I was doing an undergrad experiment on the sodium emission spectrum and I have a few queries. Of course, I was asked to analyse the famous yellow doublet which I'm on top of. I was also asked the analyse the diffuse series which was fine too. What I did notice however, was a strong red line that wasn't in the diffuse series. I was told to do somme research about its origin but I've had absolutely no success in finding any information online or in books yet. It seems that most information online just focuses on the bright yellow doublet. This wavelength wasn't even measured and I can hardly be expected to go through every possible energy difference and try to find the one that's the best match. There must be an easier explaination. We were also given, for the practical task, a list of transition levels. That got me wondering - why are there so many omissions? I was again told to do some research, specifically on why there aren't transitions between the s and d series. I was thinking of something along the line of not conserving spin etc but I can't quite put my finger on it. Thank-you for your help. In summary, you could say my questions are: -Where the hell does the strong red line in the sodium emission spectrum come from? -Why the hell are the sodium electrons so stubborn that there won't be any transitions between s and d series. Thank-you kindly in advance.