Molecules in Magnetic Fields: Fundamental Aspects

Abstract

The experimental observation of the splitting of the spectral lines of an atom in a magnetic field dates back to the early history of atomic physics (Zeeman 1896). During the past hundred years the behaviour of matter in a magnetic field has been studied extensively in different areas of physics like for example, atomic, molecular and solid state physics. However, this should not obscure the fact that almost all of these investigations deal with magnetic forces which are small in comparison with the binding energies of the underlying system: the theory of the normal, anomal Zeeman effect as well as of the Paschen-Back effect in atomic physics is based on the applicability of perturbation theory to the influence of the magnetic field on the energy levels of the atom. The relevant parameter which characterizes the so-called low and high field region is given by the ratio of the magnetic and Coulomb binding energies. For the ground state of the hydrogen atom (13.6 eV binding energy) the low field region goes up to a field strength of approximately B≈104 T. The intermediate region, for which the cyclotron and Coulomb binding energy are of comparable order of magnitude, is given for 104T ≤ B ≤ 106T whereas the high field region means B ≥ 106T. Relativistic corrections due to the external magnetic field have to be taken into account only above a threshold field strength of approximately B=108T. These values should be compared with a typical laboratory magnetic field strength of only a few Tesla (B ≤ 30T). If we consider the ground or first few excited states of an atom or molecule in a laboratory magnetic field we are, therefore, always confronted with the low field region.