selection rule

In physics, especially in the context of quantum mechanics, a selection rule is a condition constraining the physical properties of the initial system and the final system that is necessary for a process to occur with a nonzero probability. See also: angular momentum couplingIn many cases, a transition involves the emission of radiation, that is, a photon is emitted. In general, electric (charge) radiation or magnetic (current, magnetic moment radiation) can be classified into multipoles Eλ (electric) or Mλ (magnetic) of order 2λ, e.g. E1, E2, E3 for electric dipole, quadrupole or octupole. The radiation field will be a sum of the multipole contributions; however, usually one or two multipoles dominate.
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