The interaction of an electromagnetic wave with an electron bound in an atom or molecule can be described by time-dependentperturbation theory.Magnetic dipole transitions describe the dominant effect of the coupling of the magnetic dipole moment of the electron to the magnetic part of the electromagnetic wave. They can be divided into two groups by the frequency at which they are observed: optical magnetic dipole transitions can occur at frequencies in the infrared, optical or ultraviolet between sublevels of two different electronic levels, while magnetic resonance transitions can occur at microwave or radio frequencies between angular momentum sublevels within a single electronic level. The latter are calledElectron Paramagnetic Resonance (EPR) transitions if they are associated with the electronic angular momentum of the atom or molecule andNuclear Magnetic Resonance (NMR) transitions if they are associated with the nuclear angular momentum.^[1]

The Hamiltonian of a bare electron bound in an atom interacting with a time-dependentelectromagnetic field is given by thePauli equation (the theoretical description follows^[2]):

${\displaystyle H=\frac{1}{2m}[\mathbf{P}-q\mathbf{A}(\mathbf{R},t){]}^2+V(R)-\frac{q}{m}\mathbf{S}\cdot \mathbf{B}(\mathbf{R},t)}$

where${\displaystyle q}$ and${\displaystyle m}$ are the charge and mass of a bare electron,${\displaystyle \mathbf{S}}$ is the spin operator,${\displaystyle \mathbf{A}(\mathbf{R},t)}$ is the vector potential of the wave and${\displaystyle \mathbf{P}}$ is themomentum operator.The Hamiltonian can be split into a time independent and a time dependent part:

${\displaystyle H=H_0+W(t)}$

with

${\displaystyle H_0=\frac{1}{2m}{\mathbf{P}}^2+V(R)}$

the atomic Hamiltonian and the interaction with the electromagnetic wave (time-dependent):

${\displaystyle W(t)=-\frac{q}{m}\mathbf{P}\cdot \mathbf{A}(\mathbf{R},t)-\frac{q}{m}\mathbf{S}\cdot \mathbf{B}(\mathbf{R},t)+\frac{q^2}{2m}{\mathbf{A}}^2(\mathbf{R},t)}$

Since the last term is quadratic in A it can be neglected for small fields.The time-dependent part can beTaylor expanded in terms belonging to electric transition dipole (from the first term), magnetic transition dipole (from the second term), and higher order terms, such as electric quadropole and so on. The term belonging to the magnetic transition dipole is:

${\displaystyle W_{DM}(t)=-\frac{q}{2m}(\mathbf{L}+2\mathbf{S})\cdot \mathbf{B}(\mathbf{R},t)}$

Theselection rules for allowed magnetic dipole transitions are:

1.${\displaystyle \mathrm{\Delta }J=0,\pm 1(\text{except }J=0\to J=0)}$ (J:total angular momentum quantum number)

2.${\displaystyle \mathrm{\Delta }{M}_J=0,\pm 1}$ (${\displaystyle M_J}$: projection of thetotal angular momentum along a specified axis)

3. No parity change

• Electric dipole transitions only have a non-vanishing matrix element between quantum states with different parity.
• Magnetic dipole transitions and electric quadrupole transitions in contrast couple states with the same parity.^[1] The response of these two transitions is much weaker than that of electric dipole transitions.
• The electronic states of atoms and molecules normally don't have a staticelectric dipole moment but many states have a static magnetic dipole moment. The classical magnetized top model can be used to describe magnetic resonances for atoms with static magnetic dipole moment between different Zeeman-split-sublevel in a sufficient way without needing a full quantum mechanical description.^[2]

• National Institute of Standards and Technology

• Magnetism