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

We have measured cross sections, σnL, for the excitation transfer process Na(3P)+Na(3P)→Na(3S)+Na(nL), where nL is the $4D$ or $5S$ level. Our results are σ4D=23 ²±35% and σ5S=16 ²±35% at T~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 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.