Hyperfine Optical Pumping of Sodium Vapors

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 $\frac{1}{2}\pi$; (ii) the hyperfine pumping is obtained using both the $D_1$ and $D_2$ 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 ${\sigma }_{\mathrm{ex}}=(1.109\mathrm{}\pm 0.005)\times {10}^{-14\mathrm{}}$ ${\mathrm{cm}}^2$.