The effect of spin-exchange collisions on the state populations obtained in an optically pumped
Na23 vapor is studied in the limit of low light intensity. These state populations and hence the intensity of the rf Zeeman transitions are found theoretically to depend strongly on the spin-exchange cross sections. The various Zeeman rf transitions are found experimentally to be very different in intensity and this is interpreted as being partially due to spin-exchange collisions. From the intensity of the rf transitions in a magnetic field large enough to separate the six Zeeman components and from the observed relaxation time a spin-exchange cross section for two sodium atoms of $\ensuremath{\pi}{{S}_{0}}^{2}=6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$
cm2 is found.
A new method has been designed for obtaining the hyperfine optical pumping of Na vapor. The pumping lamp is placed in a magnetic field and the light emitted in the field direction is used for excitation after crossing a circular polarizer selecting one of the Zeeman lines. This method has two interesting features: (i) The atoms can be pumped either in the lower or in the upper hyperfine sublevel by simply rotating a polarizer by an angle of $\frac{1}{2}\ensuremath{\pi}$; (ii) the hyperfine pumping is obtained using both the
D1 and
D2 resonance lines without filtering losses. The pumping efficiency is good and it has been possible to measure the spin-exchange cross section between sodium atoms by studying the hyperfine relaxation rates as a function of the sodium density. The result is ${\ensuremath{\sigma}}_{\mathrm{ex}}=(1.109\ifmmode\pm\else\textpm\fi{}0.005)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$
cm2.