The spin-relaxation time of an optically oriented sodium vapor diffusing in a helium or neon buffer gas has been measured and analyzed by the method of Franzen. The diffusion coefficients for sodium in helium and neon are 1.0\ifmmode\pm\else\textpm\fi{}0.3 and 0.50\ifmmode\pm\else\textpm\fi{}0.17
cm2/sec, respectively. The disorientation cross sections for sodium colliding with helium and neon are (3\ifmmode\pm\else\textpm\fi{}4)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}26}$ and (1.8\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}24}$
cm2, respectively. The radio-frequency (rf) Zeeman transitions of the sodium atoms in the ground state have been examined. An analysis of the effects of spin-exchange collisions between sodium atoms on the intensities and line shapes of the various Zeeman transitions is presented. From these studies a spin exchange cross section of (1-3)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}14}$
cm2 is estimated for two sodium atoms colliding. The main source of error in this measurement is probably lack of knowledge of the sodium vapor pressure in the sodium-absorption cells.
Collisional damping can induce a coupling between two nonresonant light beams propagating in an atomic vapor. This coupling is sufficiently important to induce a gain process for a weak probe beam interacting with an intense pump beam. We present here the first observation of a cw oscillation generated by this process using a cell which contains sodium and helium, the pump beam being slightly detuned from the
D2 resonance line. We also show how the self-oscillation due to four-wave mixing is modified by the presence of a buffer gas.