Zeeman effect

In most atoms, there exist several electronic configurations that have the same energy, so that transitions between different pairs of configurations correspond to a single line.

The presence of a magnetic field breaks the degeneracy, since it
interacts in a different way with electrons with different quantum numbers, slightly modifying their energies. The result is that, where there were several configurations with the same energy, now there are different energies, which give rise to several very close spectral lines.

Without a magnetic field, configurations a, b and c have the same energy, as do d, e and f. The presence of a magnetic field splits the energy levels. A line produced by a transition from a, b or c to d, e or f now will be several lines between different combinations of a, b, c and d, e, f. Not all transitions will be possible -- see transition rules.

Since the distance between the Zeeman sub-levels is proportional with the magnetic field, this effect was used by astronomers to measure the magnetic field of the Sun and other stars.

There is also an "anomalous Zeeman" effect that appears on transitions where the spin of the electrons is not 0, the number of Zeeman sub-levels being even instead of odd if there's an uneven number of electrons involved. It was called "anomalous" because the electron spin was not yet discovered and so there was no good explanation for it when Zeeman discovered the effect.

If the magnetic field strength is too high, the effect is no longer linear; at even higher field strength, electron coupling is disturbed and the spectral lines rearrange. This is called Paschen-Back effect.

The Zeeman effect is named after the Dutch physicist Pieter Zeeman.

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