Line strengths of discrete transitions

In the present chapter, we consider line strengths of transitions between bound states, i.e. of lines whose only form of natural broadening is radiative, and which lie below the first ionisation potential. The simplest situation is encountered in the photoabsorption or photoexcitation of an atom, initially in its ground state , in which case one transition is observed to each excited final state / . The price one pays for this simplicity is that all excited states cannot be reached in this way because of selection rules. 

The distribution of intensities is an essential property of a spectrum. We may consider (a) the distribution in energy over the whole spectrum or (b) the distribution within an individual spectral line. 

In the presence of perturbations, the course of intensities becomes far less regular than that of transition energies, and it is generally more likely that intensities will exhibit fluctuations or departures from the expected 

At a yet finer level of understanding, one is concerned, not just about intensities of lines, but also about how the intensity is distributed within the width of a spectral line. The lineshape, or line profile, carries within it much more information about the condition of excited atoms and, possibly, their environment than could be deduced from the gross structure of spectra. 

We shall concentrate our attention on homogeneous broadening effects, i.e. those which are the same for all the atoms in an ensemble, and exclude from our discussion effects due temperature and density.2 

---