Transitions between energy levels in a quantum-mechanical
system that are not allowed to take place because
of selection of rules. In practice, forbidden transitions
can occur, but they do so with much lower probability
than allowed transitions.
The selection rule that
is violated is only an approximate value.
When spin - orbit
coupling is taken into account, the forbidden transitions
become allowed - their strength increasing with
the size of the spin orbit coupling; the selection
rule is valid for dipole radiation, i.e., in the interaction
between a quantum - mechanical system, such as an atom,
and an electromagnetic field, only the electric dipole
moment is considered.
Actual transitions may involve
magnetic dipole radiation or quadrupole radiation;
the selection only applies for an atom, molecule,
etc. in isolation and does not necessarily apply if
external fields, collisions etc., are taken into account.