Studies of Perturbation Theory and Spin Temperature by Rotary Saturation of Spins

The relationship between the Bloembergen-Purcell-Pound theory of resonance saturation and the Redfield theory is discussed in terms of the effect of a weak perturbation acting for a long time on a spin system perfectly isolated from the lattice. A simple derivation of Provotorov's rate equations is given, based on Schumacher's concepts. The theoretical ideas are applied to the ${\mathrm{F}}^{19}$ resonance in Ca${\mathrm{F}}_2$ to discuss the phenomenon of rotary saturation, in which one applies a radio-frequency field $H_1$, and modulates the static field at a frequency ${\omega }_a$ such that $\gamma H_1\simeq {\omega }_a$, where $\gamma$ is the nuclear gyromagnetic ratio. When $H_1$ is large compared to the local field, the rotary saturation is described by Redfield's concepts; when $H_1$ is comparable to the local field, the original Bloembergen-Purcell-Pound theory works.