Energy Transfer within a Spin System

The nature of the transfer, within a single nuclear spin system, of energy absorbed from an external source of radio-frequency magnetic field has been investigated by a double-irradiation technique. Energy from a high-power oscillator running at fixed frequency is absorbed by the nuclear spin system. The frequency of a second, low-level oscillator is then swept through the nuclear resonance, sampling the line shape existing in the presence of the strong rf field from the fixed-frequency oscillator. Particular spin systems investigated were the proton system in single crystalline CaS${\mathrm{O}}_4$·2${\mathrm{H}}_2$O, and the ${\mathrm{Al}}^{27}$ system in aluminum metal. In aluminum, the technique gives direct experimental verification of a completely homogeneous saturation behavior, a behavior expected from elementary considerations. It also gives further verification of the Redfield saturation theory. In CaS${\mathrm{O}}_4$·2${\mathrm{H}}_2$O, an enhancement effect is observed which allows one to determine the importance of double-flip spin-lattice relaxation processes. The technique could be usefully applied to many spin systems to determine the degree of inhomogeneity in the resonance line broadening.