Spin-exchange optical pumping of ¹²⁹Xe in the low-pressure regime

Hyperpolarized noble gases, produced via spin-exchange optical pumping (SEOP), have many applications including magnetic resonance imaging, magnetometry, and spin-polarized targets. Despite decades of research, several questions remain open in SEOP of ¹²⁹Xe : van der Waals (vdW) molecules (e.g., RbXe) cause the spin-exchange rate coefficient k_se to have a complicated dependence on gas density and composition, both of which affect the distribution of molecular lifetimes. The regime of low total gas density (≤ 0.1 amagat) has not been further explored since initial work by Happer and coworkers almost 40 years ago [1], likely because the narrow D₁ absorption line in this regime is not suited to the high-power broadband diode lasers used for most applications. While k_se is certainly larger at low density due to longer-lived vdW molecules, it has neither been fully characterized in this regime nor compared to alkali-metal spin-destruction rates to determine spin-exchange efficiency. There are also open questions concerning the molecular formation mechanism(s), whether there is an “X-factor” limit to the achievable ¹²⁹Xe polarization, as has been observed with ³He SEOP [2], and the dependence of k_se on magnetic field. We have undertaken a systematic experimental study of k_se focused on this low-density regime employing multiple experimental techniques refined over the last several years, including the frequency-shift of the alkali-metal electron paramagnetic resonance (EPR) to determine the ¹²⁹Xe polarization and direct measurements of alkali-metal density and polarization. We can measure k_se up to three different ways under similar conditions to root out systematic errors and possibly to account for unusual physics such as the X-factor. Refs: [1] X. Zeng, et al., Phys. Rev. A 31, 260 (1985); [2] W.C. Chen, et al., J. Appl. Phys. 116, 014903 (2014).