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

Abstract

We have measured cross sections, ${\ensuremath{\sigma}}_{\mathrm{nL}}$, for the excitation transfer process $\mathrm{Na}(3P)+\mathrm{Na}(3P)\ensuremath{\rightarrow}\mathrm{Na}(3S)+\mathrm{Na}(\mathrm{nL})$, where $\mathrm{nL}$ is the $4D$ or $5S$ level. Our results are ${\ensuremath{\sigma}}_{4D}=23 {\AA{}}^{2}\ifmmode\pm\else\textpm\fi{}35%$ and ${\ensuremath{\sigma}}_{5S}=16 {\AA{}}^{2}\ifmmode\pm\else\textpm\fi{}35%$ at $T\ensuremath{\sim}600$ 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.