Discovery of dumbbell-shaped Cs*Hen exciplexes in solid 4He.

We have observed several new spectral features in the fluorescence of cesium atoms implanted in the hcp phase of solid helium following laser excitation to the 62P states. Based on calculations of the emission spectra using semiempirical Cs-He pair potentials the newly discovered lines can be assigned to the decay of specific Cs*Hen exciplexes: an apple-shaped Cs(APi3/2)He2 and a dumbbell-shaped Cs(APi1/2)Hen exciplex with a well-defined number n of bound helium atoms. While the former has been observed in other environments, it was commonly believed that exciplexes with n>2 might not exist. The calculations suggest Cs(APi1/2)He7 to be the most probable candidate for that exciplex, in which the helium atoms are arranged on a ring around the waist of the dumbbell-shaped electronic density distribution of the cesium atom.

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 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.