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The availability of empty electron states above the Fermi level and the presence of strong intra-atomic correlations may be expected to lead to new features in the Auger XVV spectra of conduction bands. Here, the qualitative aspects of the problem are studied within a simplified model. The case of a low equilibrium hole density n h in the band is considered in detail and the various propagators are calculated in a low density approximation. It is found that correlation and shake up effects can be approximately factored out and the singularity exponent relevant to the problem is linear in n h . In the single particle density of states we find a peak in the high binding energy side, which appears to be consistent with the experimental XPS spectra of Fe, Co and Ni. The Auger XVV spectra are obtained in the same approximation. As in the case of closed bands ( n h = 0) two-hole resonances appear in the spectra provided that intra-atomic correlations are strong enough (quasiatomic case), but the condition for their occurrence is found to involve the total band width, rather than the width of the occupied portion of the band only. Moreover, even in the quasiatomic case, the resonances have a width that is related to the energy difference between the top of the band and the Fermi level.

Valence holes can react to various elementary excitations, such as plasmons and phonons, causing them to relax coherently in the final states of electron spectroscopies. This can produce interesting effects on the lineshapes. Here the photoemission spectra of a chemisorbate or an impurity are discussed in the framework of a model that explicitly deals with the dynamics of two different boson modes. The exact solution is given and leads to the conclusion that coherence effects are important when the two excitations give comparable contributions to the relaxation energy. The dynamical effects due to the plasmons end to enhance the vibrational structures.

Photoemission and Auger line shapes from almost completely filled bands have been widely discussed in recent years within a simplified model based on an Anderson Hamiltonian in which the virtual level shift due to the interactions is suitably compensated for. Up to now, the theory has been much more successful with XPS than with AES, and the reason for this was obscured by the lack of an exact solution and by the difficulty to assess the degree of validity of various approximate treatments that have been proposed. Here we present a Green’s function formalism that allows us to extend the closed band solution to the partially occupied case and lends itself to the exact numerical treatment of finite systems. By applying the theory to 27 and 125 atom clusters, we analyse the dependence of the spectra on hole-hole repulsionU with a degree of unfillingnh≤0.25. We also consider the case when one of the spin subbands is full as a rough model for ferromagnetic metals. Correlation effects on the one-hole density of states produce a narrowing of the band region, while a split-off structure develops below the band forU comparable to the band width. The low-density approximation is good agreement with the exact results fornh=0.1 and also fornh=0.25 for small and moderateU. Our results on the Auger line shapes justify somewhat the suggestion by Haak and Bennetet al. that split-off states observed in photoemission must be discarded before computing the two-hole spectrum. Indeed self-energy corrections must be excluded also in bandlike cases, when the simple procedure of cutting off the unwanted structure is not applicable. This arises because, in a wide range of physical situations, the Auger line shape reflects the mutual scattering of undressed final-state holes.

The RPA is extended in the framework of linear response theory to allow for the treatment of a finite free electron gas. The dielectric function taking quantum size effects into account, is calculated and shown to be finite in the limit of small wavevectors. The theory is applied to a simple free electron model for a small metal particle and analytical expressions are obtained for the electronic polarizability of a thin film and for the optical properties in the plasma resonance region.

A nonperturbative many electron theory of atom resonance ionization and ion resonance neutralization at a metal surface is presented. Particular attention is paid to the problem of the ion yield in SIMS experiments. Using a time dependent equation of motion method, the ionization probability is computed as a function of the velocity of secondary ions and the form of the dependence is sensitive to the detailed shape of the virtual level. For a lorentzian line shape the ionization probability is linear with the velocity for small velocities.

A new many-body theory of tunneling is proposed in which the time-dependent applied potential, rather than a fictitious static pseudo-Hamiltonian, provides the driving force. The theory is developed for continuous and discrete models and the exact solutions for the electron density and for the current are given at finite temperatures in the case of independent particles. However, the theory lends itself to possible extensions to include interaction effects analogous to previous static approaches. In addition to allowing for the study of transient effects, the time-dependent formulation leads directly to convenient new expressions for the current-voltage characteristics in terms of the asymptotic amplitude and phase of wave functions. The new results appear to be particularly well suited for application to semiconductor junction devices.

A theory of XVV Auger spectra of atoms in solids is proposed, that takes hole-hole repulsion into account. The interaction between the holes is taken to be localized at the atomic site. Within this model, the theory is exact. The relationship between atomic-like and band-like spectra is discussed and several previously unexplained features of the experimental spectra are shown to find a natural justification within the context of the theory.

The theoretical analyses of photoemission and Auger XVV spectra of incompletely filled bands are developed in a coherent fashion, based on the same model Hamiltonian. The model, which had been justified previously for cases when the hole density is small, is generalised to account for band degeneracy and multiplet splittings. Accordingly, the authors extend the graph theoretical expansion of the self-energy in the low-density approximation and that of the two-hole Green function in the bare ladder approximation. They work in the intermediate-coupling scheme, in order to include the spin-orbit interaction. Assuming that to a good approximation the total angular momentum J is conserved, they apply the formalism to Pd metal; they use the same Coulomb matrix and Christensen's independent-particle density of states for both spectroscopies and obtain good agreement with the experimental lineshapes.

Several physical models involving the interaction of electronic and bosonic degrees of freedom have been proposed and solved exactly in recent years by a new recursion technique. The models belong to various contexts, but most applications have been in the theory of electron and photon spectroscopies of molecules and solids. For a class of Hamiltonians which describe closed-shell systems the recurrence relations provide closed-form analytical solutions. Here the authors present the method and discuss its relationship to alternative approaches that are also based on deriving and solving recurrence relations. They review the main results obtained to date and present solutions relevant to optical spectroscopy for the first time. They demonstrate the usefulness of exact analytical solutions for understanding the nontrivial dynamical behaviour of interacting fermion-boson systems.

