Interdependence of in-cell xenon density and temperature during Rb/ ¹²⁹Xe spin-exchange optical pumping using VHG-narrowed laser diode arrays

The ¹²⁹Xe nuclear spin polarization (P _Xe) that can be achieved via spin-exchange optical pumping (SEOP) is typically limited at high in-cell xenon densities ([Xe] _cell), due primarily to corresponding reductions in the alkali metal electron spin polarization (e.g. P _Rb) caused by increased non-spin-conserving Rb-Xe collisions. While demonstrating the utility of volume holographic grating (VHG)-narrowed lasers for Rb/ ¹²⁹Xe SEOP, we recently reported [P. Nikolaou et al., JMR 197 (2009) 249] an anomalous dependence of the observed P _Xe on the in-cell xenon partial pressure (p _Xe), wherein P _Xe values were abnormally low at decreased p _Xe, peaked at moderate p _Xe (∼300 torr), and remained surprisingly elevated at relatively high p _Xe values (>1000 torr). Using in situ low-field ¹²⁹Xe NMR, it is shown that the above effects result from an unexpected, inverse relationship between the xenon partial pressure and the optimal cell temperature (T _OPT) for Rb/ ¹²⁹Xe SEOP. This interdependence appears to result directly from changes in the efficiency of one or more components of the Rb/ ¹²⁹Xe SEOP process, and can be exploited to achieve improved P _Xe with relatively high xenon densities measured at high field (including averaged P _Xe values of ∼52%, ∼31%, ∼22%, and ∼11% at 50, 300, 500, and 2000 torr, respectively).