Absorption studies of excimer transitions in Cs-noble-gas and Rb-noble-gas molecules

The new excimer emission bands from laser-excited and discharge-excited alkali-noble-gas systems we recently reported are observed in absorption for Cs and Rb, with noble-gas densities between 1 and 10 amagats. The absorption bands observed correspond to transitions from the unbound $\Sigma$ molecular ground state to bound or free $\Sigma$ and $\Pi$ excited states which are correlated with low-lying $S$ and $D$ excited alkali atoms, so that the corresponding atomic transitions are forbidden. Quantitative reduced absorption coefficients ($k_{\nu }$ divided by the vapor densities) are derived from the observed spectra and have peak values of the order of 4 × ${10}^{-38\mathrm{}}$ ${\mathrm{cm}}^5$ for the heaviest noble gas, xenon. A comparison is made with the spectra that would be expected from the adiabatic potential curves recently calculated by Pascale and Vandeplanque, with the conclusion that they have exaggerated the repulsion in certain excited $\Sigma$ states.