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Hydrogen like atoms 

- Sodium D line

You can treat sodium as an “one electron” system because of the closed noble gas body “under” the working electron (You will see later!).  Other atoms than hydrogen have more electrons around the nucleus. So the problem is now to solve the Schrödinger equation for more than one electron. The solution is only as approximation available. But the main thing is that the spins and the angular momentum of the electrons interact with each other (depending on the element).

Sodium has only one more electron than a noble gas. Because noble gases have a ”closed” shell, sodium can be considered as a one electron system. Therefore the overall sum of the angular momentums and the spins of the noble gas body is zero. So the working electron is the outside one. Because of that, the sodium spectra looks similar to the hydrogen spectra.
In table 1 you can see the picture of a sodium high pressure lamp which is very often used as street lighting. The light of sodium lamp is dominated by the very bright orange yellow light which comes from the transition.
2S1/2 – 2P1/2 (589.6 nm) and 2S1/2 – 2P3/2 (589.0 nm). 
Note: there are no transitions with n=1, 2 because they are used in the noble gas body (Neon). There are transitions involving electrons in inner shells, but the energies involved are higher, and so the  radiation (absorbed or emitted) is not visible. The term scheme of sodium is shown in table 2.  [2]

Sodium lampSodium high pressure lamp and its spectrum [1]

Sodium termscheme
The yellow Sodium-“D” Lines (fine structure)[3]


References:

[1] http://en.wikipedia.org/wiki/Image:Na-lamp-3.jpg
[2] Skriptum Experimentalphysik III, Institut für Experimentalphysik, Auflage WS 2001/2002
[3] http://de.wikipedia.org/wiki/Bild:NaD-terms.png

First published on the web: 15 November 1999 by Richard Payling

Authors of the latest version (March 2008): Lara Lobo Revilla and Karl Preiss . The text is based on a lecture given by Prof E.B. Steers at the first Gladnet training course in Antwerp Sept. 2007