Rapid-dispersal approximation in radiative atomic processes

We discuss the assumption that in the decay of an unstable system the decay products rapidly disperse immediately after their formation. This assumption, a version of the Weisskopf--Wigner assumption of a broad energy spectrum, makes it possible to obtain detailed expressions for the time development of the decaying system. It is shown to follow from quantum electrodynamics, with no further assumptions, for spontaneous emission by atoms. It is used to give simple and intuitive treatments of spontaneous emission and resonance fluorescence, in the Schroedinger picture and with the focus on photons rather than electromagnetic field values. The time dependence of the lineshape is discussed, and it is shown that the effect of detection of photons at finite times after emission is to give an overall lineshape which is not Lorentzian. copyright 1988 Academic Press, Inc