Xenon can be optically polarized by spin exchange with rubidium. While the rubidium vapor is irradiated with circularly polarized light at the D1 transition, only one of the excited states becomes populated. Collisional mixing, however, leads to scrambling of the populations. Since relaxation occurs with equal probability to the ground state, no net polarization would be observable were it not for the fact that only one of the ground states is continuously depleted.
Rubidium and xenon form van der Waals pairs whose lifetime is sufficiently long for spin-exchange through the hyperfine interaction. Highly nuclear spin-polarized xenon results. When comparing the population of the two nuclear energy levels of 129Xe with the situation in a high field NMR magnet, the huge signal enhancement achieved through optical pumping becomes obvious:
T. G. Walker and W. Happer, Rev. Mod. Phys. 69, 629 (1997).
D. Raftery, H. Long, T. Meersmann, P. J. Grandinetti, L. Reven and A. Pines, Phys. Rev. Lett. 66, 584 (1991).