Nuclear spin resonance of 129Xe doped with O2

Spin-lattice relaxation of 129Xe nuclei in solid natural xenon has been investigated in detail over a large range of paramagnetic O2 impurity concentrations. Direct measurements of the ground state magnetic properties of the O2 are difficult because the ESR (electron spin resonance) lines of O2 are rather unstructured, but NMR measurements in the liquid helium temperature region (1.4-4 K) are very sensitive to the effective magnetic moments associated with the spin 1 Zeeman levels of the O2 molecules and to the O2 magnetic relaxation. From these measurements, the value of the D[Sz2 − (1/3)S2] spin-Hamiltonian term of the triplet spin ground state of O2 can be determined. The temperature and magnetic field dependence of the measured paramagnetic O2-induced excess line width of the 129Xe NMR signal agree well with the theoretical model with the spin-Hamiltonian D = 0.19 meV (2.3 K), and with the reasonable assumption that the E[Sx2-Sy2] spin-Hamiltonian term is close to 0 meV. An anomalous temperature dependence between 1.4 K and 4.2 K of the 129Xe spin-lattice relaxation rate, T1n-1(T), is also accounted for by our model. Using an independent determination of the true O2 concentration in the Xe-O2 solid, the effective spin lattice relaxation time (which will be seen to be transition dependent) of the O2 at 2.3 K and 0.96 T is determined to be approximately 1.4 × 10−8 s. The experimental results, taken together with the relaxation model, suggest routes for bringing highly spin-polarized 129Xe from the low temperature condensed phase to higher temperatures without excessive depolarization.