Spin-spin cross relaxation and spin-Hamiltonian spectroscopy by optical pumping ofPr3+:LaF3

We report the observation of an anticrossing in solid-state laser spectroscopy produced by cross relaxation. Spin-spin cross relaxation between the ${}_{}{}^{141}{}_{}{}^{}\mathrm{-}$ and ${}_{}{}^{19}{}_{}{}^{}\mathrm{spin}$ reservoirs in ${\mathrm{Pr}}^{3+}$:${\mathrm{LaF}}_3$ and its influence on the ${}_{}{}^{141}{}_{}{}^{}\mathrm{Pr}$ NMR spectrum is detected by means of optical pumping. The technique employed combines optical pumping and hole burning with either external magnetic field sweep or rf resonance saturation in order to produce slow transient changes in resonant laser transmission. At a certain value of the external Zeeman field, where the energy-level splittings of Pr and F spins match, a level repulsion and discontinuity of the ${\mathrm{Pr}}^{3+}$ NMR lines is observed. This effect is interpreted as the ‘‘anticrossing’’ of the combined Pr-F spin-spin reservoir energy states. The Zeeman-quadrupole-Hamiltonian spectrum of the hyperfine optical ground states of ${\mathrm{Pr}}^{3+}$:${\mathrm{LaF}}_3$ is mapped out over a wide range of Zeeman magnetic fields. A new scheme is proposed for dynamic polarization of nuclei by means of optical pumping, based on resonant cross relaxation between rare spins and spin reservoirs.