Subnatural Linewidths in Atomic Spectroscopy

This chapter focuses on atomic spectroscopic techniques employed to achieve a spectroscopic resolution beyond the natural linewidth. Most of these techniques rely on the use of some kind of delayed detection scheme, which are described to understand analogies and differences between them. The two essential characteristics of these methods are the time delay producing the subnatural spectrum, and the coherence generated, which centers the resonance about the atomic transition. This second property is necessary if the method is to be used as a spectroscopic tool. Because of the delayed detection scheme, all of these methods present the major disadvantage of a concomitant loss of signal. Thus it is clear that they find their most relevant applications in cases where the spectroscopic study of the atom or molecule under consideration is already so advanced that one is interested in its most minute details. A further requirement for the application of such techniques is an excellent signal-to-noise ratio.