MODULE 1. Magnetism
MODULE 2. Particle Physics
MODULE 3. Modern Physics
MODULE 4. Semicoductor Physics
MODULE 5. Superconductivty
MODULE 6. Optics
LESSON 11. Qualitative Explanation of Zeeman Effect
Qualitative explanation of Zeeman Effect
The Zeeman Effect, named after the Dutch physicist Pieter Zeeman, is the effect of splitting a spectral line into several components in the presence of a static magnetic field. It is analogous to the Stark effect, the splitting of a spectral line into several components in the presence of an electric field
Fig Zeeman Splitting against Magnetic Field
Also similar to the Stark effect, transitions between different components have, in general, different intensities, with some being entirely forbidden (in the dipole approximation), as governed by the selection rules.
The distance between the Zeeman sub-levels is a function of the magnetic field; this effect can be used to measure the magnetic field
When the spectral lines are absorption lines, the effect is called inverse Zeeman Effect.
The total Hamiltonian of an atom in a magnetic field is
H = H0 + VM ,
where H0 is the unperturbed Hamiltonian of the atom, and VM is perturbation due to the magnetic field:
where is the magnetic moment of the atom. The magnetic moment consists of the electronic and nuclear parts; however, the latter is many orders of magnitude smaller and will be neglected here. Therefore,
where is the Bohr magneton, is the total electronic angular momentum, and g is the Landé g-factor. The operator of the magnetic moment of an electron is a sum of the contributions of the orbital angular momentum and the spin angular momentum , with each multiplied by the appropriate gyromagnetic ratio:
where gl = 1 and (the latter is called the anomalous gyromagnetic ratio; the deviation of the value from 2 is due to Quantum Electrodynamics effects). In the case of the LS coupling, one can sum over all electrons in the atom:
Where and are the total orbital momentum and spin of the atom, and averaging is done over a state with a given value of the total angular momentum.
If the interaction term VM is small (less than the fine structure), it can be treated as a perturbation; this is the Zeeman Effect proper. In the Paschen-Back effect, described below, VM exceeds the LS coupling significantly (but is still small compared to H0). In ultra strong magnetic fields, the magnetic-field interaction may exceed H0, in which case the atom can no longer exist in its normal meaning, and one talks about Landau levels instead. There are, of course, intermediate cases which are more complex than these limit cases.
If the spin-orbit interactions dominate over the effect of the external magnetic field and are not separately conserved, only the total angular momentum is. The spin and orbital angular momentum vectors can be thought of as processing about the (fixed) total angular momentum vector. The (time-)"averaged" spin vector is then the projection of the spin onto the direction of :
and for the (time-)"averaged" orbital vector:
Using = - and squaring both sides, we get
and: using = - and squaring both sides, we get
Combining everything and taking , we obtain the magnetic potential energy of the atom in the applied external magnetic field,
where the quantity in square brackets is the Landé g-factor gJ of the atom ( and ) and mj is the z-component of the total angular momentum. For a single electron above filled shells s = 1 and j = 1 ± s, the Landé g-factor can be simplified into:
In the presence of an external magnetic field, the weak-field Zeeman effect splits the 1S1/2 and 2P1/2 levels into 2 states each (mj =1/2,-1/2) and the 2P3/2 level into 4 states (mj = 3/2,1/2,-1/2,-3/2).
Fig. Zeeman Splits
The Landé g-factors for the three levels are:
gJ = 2 For (j=1/2, l=0)
gJ = 2/3 For (j=1/2, l=1)
gJ = 4/3 For (j=3/2, l=1).
Note in particular that the size of the energy splitting is different for the different orbital, because the gJ values are different. On the left, fine structure splitting is depicted. This splitting occurs even in the absence of a magnetic field, as it is due to spin-orbit coupling. Depicted on the right is the additional Zeeman splitting, which occurs in the presence of magnetic fields.