A quantum mechanical model of ionic chemisorption on small metal particles is presented. The model takes into account the size dependence of the metal electronic properties and treats ionic and covalent bond contributions in a selfconsistent fashion. The binding energy and the charge at the adatom are found to be strongly size dependent.

When a chemisorbed atom undergoes a CVV Auger transition, the surface polarization modes respond no faster than the surface plasmon frequency. Hence we must expect deviations from the current description of CVV line shapes that assumes instantaneous screening. Here we propose an extended theory than includes dynamicalscreening effects. Our description applies to systems that have filled valence bands in the ground states. The absence of a translational symmetry implies a model Hamiltonian that is different and more involved than the one appropriate for homogeneous solids. The exact solution leads to a complex phenomenology that we illustrate by selected examples.

To understand the electronic processes that occur during desorption, we must allow simultaneously for the intrinsic nonadiabatic character of electron states and for correlation-induced features like two-hole resonances. A recently proposed theory describes the Menzel-Gomer-Redhead and Knotek-Fiebelman mechanisms on equal footing and reveals a complex interplay of electronic and kinematic effects on the ion yield. Here, I wish to show how in the same scheme we can also obtain the yield of ions that have suffered (partial or total) re-neutralization in the course of desorption. Closed-form expressions for the relevant escape probabilities are given in terms of one-hole and two-hole Green’s functions. The surface molecule limit of the theory is worked out in detail assuming that the hopping parameters decay exponentially in time during desorption. It is shown that, in general, when a given species can be desorbed by different mechanisms, we can discriminate them experimentally by the energy and angular dependence of the yield.

We propose a soluble quantum mechanical model for the photoemission spectrum of a surface molecule interacting with the surface plasmon field. Plasmon polarization and hole recoil are included in our model. These terms should be particularly important in the XPS spectra of chemisorbed electronegative species like F and Cl. Further our exact solution implies the existence of a new, plasmon mediated, chemisorption mechanism. This new kind of chemisorption bond is involved in the absorption of closed shell species, like Xe, on transition metals, and explains the microscopic physical mechanism underlying well known phenomenological arguments like the “charge transfer no bond” model of chemisorption.

A theory of XPS and Auger lineshapes from almost completely filled bands, that had previously been proposed in the context of a graph-theoretical expansion, is applied to a finite cluster and solved exactly. The new method avoids the calculation of complex vertex corrections and reveals the full potential of the theory. We find that while the holes observed in photoemission are dressed quasi-holes, those observed in Auger CVV spectra are essentially bare. This explains certain discrepancies between calculated and experimental spectra, that had been attributed to intrinsic limitations of the theoretical set-up, but actually were due to the mathematical approximations. A simple analytic approach based on the use of bare propagators is found to agree closely with the exact numerical results.

Auger CW and Coster-Kronig spectra in gas phase of boron trifluoride, boron trichloride, boron tribromide and hydrogen bromide have been obtained by 2 keV electron impact. The fluorine spectrum can be understood as quasi-atomic by the Thomas—Weightman Δ U/W criterion, while in the spectra of the heavier halides some of the final states are delocalized. The boron spectrum from boron trifluoride is definitely band-like, but it also shows two prominent quasi-atomic structures induced by fluorine. A simplified model allows the nature of these “partner resonances” to be understood and their intensities calculated; a semi-quantitative agreement with experiment is obtained without adjustable parameters. We provide evidence that these resonances are a general feature of the CW spectra of ionic molecules and identify them in the boron spectrum, from boron tribromide, as well as in the literature data.

The Auger lineshapes of chemisorbed atoms are determined by the dynamical response of the substrate and if the surface plasmon energy is comparable to the adatom-substrate hopping integral, the final state holes cannot be described as interacting through a statically screened repulsion U. We propose a generalized model of CVV lineshapes that explicitly includes the dynamics of a dispersionless surface plasmon. The exact solution is given for any shape of the unperturbed virtual level and physical implications are pointed out.

A simplified, soluble model of photoemission from valence states with a localized electron-phonon coupling is discussed. When the number of phonons produced is large the model predicts a temperature-dependent enhancement of the spectral features near the band edges due to hole-phonon interference effects.

If two holes are suddenly created in the same band and at the same atomic site e.g. by an Auger process in a solid, their density of states N (ω) will depend on their Coulomb interaction. In a tight binding model, we present the exact N (ω), in the limit of zero bandwidth. In the case of a general band, we give an exact integral equation that allows calculating N (ω) once the single electron density of states is known. The interaction is shown to produce a characteristic distortion of N (ω) and hence of the Auger spectrum.

ABS>The problem of neutron-proton mass difference is examined by using as a basia the expression of the Comptor scattering amplitude derived with a dispersion relation approach. (J.H.M.)

The interaction between a small metal particle and an external charge is calculated in the linearized Fermi-Thomas approximation. It is suggested that the size dependence of the 'image potential' could play an important role in chemisorption and catalysis.

The current theoretical formulations of optical absorption by small metal particles are critically reviewed. Several inconsistencies of the usual dielectric-function calculations are noted, and an improved expression is proposed. The results indicate that optical spectra should have a bearing on the difficult question of the level distribution in small particles. Finite ionization potential effects are shown to be important for small particles and are included in the dielectric formulation. The new results agree with hydrodynamic theories that state that the position of the anomalous absorption peak essentially depends on the degree of diffuseness of the metal-particle surface.