How Does the ΔS = 0 Selection Rule Arise?

[pfollansbee]

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!

 

[atyy]

http://math.ucr.edu/home/baez/spin/node10.html
http://quantummechanics.ucsd.edu/ph130a/130_notes/node422.html

 

[DrDu]

Have a look how the spin couples to the EM field in the non-relativistic limit:
http://en.wikipedia.org/wiki/Pauli_equation
Try to estimate the relation of a d*E term (responsible for electric dipole transitions) to the Stern Gerlach term e.g. for visible light.