Static Spin Temperature Experiments and the Approach to Thermal Equilibrium in the Rotating Reference Frame
Nuclear magnetic resonance experiments were performed which further verify Redfield's assumption that, for large rf fields H1(ν), a nuclear spin system is properly described by a spin temperature in a reference frame rotating about the Zeeman field with frequency ν. As with similar experiments recently performed by Slichter and Holton, the interpretation of our results does not require any assumption about the spin-lattice relaxation mechanism. The measurements were made on the Na23 spins in NaCl and the F19 spins in CaF2. In both materials the agreement between experiment and theory was very good. From the sodium chloride results it is concluded that quadrupole interactions, caused by strains and imperfections, produce an unobservably small increase in the local field Hl, in the crystals used. Also studied was the rate at which the Na23 spin system in NaCl approaches thermal equilibrium in the rotating frame. In these experiments the magnetization Mz was measured as a function of the duration of a saturating pulse of amplitude 2H1(ν). The measurements were made over a time interval which was much shorter than the spin-lattice relaxation time. Our results provide quantitative verification of a recent calculation by Provotorov, who has shown that when H1<<Hl, Mz(τa) approaches its equilibrium value (in the rotating frame) at a rate determined by a master equation. The master equation derived by Provotorov is in disagreement with the predictions of elementary perturbation theory except when the frequency of the rf field is equal to the Larmor frequency (or when τa is very short).