Spectra of alkali metal atoms

The relatively large energy separation between configurations in which the valence electron is in orbitals differing only in the value of I (e.g. Ix Sx s 3p, s 3d ) is characteristic of all atoms except hydrogen (and one-electron ions). 

The selection rules governing the promotion of the electron to an excited orbital, and also its falling back from an excited orbital, are 

Some excited configurations of the lithium atom, involving promotion of only the valence electron, are given in Table 7.4, which also lists the states arising from these configurations. Similar states can easily be derived for other alkali metals. 

In the sodium atom pairs of 3/2 states result from the promotion of the 3s valence electron to any np orbital with n 2. It is convenient to label the states with this value of n, as n P 1/2 and n f 3/2, the n label being helpful for states that arise when only one electron is promoted and the unpromoted electrons are either in filled orbitals or in an x orbital. The n label can be used, therefore, for hydrogen, the alkali metals, helium and the alkaline earths. In other atoms it is usual to precede the state symbols by the configuration of the electrons in unfilled orbitals, as in the 2p3p state of carbon. 

The splitting of the 3 P 2, 3 Py2 states of sodium is 17.2 cm and this reduces to 5.6 cm2.5 cmand 1.3 cmfor n = 4, 5 and 6 respectively. The splitting decreases rapidly withL as exemplified by the splitting of only 0.1 cm for the 3 D /2, 3 D /2 states. All these P and multiplets are normal, the state with lowest J lying lowest in energy. 

The 32Pl/2, 32P2/2 excited states involved in the sodium D lines are the lowest energy excited states of the atom. Consequently, in a discharge in the vapour at a pressure that is sufficiently high for collisional deactivation of excited states to occur readily, a majority of atoms find themselves in these states before emission of radiation has taken place. Therefore 

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