Exactly what the title says. I know that there exists a selection rule that ΔS=0, but I cannot find a quantum mechanical explanation for these transitions being forbidden. I know that it has something to do with the electric dipole, but I have not been able to find anything explicit. In reference to my earlier thread, I want to state mathematically why singlet->triplet transitions are forbidden and then use spin orbit coupling to show why these transitions still happen (though with less probability) due to singlet/triplet mixing. My best clue so far: http://en.wikipedia.org/wiki/Selection_rule#Electronic_spectra Anything would be excellent, so I can get a better starting point. A section from a book would be preferable though so that I could cite it. I've looked in Sakurai, Blinder, and Atkins/Friedman so far with no luck. Thanks again!