Optical Detection of Nonradiating Alkali Atoms in Solid Helium

We have detected by optical means nonfluorescing ⁸⁵Rb and ⁸⁷Rb atoms implanted in a body centered cubic ⁴He crystal. In contrast to cesium the resonance fluorescence of rubidium is strongly quenched by the helium matrix, and the weak resonance absorption of the two Rb isotopes was detected using a double resonance technique. From a comparative study of the (optically detected) magnetic resonance spectra of ⁸⁵Rb, ⁸⁷Rb, and ¹³³Cs we infer their effective gF factors and conclude that they are not perturbed by the He matrix at a level of 2×10^-4. We show further that optical pumping of Rb proceeds via depopulation, whereas for Cs it proceeds via repopulation.