Selection rule

Summary

In physics and chemistry, a selection rule, or transition rule, formally constrains the possible transitions of a system from one quantum state to another. Selection rules have been derived for electromagnetic transitions in molecules, in atoms, in atomic nuclei, and so on. The selection rules may differ according to the technique used to observe the transition. The selection rule also plays a role in chemical reactions, where some are formally spin-forbidden reactions, that is, reactions where the spin state changes at least once from reactants to products. In the following, mainly atomic and molecular transitions are considered. Overview In quantum mechanics the basis for a spectroscopic selection rule is the value of the transition moment integral :\int \psi_1^* , \mu , \psi_2 , \mathrm{d}\tau,, where \psi_1 and \psi_2 are the wave functions of the two states, "state 1" and "state 2", involved in the transition

Official source

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