Thanks axlm, that was very helpful - I have one more raised question, if you don't mind?
For phosphorescence, you say there is a radiative transition back to a singlet. My understanding of a "forbidden" transition was that it could only be done through an intermediate generated through...
Does anyone have a good understanding of why the spin selection rule (delta S = 0) holds for Russell-Saunders coupling in smaller atoms, but not for jj-coupling in larger atoms? For example, a transition from a triplet 2P to singlet 1S in helium is "disallowed," but a transition from a triplet...