Discovery of Dumbbell-Shaped ${\mathrm{C}\mathrm{s}}^{*}{\mathrm{H}\mathrm{e}}_n$ Exciplexes in Solid ${}^4\mathrm{H}\mathrm{e}$

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.

Figure 1

Measured emission spectrum (dots) of matrix-isolated Cs atoms in the hcp phase of solid ${}^4\mathrm{H}\mathrm{e}$. The fluorescence emission was observed following resonant $D_2$ excitation of the atoms. For comparison, calculated emission lines (solid curves) of $\mathrm{C}\mathrm{s}(A{\Pi }_{3/2}){\mathrm{H}\mathrm{e}}_2$, $\mathrm{C}\mathrm{s}(A{\Pi }_{1/2}){\mathrm{H}\mathrm{e}}_6$, and $\mathrm{C}\mathrm{s}(A{\Pi }_{1/2}){\mathrm{H}\mathrm{e}}_7$ exciplexes are also presented.

Figure 2

Adiabatic potentials of the ${\mathrm{C}\mathrm{s}}^{*}{\mathrm{H}\mathrm{e}}_2$ system, including the spin-orbit interaction. The two helium atoms are located at $r$ and $-r$ on the quantization (rotationally symmetry) axis, indicated in the pictographs by a solid line. The shape of the electronic density distribution of the cesium atom changes significantly as helium atoms (filled circles) approach.

Figure 3

Adiabatic potentials of the ${\mathrm{C}\mathrm{s}}^{*}{\mathrm{H}\mathrm{e}}_7$ system. The seven helium atoms are located on a ring of radius $r$ concentric with the symmetry axis. Only the $A^2{\Pi }_{1/2}$ potential has a binding attractive well.

Figure 4

Energy dependencies of the $\mathrm{C}\mathrm{s}(A{\Pi }_{1/2}){\mathrm{H}\mathrm{e}}_n$ exciplexes as a function of the number $n$ of helium atoms. Shown are the minimal energies ${ϵ}_{\mathrm{m}\mathrm{i}\mathrm{n}}$ (open circles) of the potential wells, the barrier heights ${ϵ}_{\mathrm{m}\mathrm{a}\mathrm{x}}$ (open squares) and the total zero-point energies ${ϵ}_0$ (filled circles). All energies are given with respect to the dissociation limit, i.e., the electronic energy of the $6P_{1/2}$ state (see Fig. 3). The temperature of the helium matrix corresponds to $\mathrm{k}\mathrm{T}=1.0{\mathrm{c}\mathrm{m}}^{-1}$.