Presentation d'une methode permettant d'obtenir pour les couches minces une fonction dielectrique simplifiee incluant des effets dimensionnels quantiques pour les excitations de paires electron-trou. Par une generalisation convenable au cas des spheres, on analyse la photo-absorption de ces systemes

A phenomenological theory is proposed to study the electrodynamics of the interface between two semi-infinite electron gases including the effects of charge transfer and of non specularity in reflection at or transmission across the interface. These effects are described in terms of phenomenological parameters having the nature of statistical fractions and vary between 0 and 1.

The applicability of the hydrodynamical model, obtained from the high-frequency expansion of the density-density response function, for calculating the surface plasmon dispersion relation and plasmons of higher polarity is analyzed. Of crucial importance to get any result for the dispersion coefficients is that the unperturbed electron density goes to zero at some point. The coefficients obtained for different diffuse electron profiles can be highly unphysical due to "local" plasmons in the tail of the electron gas. This is also the case for the multipolar surface plasmons. A simple theory which includes damping of the local plasmon is found to give very good agreement for the damping of the surface plasmon when compared to more extensive calculations.

The normal modes of a thin free electron like metal film are calculated within a specular model. When the film is described by different dielectric functions the normal modes dispersion relations change. In particular the effects of single-particle excitations are studied by using a dielectric function calculated for a thin film, i.e., a dielectric function calculated for a system having a quasi-discrete excitation spectrum. Single-particle effects are seen both in the dispersion relations and in the surface loss function. The latter clearly exhibits the Landau damping of the surface plasmons.

The photoelectric yield spectrum, proportional to the absorptance, of thin metal films is calculated. The film is described by either a dielectric function having a discrete single-particle excitation spectrum or, for comparison, a continuous spectrum (Lindhard). A numerical example of a 15 Å thick potassium film on a substrate is given to illustrate the changes in absorptance when the dielectric response is described differently. In the frequency derivative of the absorptance spectrum structure due to bulk plasmon resonances, well in accordance with experimental data, as well as single-particle excitations, not yet looked for experimentally, are found. We predict structure due to single particle excitations in electron energy-loss spectra.

An analysis of the possible excitations in the outer part of the electronic structure of YV${\mathrm{O}}_{4}$, based on the measurement of the characteristic electron-energy losses and x-ray photoelectron spectra is presented. The loss structures below 12 eV and above 22 eV are consistent with the molecular-orbital diagram of the oxyanion V${\mathrm{O}}_{4}^{3\ensuremath{-}}$ obtained within the cluster-model approach. Plasmonlike collective excitations are found at an energy some 5 eV below the "free-valence-electron" value. Deeper O $2s$ electrons appear to be excited by electrons preferentially not in single-loss events, but together with some molecular orbitals or plasmons. Deeper V $3p$ electrons are also excited, with losses that exceed by a few eV the binding energies of these electrons. Yttrium states are found in the photoelectron spectra, but their excitation by electrons, although possible in principle, is not supported by our data.

The absorptance of a thin metal film is calculated using a dielectric function with a discrete single particle excitation spectrum. The inclusion of the quantum size effects give rise to large anomalous absorption peaks at the renormalized single particle excitation energies. A numerical example of a 15 Å thick potassium film on a substrate is discussed and compared with a calculation based on a dielectric function with a continuous excitation spectrum (Lindhard).

As Cs atoms are adsorbed, the electronic surface band of the Cu(111) surface is shifted towards lower energies. Good agreement with experiments is obtained by using a model which takes into account the relation between the surface state energy and the work function. Two other simple models are also discussed.