Influence of a magnetic field on line intensities in the optical spectra of free molecules

Results are derived, by an analysis based on irreducible Cartesian tensors, for the change in line intensities in the optical spectrum of a free molecule under application of a static magnetic field. Consideration is specifically given to the cases of electric dipole-allowed, magnetic dipole-allowed and electric quadrupole-allowed transitions, and distinctions are drawn between their behaviour. Magnetochromic effects are identified in connection with both plane and circularly polarised light. In the case of an electric dipole-forbidden transition excited by application of a plane-polarised beam, the differential absorption signal should be sufficiently large to be readily measurable. Suitable candidates for study are the analogues in chiral systems of magnetic dipole-allowed transitions in parent molecules of higher symmetry.