Zeeman Effect

Zeeman Effect
The Zeeman effect is the name given to the splitting observed in spectral lines when the source of those lines is placed in an external magnetic field. Splitting of a single spectral line into three components in the presence of an external magnetic field is called the normal Zeeman effect. Any time the g-values of the upper and lower states are the same, this pattern results. When the g-values are different, much more complicated patterns are possible. These, for historical reasons, are called the anomalous Zeeman effect.
g is the Land´e g-value defined as:

……(8)

When S=0, gJ =1 because then J must equal L. In this case, the magnetic
moment is independent of L, and so all singlet terms are split to the same
extent. This uniform splitting results in the normal Zeeman effect. When
S ? 0, the value of gJ depends on the values of L and S, and so different terms are split to different extents.


















Normal Zeeman effect











Anomalous Zeeman effect: spin orbit coupling


When the applied field is very strong, the coupling between L and S may be broken in favour of their direct coupling to the magnetic field. The individual angular momenta, and therefore their magnetic moments, now precess independently about the field direction . As the electromagnetic field couples to the spatial distribution of the electrons (recall the form of the transition dipole moment), not to the magnetic moment due to the spin, the presence of the spin now makes no difference to the energies of the transitions. As a result, the anomalous Zeeman effect gives way to the normal Zeeman effect. This switch from the anomalous effect to the normal effect is called the Paschen–Back effect.