Discovery of Dumbbell-Shaped Cs^*He_n 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^*He_n exciplexes: an apple-shaped Cs(AΠ_3/2)He₂ and a dumbbell-shaped Cs(AΠ_1/2)He_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 Cs(AΠ_1/2)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.