Nuclear spin relaxation of 129Xe due to persistent xenon dimers

We have measured longitudinal nuclear relaxation rates of Xe-129 in Xe-N-2 mixtures at densities below 0.5 amagats in a magnetic field of 8.0 T. We find that intrinsic spin relaxation in this regime is principally due to fluctuations in the intramolecular spin-rotation (SR) and chemical-shift-anisotropy (CSA) interactions, mediated by the formation of Xe-129-Xe persistent dimers. Our results are consistent with previous work done in one case at much lower applied fields where the CSA interaction is negligible and in another case at much higher gas densities where transient xenon dimers mediate the interactions. We have verified that a large applied field suppresses the persistent-dimer mechanism, consistent with standard relaxation theory, allowing us to measure room-temperature gas-phase relaxation times T-1 for Xe-129 greater than 25 h at 8.0 T. These data also yield a maximum possible low-field T-1 for pure xenon gas at room temperature of 5.45 +/- 0.2 h. The coupling strengths for the SR and CSA interactions that we extract are in fair agreement with estimates based both on previous experimental work and on ab initio calculations. Our results have potential implications for the production and storage of large quantities of hyperpolarized Xe-129 for use in various applications.