"Zeeman" and "Dipolar" Spin Temperatures during a Strong rf Irradiation

Experiments have been performed on a single crystal of Ca${\mathrm{F}}_2$ in a large constant magnetic field (7 kG), irradiated at the exact NMR frequency of ${\mathrm{F}}^{19}$ with transverse rf magnetic fields much larger than the local field. The results demonstrate the existence, in the steady state under very strong irradiation, of two independent quasi-invariants of the motion and of two corresponding temperatures which are the rotating-frame analogs of the usual laboratory-frame Zeeman and dipolar energies and temperatures in the absence of irradiation. Using pulse techniques, Zeeman and dipolar order have been transferred reversibly back and forth between the laboratory frame (without rf field) and the rotating frame (with the large rf field on). No loss of order or mixing between Zeeman and dipolar order has been observed for irradiation times up to 13 msec with a rotating component of the rf field of 25 G (the local field in Ca${\mathrm{F}}_2$ amounts to a few gauss). The rotating-frame analog of the Strombotne-Hahn oscillations has also been observed.