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.

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... 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.