Emission spectra of alkali-metal(K,Na,Li)−Heexciplexes in cold helium gas

We have observed emission spectra of excimers and exciplexes composed of a light alkali-metal atom in the first excited state and ${}^4\mathrm{He}$ atoms [${\mathrm{K}}^{*}{\mathrm{He}}_n$ $(n=1\mathrm{}-6),$ ${\mathrm{Na}}^{*}{\mathrm{He}}_n$ $(n=1\mathrm{}-4),$ and ${\mathrm{Li}}^{*}{\mathrm{He}}_n$ $\mathrm{}(n=1,2\mathrm{})\mathrm{}]$ in cryogenic He gas (the temperature $2\mathrm{K}<T<\mathrm{}100\mathrm{}\mathrm{K})$ in the spectral range from the atomic D lines to $6300{\mathrm{cm}}^{-1}.$ Differently from exciplexes with heavier alkali-metal atoms, the spectra for the different number of He atoms were well separated, so that their assignment could be made experimentally. Comparing with the spectra of ${\mathrm{K}}^{*}{\mathrm{He}}_n,$ we found that the infrared emission spectrum of the K atom excited in liquid He was from ${\mathrm{K}}^{*}{\mathrm{He}}_6.$ To confirm the assignment, we have also carried out ab initio calculation of adiabatic potential curves and peak positions of the emission spectra of the exciplexes.