Effect of Imprisonment of Radiation in Sodium Vapor on the Measured Lifetime of the3 P2States

The effect of imprisonment of radiation in sodium vapor on the measured lifetime of the $3 {}_{}{}^2{}_{}{}^{}P$ states has been investigated in an experiment in which the atoms in the vapor were excited with 10-nsec pulses of resonance radiation and the elapsed times between the exciting and the fluorescent pulses were recorded using a time-to-amplitude converter and kicksorter. At vapor densities below ${10}^{10}$ atoms/${\mathrm{cm}}^3$, where there is no multiple scattering, the measured lifetime reaches a constant value of (1.63±0.04)×${10}^{-8\mathrm{}}$ sec, equal to the natural lifetime of the $3 {}_{}{}^2{}_{}{}^{}P$ states. In the density range ${10}^9$-${10}^{13}$ atoms/${\mathrm{cm}}^3$, the variation of the effective lifetime with the vapor density is in good agreement with the predictions of Milne's theory of radiation trapping; agreement with Holstein's theory is observed only in the range ${10}^{12}$-${10}^{13}$ atoms/${\mathrm{cm}}^3$. The results of a subsidiary Hanle experiment show that a multiply scattered photon of sodium resonance fluorescence loses its original polarization and that, at high sodium vapor pressure, coherence is not preserved.