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. 

An obvious difference between the emission spectra of most atoms and those we have considered so far is their complexity, the spectra showing very many lines and no obvious series. An extreme example is the spectrum of iron, which is so rich in lines that it is commonly used as a calibration spectrum throughout the visible and ultraviolet regions. 

However complex the atom, we can use the Russell-Saunders coupling approximation (or jj coupling, if necessary) to derive the states that arise from any configuration. The four general selection mles that apply to transitions between these states are as follows. 

Previously we have considered the promotion of only one electron, for which Af = 1 applies, but the general mle given here involves the total orbital angular momentum quantum number L and applies to the promotion of any number of electrons. 

even and odd refer to the arithmetic sum over all the electrons and this selection mle is called the Laporte mle. An important result of this is that transitions are forbidden between states arising from the same configuration. For example, of the terms given in Equation (7.18) arising from the configuration of the carbon atom, 

4 The 22Pi/2 22S1//2 and 22S1/,2 — 22P1//2 transitions observed in the hydrogen atom violate the Laporte rule because they are magnetic dipole transitions the rule applies only to electric dipole transitions. 

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