Specific parts of the emission spectrum of a sodium lamp

[Einstein2nd]

Homework Statement

Part a) The emission spectrum for Sodium lamp contains a reasonably bright red line in its spectrum. What is its origin?

Part b) The possible transitions between electron energy levels for Sodium were analysed. There were no transitions between the s series (s subshell) and d series (d subshell). Why is that?

Homework Equations

None really.

The Attempt at a Solution

Part a) I analysed the diffuse d series and this red line isn't in it. I also analysed the origin of the famous yellow doublet but that really isn't overly relevant. I've done a heap of browsing the web and books and am yet to find anything at all. Most sources simply talk about the yellow doublet

Part b) As I said, I analysed the diffuse d series but these are all transitions between d orbitals and the 3p energy level. I'm thinking something along the lines of not conserving angular momentum or spin or something would prevent transitions between d and s series but I'm really not sure. As with Part a), the web and books have been useless so far so this is a bit of a last resort.

Thank you kindly in advance.

 

[Einstein2nd]

I found a database with all known wavelengths but I have no real idea of where the red wavelength is so it's so hard to tell.

 

[Einstein2nd]

For part b) I was told it's not to do with conservation of spin. Conservation of angular momentum perhaps?