Origin of additional satellites in electron paramagnetic resonance spectra of rare earth ion pairs

Electron Paramagnetic Resonance (EPR) spectrum of pairs of two identical rare earth ions is considered in the case where the two ions feel slightly different crystal fields giving different g factors. When the differences Δg between the g factors give a Zeeman difference term ΔgβB0 of the order of magnitude of the interaction between the two ions, the pair spectrum is composed of four lines instead of two: the usual doublet structure, and two additional satellites around the main central transitions. It is shown that for rare earth ions, the shape of the EPR pair spectrum is very sensitive to small g factor differences. This situation is illustrated by the case of neodymium pairs in the SrAl12O19 host.