Interaction of Electromagnetic Radiation with Atomic Systems

Abstract

So far, we have considered quantum systems in external oscillating magnetic (or electric) fields, where the perturbing fields were considered as classical oscillating fields. This approach would permit us to study induced absorption or emission processes in an atomic system, particularly if the intensity of the electromagnetic fields is such that the density of photons is very great in the region of interest. To study processes involving a small number of photons interacting with an atomic system, we must first quantize the electromagnetic or radiation field itself. So far, our time-dependent perturbation theory would not permit us to study the spontaneous emission of photons by an atomic system, i.e., a situation in which the initial state consists of an atom in an excited state and no photons, and the final state is the atom in a lower state (perhaps the ground state) and one photon of the appropriate energy. In order to treat this problem rigorously, we need to quantize the radiation field.