Explanation of some `Forbidden' Transitions in Paramagnetic Resonance

The abnormal intensity of some `forbidden' hyperfine transitions in the paramagnetic resonance spectra of ions with S = 1 or more is shown to be due to cross terms in the spin Hamiltonian. These arise from off-diagonal terms in the electronic and nuclear interactions, and the computed transition intensities agree closely with measurements on vanadium fluosilicate. In a powder an apparent doubling of the hyperfine transitions is shown to be due to the combined effect of these terms and others due solely to the electronic splitting.