Efficiency of Spin Exchange between Rubidium Spins andXe129Nuclei in a Gas

By directly observing the nuclear polarization of ${}_{}{}^{129}{}_{}{}^{}\mathrm{Xe}$, the efficiency $\eta$ of spin exchange between optically pumped Rb spins and ${}_{}{}^{129}{}_{}{}^{}\mathrm{Xe}$ nuclei has been measured. It is found that $\frac{1}{\eta }=23\mathrm{}\pm 4$ rubidium $D_1$ resonance-line photons are required to polarize a ${}_{}{}^{129}{}_{}{}^{}\mathrm{Xe}$ nucleus when long-lived van der Waals molecules are unimportant. The binary spin-exchange cross section deduced from our measurements is ${\sigma }_{\mathrm{ex}}=(7.3\mathrm{}\pm 1.1)\times {10}^{-21\mathrm{}}$ ${\mathrm{cm}}^2$.