Pourfathi, M., et al., Propagation of Dynamic Nuclear Polarization across the Xenon Cluster Boundaries: Elucidation of the Spin-Diffusion Bottleneck. J. Magn. Reson., 2013(0).
Earlier dynamic nuclear polarization (DNP) experiments with frozen xenon/1-propanol/trityl mixtures have demonstrated spontaneous formation of pure xenon clusters above 120 K, enabling spectrally-resolved real-time measurements of 129Xe nuclear magnetization in the clusters and in the surrounding radical-rich matrix. A spin-diffusion bottleneck was postulated to explain the peculiar time evolution of 129Xe signals in the clusters as well as the apparent discontinuity of 129Xe polarization across the cluster boundaries. A self-contained abinitio model of nuclear spin diffusion in heterogeneous systems is developed here, incorporating the intrinsic T 1 relaxation towards the temperature-dependent equilibrium along with the spin-diffusion coefficients based on the measured NMR line widths and the known atomic densities in each compartment. This simple model provides the physical basis for the observed spin-diffusion bottleneck and is in a good quantitative agreement with the earlier measurements. A simultaneous fit of the model to the time-dependent NMR data at two different DNP frequencies provides excellent estimates of the cluster size, the intrinsic sample temperature, and 129Xe T 1 constants. The model was also applied to the NMR data acquired during relaxation towards thermal equilibrium after microwaves were turned off to estimate T 1 relaxation time constants inside and outside the clusters. Fitting the model to data during and after DNP provides estimates of cluster size that are in complete agreement.