Collisional EPR Frequency Shifts in Cs-Rb-Xe Mixtures

Spin-exchange optical pumping (SEOP) generates large non-thermal nuclear polarizations in certain non-zero-spin noble gases. The collisionally modulated Fermi-contact hyperfine interaction between the alkali-metal valence electron and the noble-gas nucleus is crucial to SEOP physics, which is incompletely understood, especially for heavy noble gases like Xe. One current question is whether Rb, Cs, or a mixture of the two is ideal for SEOP of $^{129}$Xe. The magnetization of hyperpolarized $^{129}$Xe generates a frequency shift in the alkali-metal EPR hyperfine spectrum that is directly proportional to the electron-wavefunction overlap characterized by the enhancement factor $\kappa_0$ [1]. We performed near-simultaneous measurements of the $^{87}$Rb and $^{133}$Cs EPR shifts caused by sudden destruction of $^{129}$Xe hyperpolarization in a ``hybrid” Rb-Cs SEOP vapor cell. Our preliminary result for the shift ratio is about 1.5 to 1.6 at 110 $^{\circ}$C, suggesting that $(\kappa_0)_{\rm CsXe}$ is about 25\% to 35\% larger than $(\kappa_0)_{\rm RbXe}$; the latter has been previously measured to be $493\pm 31$ [2]. [1] S.R. Schaefer, et al., Phys Rev. A {\bf 39}, 5613 (1989). [2] Z.L. Ma, et al., Phys. Rev. Lett. {\bf 106}, 193005 (2011).