Collisional excitation transfer and quenching in Rb(5P) -methane mixtures

Abstract

We have examined fine-structure mixing between the rubidium $5\phantom{\rule{0.16em}{0ex}}^{2}P_{3/2}$ and $5\phantom{\rule{0.16em}{0ex}}^{2}P_{1/2}$ states along with quenching of these states due to collisions with methane gas. Measurements are carried out using ultrafast laser-pulse excitation to populate one of the Rb $5{\phantom{\rule{0.16em}{0ex}}}^{2}P$ states, with the fluorescence produced through collisional excitation transfer observed using time-correlated single-photon counting. Fine-structure mixing rates and quenching rates are determined by the time dependence of this fluorescence. As $\mathrm{Rb}(5\phantom{\rule{0.16em}{0ex}}^{2}P$) collisional excitation transfer is relatively fast in methane gas, measurements were performed at methane pressures of $2.5--25$ Torr, resulting in a collisional transfer cross section ($5\phantom{\rule{0.16em}{0ex}}^{2}P_{3/2}\ensuremath{\rightarrow}5\phantom{\rule{0.16em}{0ex}}^{2}P_{1/2}$) of $(4.23\ifmmode\pm\else\textpm\fi{}0.13)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}$. Quenching rates were found to be much slower and were performed over methane pressures of $50--4000$ Torr, resulting in a quenching cross section of $(7.52\ifmmode\pm\else\textpm\fi{}0.10)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}$. These results represent a significant increase in precision compared to previous work, and also resolve a discrepancy in previous quenching measurements.