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 6P to singlet 6P in Hg is allowed. Some explanations say that photons do not directly affect spin, but what about radio waves flipping spin in NMR/ESR? Also, does anyone know why you can't go from, say, triplet 3S to singlet 2S? It seems that a spin flip 1/2 -> -1/2 should account for a photon's angular momentum, so this would be allowed? I appreciate any input.