306 Appendix C: Magnetic dipole transitions
the gas density is low and there are vast clouds of atomic hydrogen;
even weak emission at a wavelength of 21 cm gives enough microwave
radiation to be picked up by radio telescopes.
The selection rules for magnetic dipole radiation transitions are given
in the table below, together with the electric dipole transitions for com-
parison.Electric dipole transitions Magnetic dipole transitions
∆J=0,±1∆J=0,± 1
(but notJ=0toJ′= 0) (but notJ=0toJ′=0)
∆MJ=0,±1∆MJ=0,± 1
Parity change No parity change
∆l=±1∆l= 0 No change of}
Any ∆n ∆n= 0 configuration
∆L=0,±1∆L=0
∆S=0 ∆S=0In certain circumstances magnetic dipole transitions can give rise to
visible transitions in atoms, and there are also electric quadrupole (E2)
transitions arising from the breakdown of the dipole approximation.
These forbidden transitions, i.e. forbidden for electric dipole transitions,
are detailed in Corney (2000).