Nuclear Double Resonance in the Rotating Frame

A double nuclear resonance spectroscopy method is introduced which depends upon effects of magnetic dipole-dipole coupling between two different nuclear species. In solids a minimum detectability of the order of ${10}^{14}$ to ${10}^{16}$ nuclear Bohr magnetons/cc of a rare $b$ nuclear species is predicted, to be measured in terms of the change in a strong signal displayed by an abundant $a$ nuclear species. The $a$ magnetization is first oriented by a strong radio-frequency field in the frame of reference rotating at its Larmor frequency. The $b$ nuclear resonance is obtained simultaneously with a second radio-frequency field; and with the condition that the $a$ and $b$ spins have the same Larmor frequencies in their respective rotating frames, a cross relaxation will occur between the two spin systems. The cross-relaxation interaction, which lasts for the order of a long spin-lattice relaxation time of the $a$ magnetization, is arranged to produce a cumulative demagnetization of the $a$ system when maximum sensitivity is desired. Final observation of the reduced $a$ magnetization indicates the nuclear resonance of the $b$ system. The concepts of uniform spin temperature, when it is valid, and of nonuniform spin temperature where spin diffusion is important, are applied. The density matrix method formulates the double resonance interaction rate in second order. Preliminary tests of the double resonance effect are carried out with a nuclear quadrupole system.