I. NMR studies of environmental photocatalysis. II. High-field optical pumping.

摘要

Part I describes experiments designed to study photocatalytic TiO2 powders with in-situ SSNMR techniques. UV photons are used to initiate the photodecomposition of trichloroethylene, which was monitored with 13C NMR spectroscopy. Packing the SSNMR rotor with TiO2 was ineffective because of light scattering and adsorption by the outer portions of the catalyst. Reaction intermediates were found to migrate to the dark interior and alter the dynamics of the reaction. These dark regions were eliminated by coating the catalyst onto transparent, microscopic optical fibers. Subsequent NMR analysis verified that the fibers are able to deliver photons to the sample interior, and the TiO2 coated fibers acted as efficient photocatalysts. Part II focuses on the production of hyperpolarized Xe from spin exchange with optically pumped Rb atoms. While the current state of the art relies on low magnetic fields for this process, it was of interest to investigate Xe-Rb spin exchange at high field, 47000 G. From variable pressure and temperature studies, the spin exchange efficiency was found to be slightly lower the comparable low field experiments. These results were explained in terms of a relaxation theory developed by Bloembergen, Pound and Purcell. Measurements of the Xe polarization as a function of pressure and temperature were also lower than theoretical predictions made with a low field optical pumping model. The failure of the low field model was attributed to the omission of a high field relaxation mechanism, modulation of the hyperfine interaction.