Abstract
Two possible techniques for determining sodium densities in nonhomogeneous optically thick plasmas are investigated. The first one is single‐mode dye‐laser scattering in the near wings of the sodium D1 and D2 resonance lines. The second technique is near‐resonant two‐photon absorption with two counterpropagating single‐mode dye‐laser beams and detection of the fluorescence. In both cases, the laser frequency(ies) is (are) detuned far enough from (the intermediate) resonance to make the plasma optically thin for the laser beam(s) and for the directly scattered light, hence for the one‐step two‐quantum transitions. In the scattering experiment, the polarization properties of the scattered light are used for discrimination against the stepwise absorption‐reemission (the collision‐induced fluorescence). In the two‐photon absorption experiment, the residual Doppler broadening is small enough to make a clear distinction between the direct two‐quantum and the collision‐induced stepwise one‐quantum transitions. It is shown that the near‐wing scattering directly yields local sodium densities. This is not the case for the two‐photon absorption. The technique of near‐resonant scattering is also applicable to other metal vapors in discharges and neutral gases.