Doublets in the sodium spectrum

The yellow lines of the sodium doublet are used widely, from the optics experiments of introductory physics to advanced research on fundamental atomic physics. The spectrum of atomic sodium results from numerous radiative transitions between its excited states, and the diagram in Figure 10.1 illustrates some of the more intense ones.

Figure 10.1: Many of the low energy levels of the sodium atom are shown. The 3p state is double because of the magnetic moment of the spinning electron interacts with the magnetic moment from its orbital motion. The existence of two states causes several spectral lines to be double, each pair with the same energy splitting. (From H.G. Kuhn's Atomic Spectra).

Notice that the yellow lines correspond to transitions from two nearby states, labeled $3p \; ^2 P_{1/2}$ and $3p \; ^2 P_{3/2}$, to the lowest ground state of the atom, $3s \; ^2 S_{1/2}$. The difference in the energies of these two lines gives the difference in the energies of the two $3p$ levels. This doublet arises from the interaction of the spin magnetic moment of the outermost electron with its own orbital magnetic moment, the so called spin-orbit interaction. You may want to read about this in a textbook on modern physics.

Notice too that there are several other lines, actually a whole series of them, that have the same separation. In this experiment you will measure two others, the red doublet $5s$ - $3p$, and the yellow-green doublet $4d$ - $3p$. You may also be able to locate and measure the fainter $6s$ - $3p$ blue-green doublet. Each pair of lines have different upper levels, but the lower levels of are always the two $3p$ states. The difference in the energies of the photons for each line of the doublet will be just the energy separation of the two $3p$ levels.