Abstract：Energy pooling (EP) was observed in Rb vapor following pulsed optical excitation to the5P1/2 state. The 5P3/2 state was populated by the energy transfer process: Rb(5P1/2)+Rb(5S1/2)→Rb(5P3/2)+Rb(5S1/2). The resulting densities of Rb atoms at the 5P1/2 level were obtained from the absorption of narrow spectral line from a Rb hollow cathode lamp, connecting the 5P1/2 state to 7S state. Since the effective lifetimes of the 5P1/2 and 5P3/2 states are approximately equal, the densities of the 5P3/2 level were obtained from the D2 to D1 fluorescence ratios where D1 and D2 are lines of the 5P1/2→5S1/2 and 5P3/2→5S1/2 transition. Because the time of the fine structure exchanging is much shorter than the lifetime of the 5D state, the fluorescence originating from the 5D state produced by the 5P1/2+5P3/2 and 5P3/2+5P3/2 processes follows the instantaneous production rate of the 5P1/2+5P1/2 process. It is clear that 5P1/2+5P3/2 and 5P3/2+5P3/2 collisions can significantly influence the results obtained for the 5P1/2+5P1/2 rate since the energy defect for 5D state is much smaller for 5P1/2+5P3/2 and 5P3/2+5P3/2 collisions than for 5P1/2+5P1/2 collisions. Effective lifetimes of the 5P levels were calculated using radiation trapping theory. The time-integrated populations and signals were studied and analyzed. The resulting fluorescence included the direct component emitted in the decay of the optically excited 5P1/2 state and the sensitized component arising from the collisions for populating 5D state at different cell temperature. These relative intensities were combined with the measured excited atom densities to yield absolute energy-pooling rate coefficients. The cross sections (in units of 10-14 cm2) for the energy-pooling collisions [i.e., 5P1/2+5P1/2,5P1/2+5P3/2,5P3/2+5P3/2] are 0.78, 2.9 and 3.1, respectively. The dependence of the rates upon energy defect ΔE was examined, but the 5D3/2 level was approximately equally populated in 5P3/2+5P3/2(ΔE=68 cm-1) and 5P3/2+5P1/2(ΔE=306 cm-1) collisions. The 5P1/2+5P3/2 collisions are as efficient as 5P3/2+5P3/2 for populating 5D3/2 state.
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