The natural line broadening resulting from electromagnetic or acoustic radiative processes, the only causes of broadening discussed so far, by no means exhausts the mechanisms available and indeed is usually so minor an effect as to be of small practical importance. We may distinguish broadening due to different behaviour on the part of different members of an ensemble from broadening exhibited by each member on its own. In the first class are Doppler broadening and broadening due to variations in environment, to which may be added the effects of slight differences between superficially similar systems (e.g. different isotopes). In the second class, in addition to radiative processes, must be counted anything, especially collision with other atoms, which interrupts or distorts the wavetrain emitted by a single system so as to widen its spread of Fourier components. There is a very large literature on these effects, whose detailed analysis is both taxing and controversial. No attempt will be made here to go beyond an elementary discussion and illustration of some of the leading ideas, with examples of how the line-width may be reduced or its effects mitigated for the purpose of high-precision measurements of the central frequency. We start with the second class of processes, and for our purpose the two-level system provides an adequate model, with ammonia as a practical realization.

The absorption spectrum of ammonia at a rather low pressure, 1.2 torr, in the wavelength range from 1.1 to 1.5 cm is shown in fig. 18.8, each line resulting from transitions between pairs of levels in different rotational states of the molecule, as defined by the pairs of quantum numbers above each line.