband plasmons.

We present a comprehensive theoretical investigation of the electron-phonon contribution to the lifetime broadening of the surface states on Cu(111) and Ag(111), in comparison with high-resolution photoemission results. The calculations, including electron and phonon states of the bulk and the surface, resolve the relative importance of the Rayleigh mode, being dominant for the lifetime at small hole binding energies. Including the electron-electron interaction, the theoretical results are in excellent agreement with the measured binding energy and temperature dependent lifetime broadening.

We present self-consistent pseudopotential calculations of surface and image-potential states on MgB2(0001) for both B-terminated (B−t) and Mg-terminated (Mg−t) surfaces. We find a variety of very clear surface and subsurface states as well as resonance image-potential states n=1,2 on both surfaces. The surface layer density of states (DOS) at EF is increased by 55% at the B−t and by 90% at the Mg−t surface compared to DOS in the corresponding bulk layers.

We have investigated the lifetimes of image-potential states on the Pt(111) surface with time-resolved two-photon photoemission and first-principles calculations. Electrons populating the first two image-potential states decay into bulk states after 26±7 and 62±7 fs, respectively. This is in agreement with results of theoretical calculations. Oxygen adsorption reduces the image-potential state lifetimes by a factor of 2. This is caused by a strong change of the electronic structure near the Fermi level.

First-principles calculations' of the inelastic lifetime of low-energy electrons in Al, Mg, Be, and Cu are reported. Quasiparticle damping rates are evaluated from the: knowledge of the electron self-energy, which we compute within the GW approximation of many-body theory. Inelastic lifetimes are then obtained along various directions of the electron wave vector, with full inclusion of the band structure of the solid. Average lifetimes are also reported, as a function of the electron energy. In Al and Mg, splitting of the band structure over the Fermi level yields electron lifetimes that are smaller than those of electrons in a free-electron gas. Larger lifetimes are found in Be, as a result of the characteristic dip that this material presents in the density of states near the Fermi level. In Cu, a major contribution from d electrons participating in the screening of electron-electron interactions yields electron Lifetimes that are well above those of electrons in a free-electron gas with the electron density equal to that of valence (4s(1)) electrons.

A metal surface generates electron states that do not exist in a bulk metal. These so-called surface states can be classified into two categories, crystal-induced (intrinsic) states and image-potential-induced (image) states. Here we present a survey of current investigations on the origin of the decay of these surface states. Inelastic lifetimes are obtained from the knowledge of the quasiparticle self-energy, which we compute, within the GW approximation of many-body theory, by going beyond a free-electron description of the surface. Surface-state lifetimes in noble metal surfaces are presented. Our results show that actual lifetimes are highly sensitive to the details of the surface response and to the presence of the intrinsic surface state itself.

Using time-resolved two-photon photoemission we have measured the linewidths and lifetimes of the unoccupied Shockley surface state and first image-potential state on Pd(111). Calculated values are in good agreement with the experimental results.

First-principles calculations of the inelastic lifetime of low-energy electrons in Al, Mg, Be, and Cu are reported. Quasiparticle damping rates are evaluated from the knowledge of the electron self-energy, which we compute within the GW approximation of many-body theory. Inelastic lifetimes are then obtained along various directions of the electron wave vector, with full inclusion of the band structure of the solid. Average lifetimes are also reported, as a function of the electron energy. In Al and Mg, splitting of the band structure over the Fermi level yields electron lifetimes that are smaller than those of electrons in a free-electron gas. Larger lifetimes are found in Be, as a result of the characteristic dip that this material presents in the density of states near the Fermi level. In Cu, a major contribution from d electrons participating in the screening of electron-electron interactions yields electron lifetimes that are well above those of electrons in a free-electron gas with the electron density equal to that of valence (4s1) electrons.