Emission, Absorption and Scattering of Light by Atomic Systems

Abstract

Problems concerning the interaction of light and microparticles are in some respects outside the scope of quantum mechanics. They cannot be discussed without the use of supplementary ideas relating to laws of creation and annihilation of electromagnetic fields. We can, however, make considerable progress on the basis of Einstein’s semiphenomenological theory of radiation, which is essentially founded on the laws of conservation of energy and momentum in interactions between quantum systems and the electromagnetic radiation field. For the behaviour of a quantum system in a given electromagnetic field is entirely a mechanical problem. We can therefore use the theory of quantum transitions to calculate the probability that an atom enters an excited state, or returns from an excited state to a lower one, under the action of incident light. In the former case the energy of the atom increases by an amount E _m − E _n , where E _n is the energy of the initial state and E _m that of the excited state, and in the latter case it decreases by this amount. Let us first consider the former process.