Absorption, emission, and linebreadths: A semihistorical perspective

The development of the theory of the interaction of electromagnetic radiation with atoms and molecules is outlined. A fully classical analysis of absorption and emission is formulated in which particular attention is paid to questions of detailed balance and to the sum rules obeyed by the susceptibility. Collision broadening is introduced through time-dependent dipole moment correlation functions, and brief discussions of interruption broadening and various frequency modulation models are given. The corresponding quantum mechanical analysis is presented with emphasis on the points in common with the classical approach. The importance of correspondence principles in bridging the gap between classical and quantum mechanical theories is stressed.