Anisotropic relaxation in spin-exchange collisions of alkali atoms

Spin-exchange optical pumping [1] uses spin-exchange collisions between He atoms and optically pumped Rb atoms to produce large quantities of highly spin-polarized He for a variety of applications, including medical imaging [2] and spin-polarized targets [3]. The efficiency of polarized He production is determined by two fundamental rates: the Rb-He spin-exchange rate and the Rb spinrelaxation rate. The measured spin-exchange rates [4] are in fairly good agreement with theory [5]. At the high temperatures needed for the He spin-exchange rates to exceed the wall relaxation rates, the limiting spinrelaxation occurs in collisions between the Rb atoms. While collisions between alkali atoms nominally conserve the spin-polarization, Bhaskar et al. discovered some time ago that rapid spin-relaxation occurs in highdensity optically pumped Cs, with surprisingly large inferred spin-relaxation cross sections in excess of 1 Å [6]. The corresponding cross sections for Rb-Rb relaxation [8,9,4,10], while smaller, still limit the efficiency of He production. The yet smaller cross section for K-K [8,11] relaxation suggests that, if technical difficulties are surmounted, K may be the optimum partner for spinexchange with He [4]. Table I is a summary of the existing data on alkali-alkali spin relaxation. The physical origin of spin-relaxation in collisions between alkali atoms is not understood. The likely source of spin-relaxation in alkali-alkali collisions is the anisotropic spin-spin interaction that couples the electron spin S to the interatomic axis R: