Statistical Theory of Spin Resonance Saturation

In the high-temperature limit for a macroscopic spin system the equivalence of canonical and micro-canonical averages is shown to imply that a diagonal element of any extensive operator is simply related to the corresponding energy eigenvalue. This relation permits a steady-state solution of the density matrix transport equation appropriate to spin resonance saturation in the limit of weak spin-lattice relaxation, strong saturation, and homogeneous broadening. The solution corresponds to a thermal distribution with respect to the transformed Hamiltonian in the rotating coordinate system, as had been conjectured previously.