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

The Xe-129 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/Xe-129 SEOP, we recently reported [P. Nikolaou et al., JMR 197 (2009) 2491 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) (similar to 300 torr), and remained surprisingly elevated at relatively high P-Xe values (>1000 torr). Using in situ low-field Xe-129 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/Xe-129 SEOP. This interdependence appears to result directly from changes in the efficiency of one or more components of the Rb/Xe-129 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 similar to 52%, similar to 31%, similar to 22%, and similar to 11% at 50, 300, 500, and 2000 torr, respectively). (C) 2010 Elsevier Inc. All rights reserved.