Everything about Selection Rule totally explained
In
physics and
chemistry, 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's necessary for a process to occur with a nonzero
probability.
See also:
angular momentum coupling
In 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 2
λ, for example 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.
The emitted particle carries away an angular momentum λ, which for the photon must be
at least 1, since it's a vector particle (for example, it has
JP = 1
−). Thus there's no E0 (electric monopoles) or M0 (
magnetic monopoles) radiation (the latter is forbidden because magnetic monopoles don't seem to exist).
Since the total angular momentum has to be conserved during the transition, we've that
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