Collisional EPR Frequency Shifts in Cs-Rb-Xe Mixtures
We report a measurement of the ratio of dimensionless enhancement factors $κ_0$ for Cs-$^{129}$Xe and Rb-$^{129}$Xe in the temperature range $115-140\, ^{\circ}$C; both pairs are used in spin-exchange optical pumping (SEOP) to produce hyperpolarized $^{129}$Xe. $κ_0$ characterizes the amplification of the $^{129}$Xe magnetization contribution to the alkali-metal electronic effective field, compared to the case of a uniform continuous medium in classical magnetostatics. The measurement was carried out in "hybrid" vapor cells containing both Rb and Cs metal in a prescribed ratio, producing approximately the same vapor density for both. Alternating measurements of the optically detected EPR frequency shifts caused by the SEOP polarization and subsequent sudden destruction of the same quantity of $^{129}$Xe magnetization were made for $^{133}$Cs and either $^{87}$Rb or $^{85}$Rb. An important source of systematic error caused by power fluctuations in the pump laser that produced variable light shifts in the EPR frequency was characterized and then mitigated by allowing sufficient warm-up time for the pump laser. We measured $(κ_0)_{\rm CsXe}/(κ_0)_{\rm RbXe} = 1.215 \pm 0.007$ with no apparent temperature dependence. Based on our previous measurement $(κ_0)_{\rm RbXe}=518 \pm 8$, we determine $(κ_0)_{\rm CsXe} = 629 \pm 10$, more precise than but consistent with a previous measurement made by another research group.