Principles and Techniques of Laser Spectroscopy

Abstract

To understand the concepts of laser action, we first need to appreciate the nature of the stimulated emission process on which it is based. Molecules in excited states generally have very short decay lifetimes (often between 10⁻⁷s and 10⁻⁹s) and by releasing energy they rapidly undergo relaxation processes. In this way, they undergo transitions to more stable states of lower energy; there are many different mechanisms for the release of energy, some of which are radiative, in the sense that light is emitted, and some of which are non-radiative. However, although chemical distinctions can be made between different types of radiative decay such as fluorescence and phosphorescence (see Chap. 5), the essential physics is the same — photons are emitted which match the energy difference between the initially excited state and the final state involved in the transition. Since this kind of photon emission occurs without any external stimulus, it is referred to as spontaneous emission.