- where does the R21, Q21, etc. branches come from?
I have a bit basic understanding of spectrum, but quite confused with LIF spectrum. It seems existing 12 main branches of Laser induced fluorescence, such P1, P2, Q1, Q2, P21, R21，and so on. I know there should be O P Q R S branches of the rotational transitions, but where these so many subscripts, 1, 2, 21, 12 com from？ thanks for any help.