Direct Field Effects in Electron Paramagnetic Resonance Hyperfine Spectra

The electron paramagnetic resonance spectrum exhibiting hyperfine effects from one nucleus is discussed for the case when the nuclear spin interacts appreciably not only with the electron magnetic moment but also directly with the applied magnetic field. The predicted hyperfine structure consists, in general, of 2S (2I+1)2 lines. The angular dependence of the intensities and splittings is investigated; nodes in the splitting are found to be possible even when the eigenvalues of the hyperfine splitting tensor are nonzero. When the direct-field interaction is large, 2I+1 principal hyperfine lines occur, flanked by weaker ``outrider'' lines. Another mechanism for producing outrider lines in high direct fields, the nuclear quadrupole interaction with an electric field gradient, is discussed. Magnetic resonance studies of color centers in germanium-doped quartz demonstrate some of the predicted effects.