Why Do Sodium D Lines Split and Lack Certain Transitions?

[t_n_p]

Homework Statement

a) The strong yellow line (sometimes called the sodium D lines) is actually a doublet which can be resolved with the prism spectrometer, Can you explain the origin of the splitting?

b)
http://img266.imageshack.us/img266/4761/fig1cr4.png
Why are there no transitions shown in figure 1 from D->S or S->D?

The Attempt at a Solution

For a) is it because the energy difference between the two spin orientations is responsible for the fine structure doubling of many atomic spectral lines (in this case, sodium being one of them)? The spin-orbit doubling of the sodium 3s level is the origin of the splitting?

I'm not sure about b), can someone please explain?

Thanks

 

[genneth]

a) is correct.

b) is because of selection rules --- conservation of angular momentum and parity.