ABSTRACT
N o resonance line is infinitely narrow, even in theory, because spontaneous emission always introduces a finite lifetime to the upper state
and therefore (by the uncertainty principle) a natural width to the transition. In practice, a number of other processes such as collisions, Doppler broadening, power broadening, field inhomogeneities, etc., also add width to resonance lines. It is important to study the effect of these processes on the frequency dependence of the transition probability (the lineshape) for two important reasons: (i) it is not possible to find the resonance frequency ω◦ with high precision unless the line (or especially several overlapping lines) can be fitted accurately, and (ii) it is not possible to estimate either the magnitude of the “signal” or its dependence on the intensity of the driving radiation without understanding the broadening mechanisms.
