Polarization ofTm169andF19in CaF2:Tm2+by Optical Pumping

Nuclear polarization of ${}_{}{}^{169}{}_{}{}^{}\mathrm{Tm}(I=\frac{1}{2})$ is achieved in a crystal of Ca${\mathrm{F}}_2$ containing a small fraction of paramagnetic ${\mathrm{Tm}}^{2+}$ ions ($S=\frac{1}{2}$) by optically pumping with circularly polarized light in the region 5400-6000 Å, where there is a large magnetic circular dichroism. By optical pumping alone at $T=1.65\mathrm{}$ ° K and in a field $H=750\mathrm{}$ G a nuclear polarization of 9% is observed. If the $I_{+}{S}_{-}$ rf transition is simultaneously saturated, the polarization increases to 18%. The polarization is reversed by changing from left- to right-circularly polarized light. These results are understood in terms of a model in which a large degree (∼90%) of nuclear-spin memory exists in the optical-pumping cycle. The experiment proves the feasibility of significant nuclear polarizations in solids by optical pumping. A small polarization of the abundant ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ nuclei is produced by a three-spin cross-relaxation process with two optically pumped ${}_{}{}^{169}{}_{}{}^{}\mathrm{Tm}_{}^{2+}{}_{}{}^{}$ ions.