Electron spectroscopies and inelastic processes in nanoclusters and solids: Theory and experiment

The recent, very significant developments in high intensity and brightness electron and photon sources have opened new possibilities of applying electron spectroscopies, such as photoemission, Auger and electron energy loss, to the study of many interesting features in the dynamics of atoms, molecules and condensed-matter systems. In the last few years it has become possible to obtain electron spectra with an overall energy resolution (electron/photon source and electron spectrometer) considerably smaller than the linewidth of the investigated level and to study quantitatively the combined effects of the intrinsic dynamical properties of the system, of features of the incident beam and of the electron spectrometer on the spectral lineshape. For all these reasons, it is important to have theoretical methods that are able to analyze the dynamics of systems at any level of aggregation under the influence of an incident radiation and, simultaneously, to predict spectral lineshapes quantitatively by correlating their features with internal dynamics of the perturbed system. In this report, we present experiments and a critical overview of theoretical methods for interpreting electron spectra of atoms, molecules and solid-state systems. The general theoretical framework for this analysis is resonant multichannel scattering theory. Electron spectroscopies are, in fact, based on scattering processes in which the initial state consists of a projectile, typically photons or electrons, exciting a target to a resonant state, which has long lifetimes if compared to the collision time. This metastable state is embedded in the continuum of final states characterized by the presence of a few fragments, whose observation provides useful information on the properties of the system under study. Even if the general theory of scattering and decay phenomena has been largely developed, its specific application to electron spectroscopies in condensed matter and, in several cases also to atoms and molecules, presents difficulties that have hindered the production of high quality theoretical spectra until recently. This is mainly due to computational problems related to treating a large number of decay channels, which prevent one from using numerical techniques for representing the electron as it moves outward through the field of the ionized system. Furthermore, another issue is represented by the need to account for shake processes and extrinsic energy losses due to the coupling with collective excitations. In this work we present a theoretical method which does not suffer from the limitations of previous approaches, and allows one accurately to reproduce the experimental results in solids. This method provides an extension to condensed matter of Fano’s formulation of the interaction between discrete and continuum states. It includes the combined effects of intrinsic and extrinsic features on spectral lineshapes so that computed spectra are directly comparable to acquired spectra, avoiding background subtraction or deconvolution procedures. This approach is sufficiently general to be applied not only to the analysis and interpretation of autoionization, Auger and photoemission spectra, but also to the study of other processes since its central feature is the ability of calculating accurate wavefunctions for continuum states of extended systems.