Interaction Between Matter and Radiation
In the previous chapters we have provided a quantum description of the radiation field in vacuum (Chap. 4) and of isolated atomic systems (Chaps. 5– 9). We now move on to describe their mutual interaction by introducing some general methods, also based on quantum mechanics, with which we will be able to handle many phenomena associated with the absorption, emission and scattering of radiation, typical of laboratory and astrophysical plasmas. The combination of these methods is now commonly referred to by the name of quantum electrodynamics and represents one of the most successful achievements of theoretical physics, both from the point of view of the precision of the results obtained, and the elegance of the formalism. We will illustrate in this chapter the fundamental concepts and their simplest applications by considering only first order phenomena, i.e. phenomena involving the emission and the absorption of a single photon. We will formally derive the equations for the evolution of the populations of an atomic system in the presence of the radiation field (statistical equilibrium equations) and the equations for the evolution of the radiation field in the presence of an atomic system (equation of radiative transfer). The most relevant second order phenomena (where two photons are involved) will be treated in Chap. 15.