Application of High Frequency EPR to Integer Spin Systems: Unusual Behavior of the Double-Quantum Line

A high-frequency EPR study is presented of the integer spin system Ni(II) in the host of Zn-tris(ethylenediamine) dinitrate. This complex shows a broad structureless line at X band. At D band (130 GHz) however, a standard triplet powder spectrum is observed. Superimposed on this spectrum is a “double-quantum” line at the expected position. The microwave power dependency of this line is found to be in contradiction with existing theory on multiple-quantum transitions. The line in our spectra shows a single quantum line behavior in intensity and linewidth as a function of the microwave frequency from 35 to 130 GHz. A literature survey leads to the conclusion that most double-quantum features in high-spin systems do not follow theP3/2power dependency predicted by second-order perturbation theory. In our case the double-quantum line is assigned to an enhancement effect due to the simultaneous excitation of two single-quantum transitions.