Reverse energy pooling in a K-Na mixture

Abstract

We report experimental rate coefficients for the reverse heteronuclear energy-pooling collisions $\mathrm{K}(5D)+\mathrm{Na}(3S)\ensuremath{\rightarrow}\mathrm{K}(4P)+\mathrm{Na}(3P)$ and $\mathrm{K}(7S)+\mathrm{Na}(3S)\ensuremath{\rightarrow}\mathrm{K}(4P)+\mathrm{Na}(3P)$ at thermal energies. Both reactions are exothermic and very high rates were observed showing that reverse exothermic energy-pooling is an order of magnitude more efficient than the corresponding forward endothermic energy-pooling reactions. This is in accordance with the general behavior of the exothermic and endothermic energy-pooling rate coefficients in alkali-metal atoms. In the experiment the potassium atoms were excited in two steps to either the $5D$ or $7S$ state via the $4P$ level using two broadband cw dye lasers. A double-modulation technique has been used to select the fluorescence contributions at the $\mathrm{Na}{(3P}_{J})$ exit channels due only to the above reactions. The ground-state sodium and potassium atom densities were measured by the absorption of lines from a K-Na hollow-cathode lamp. The measured densities and fluorescence intensities have been used to obtain absolute reverse energy-pooling rate coefficients. The contribution to the rate coefficients from other processes are discussed.