Emission spectra of alkali-metal (K,Na,Li)-He exciplexes 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 ⁴He atoms [K^*He_n (n=1-6), Na^*He_n (n=1-4), and Li^*He_n (n=1,2)] in cryogenic He gas (the temperature 2K<T<100K) in the spectral range from the atomic D lines to 6300cm^-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 K^*He_n, we found that the infrared emission spectrum of the K atom excited in liquid He was from K^*He₆. 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.