Collisional excitation transfer and quenching in $\mathrm{Rb}\left(5P\right)$-methane mixtures

We have examined fine-structure mixing between the rubidium $5{}^{2}P_{3/2}$ and $5{}^{2}P_{1/2}$ 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 $5{}^{2}P$ 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 $\mathrm{Rb}(5{}^{2}P$) 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 ($5{}^{2}P_{3/2}\to 5{}^{2}P_{1/2}$) of $(4.23\pm 0.13)\times {10}^{-15}{\mathrm{cm}}^2$. 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 $(7.52\pm 0.10)\times {10}^{-19}{\mathrm{cm}}^2$. These results represent a significant increase in precision compared to previous work, and also resolve a discrepancy in previous quenching measurements.

Figure 1

Rubidium energy levels involved in this experiment. Excitation is performed on either the $D_2$ transition at 780 nm or the $D_1$ transition at 795 nm. Collisional excitation transfer between Rb $5P$ states is denoted by $R$, the natural radiative decay by $\gamma$, and quenching by $Q$.

Figure 2

Schematic diagram of the experimental setup. Ultrafast laser pulses excite Rb atoms to the state of interest. Time-correlated single-photon counting is employed to observe the photons emitted due to collisional excitation transfer in time.

Figure 3

The distribution of photons in time observed at 780 nm (open circles) after excitation of Rb at 795 nm with methane gas present at the pressures shown. The time axis is calibrated with respect to the laser pulse, and the solid lines are fits to the data with a functional form given by Eq. (4). The lower plot illustrates the normalized residuals obtained from fitting the 15-Torr ${\mathrm{CH}}_4$ data set.

Figure 4

Pressure dependence of the $\mathrm{Rb}(5{}^{2}P$) mixing rate $R_{21}$ over a range of methane pressures from 2.5 to 25 Torr. This data set was taken with laser excitation at 795 nm and fluorescence detection at 780 nm. A linear fit to the data points (solid line) results in a value of ${\sigma }_{21}=(3.95\pm 0.02)\times {10}^{-15}{\mathrm{cm}}^2$ (statistical error only). Error bars are not shown as they are within the size of the data points.

Figure 5

Comparison of the ${\sigma }_{21}$ cross section determined in this work (red data point at a temperature of 298 K) with the previous determinations given in Table 1.

Figure 6

The distribution of photons in time observed at 795 nm after excitation of Rb at 780 nm with methane gas present at the pressures shown. The upper plot is presented on a semilogarithmic scale with fits to the data (an exponential decay with a constant background) shown as solid lines. The lower plot illustrates the normalized residuals obtained from fitting the 500-Torr ${\mathrm{CH}}_4$ data set.

Figure 7

Pressure dependence of the Rb $5P_{3/2}$ decay rate over a range of methane pressures from 50 to 4000 Torr. This data set was taken with laser excitation at 795 nm and fluorescence detection at 780 nm. A linear fit to the data points (solid line) results in a value of ${\sigma }_Q=(7.60\pm 0.11)\times {10}^{-19}{\mathrm{cm}}^2$ (statistical error only).