Electron Impact Excitation

Abstract

The case study part of this book begins in Sect. 7.1 with the presentation of a few examples where either unpolarized or spin-polarized electron beams are used to excite unpolarized atoms. Only the emitted light and its polarization, but not the scattered electrons, are observed. The intensity of the light corresponds to the so-called optical excitation function. After proper normalization and accounting for a possible anisotropy in the radiation, excitation functions are often used to determine absolute cross sections for the process of interest. Furthermore, the light polarization components correspond to the “angle-integrated Stokes parameters.” They are determined by the angle-integrated state multipoles that represent alignment and orientation of the excited target state, averaged over all projectile scattering angles. Angle-differential observation of Stokes and STU parameters is discussed in Sect. 7.2. The discussion focuses on some benchmark cases, involving light targets such as helium and sodium and heavy targets such as mercury and cesium. Special attention is given to the noble-gas targets Ne–Xe. These continue to represent a major challenge to theory, partly due to difficulties associated with the structure calculation.