ELECTRON-SOLID INTERACTIONS - THEIR NATURE AND CONSEQUENCES

During the past two decades electron scattering and emission spectroscopies have become utilized almost universally to characterize the properties of solid surfaces. Two major trends have created this situation: The increasing availability of suitable, convenient experimental apparatus, and the recognition and exploitation of those features of electron-solid interactions which render these measurements sensitive to surface properties. Whereas prior Welch Award lectures have dealt with the former trend, the latter is examined herein. Three concepts are central to an understanding of the interaction of ″low-energy″ , 10 APP 1STH E APP 1STH 10**3-eV electrons with the constituents of solids. First, their strong Coulombic interaction with the valence electrons of the solid renders their mean free paths for inelastic collisions of atomic dimensions, i. e. , 2 APP 1STH lambda APP 1STH 20 A. Thus, elastically scattered or emitted electrons in this energy range must have emanated from the uppermost few atomic layers of the solid. Second, the interactions of these electrons with the core electrons are comparable to those with the valence electrons. Therefore, multiple elastic scattering as well as rapid energy loss characterizes their scattering and emission. Third, resonant processes associated with electronic states localized at a surface can be as important as inelastic processes in establishing surface sensitivity. Examples of this phenomenon are found in resonant field emission through adsorbates and resonant photoemission associated with the excitation of transitions characteristic of adsorbed complexes. The major portion of this article is devoted to an indication of how exploitations of these three concepts have led to quantitative surface analysis methodologies for metals and covalent solids. In the case of polymers and molecular solids, however, one further concept is required: the localization of electronic states caused by fluctuations in relaxation energy. These fluctuations lead to localized molecular ion (or exciton) states rather than extended band states. The final portion of this article is devoted to exploring the consequences of this additional concept and how it led to the invention of yet another electron spectroscopy, contact charge exchange spectroscopy, for the examination of molecular ion states in polymers.