Discovery of Dumbbell-ShapedCs*HenExciplexes in SolidHe4

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 $6^{2}P$ 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 ${\mathrm{C}\mathrm{s}}^{*}{\mathrm{H}\mathrm{e}}_n$ exciplexes: an apple-shaped $\mathrm{C}\mathrm{s}(A{\Pi }_{3/2}){\mathrm{H}\mathrm{e}}_2$ and a dumbbell-shaped $\mathrm{C}\mathrm{s}(A{\Pi }_{1/2}){\mathrm{H}\mathrm{e}}_n$ 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 $\mathrm{C}\mathrm{s}(A{\Pi }_{1/2}){\mathrm{H}\mathrm{e}}_7$ 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.