Optical detection of nonradiating alkali atoms in solid helium -: art. no. 123002

We have detected by optical means nonfluorescing Rb-85 and Rb-87 atoms implanted in a body centered cubic He-4 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 Rb-85, Rb-87, and Cs-133 we infer their effective g(F) factors and conclude that they are not perturbed by the He matrix at a level of 2 x 10(-4). We show further that optical pumping of Rb proceeds via depopulation, whereas for Cs it proceeds via repopulation.