Interaction potential energies and quasi-continuum vacuum UV emission of rare-gas — alkali ionic excimers

An ab initio calculation of the potential energies was made for the electronic states of ionic excimer molecules consisting of rare gases (Rg = Ne, Ar, Kr and Xe) and alkali metals (A = K and Rb). The calculation included the ground 1¹Σ⁺ states and the first excited singlet 2¹Σ⁺, which correspond to the Rg(¹S) + A⁺(¹S) and Rg⁺(²P) + A(²S) separate limits, respectively. The calculated results predict that the emission based on transitions from the 2¹Σ⁺(ν'=0) states to the higher ν'' levels of the 1¹Σ⁺ states is characterised by the highest relative transition probabilities, which are promising for lasing in the vacuum and extreme ultraviolet (VUV/XUV) spectral ranges. Moreover, bound — free VUV emission bands were observed for electron-beam-pumped gas mixtures of Xe or Kr with a hot Rb or Cs alkali vapour. The observed four emission bands were assigned to the 2¹Σ⁺ → 1¹Σ⁺ transitions in the rare-gas — alkali ionic excimers Xe⁺Rb, Kr⁺Rb, Xe⁺Cs, and Kr⁺Cs, respectively.