Cross sections for energy transfer in collisions between two excited sodium atoms

We have measured cross sections, ${\sigma }_{\mathrm{nL}}$, for the excitation transfer process $\mathrm{Na}\mathrm{}\left(3P\right)+\mathrm{Na}\mathrm{}\left(3P\right)\mathrm{}\to \mathrm{Na}\mathrm{}\left(3S\right)+\mathrm{Na}\mathrm{}\left(\mathrm{nL}\right)\mathrm{}$, where $\mathrm{nL}$ is the $4D$ or $5S$ level. Our results are ${\sigma }_{4D}=23\mathrm{} {\AA }^2\pm 35\%$ and ${\sigma }_{5S}=16\mathrm{} {\AA }^2\pm 35\%$ at $T\sim 600$ K. To obtain these cross sections we have used pulsed excitation and measured the intensities of $4D$, $5S$, and $3P$ fluorescence emissions, and the spatial distribution of excited atoms resulting from radiation diffusion, as well as the excited atom density as a function of time. Additionally, we have accounted for (time-dependent) radiation trapping of $3P$ and $\mathrm{nL}$ level radiation and for the resulting anisotropies of these fluorescence emissions. Comparisons of our results with theory have been made, and their relevance to other experiments is discussed.