Modern Spectroscopy

222 7 ELECTRONIC SPECTROSCOPY
Figure 7.10 (a) A simple triplet and (b) a compound triplet in the spectrum of an alkaline earth
metal atom
also obey Equation (7.20). In other respects, the alkaline earth metal spectra resemble that of
helium quite closely. All the atoms have an ns 2 valence orbital configuration and, when one
of these electrons is promoted, give a series of singlet and triplet states.
The fine structure of a 3 P 3 S transition of an alkaline earth metal is illustrated in Figure
7.10(a). The D J selection rule (Equation 7.22) results in a simple triplet. (The very small
separation of 2 3 P 1 and 2 3 P 2 in helium accounts for the early description of the lowresolution
spectrum of triplet helium as consisting of ‘doublets’.)
A 3 D 3 P transition, shown in Figure 7.10(b), has six components. As with doublet states
the multiplet splitting decreases rapidly with L so the resulting six lines in the spectrum
appear, at medium resolution, as a triplet. For this reason the fine structure is often called a
compound triplet.
7.1.6 Spectra of other polyelectronic atoms
So far we have considered only hydrogen, helium, the alkali metals and the alkaline earth
metals but the selection rules and general principles encountered can be extended quite
straightforwardly to any other atom.