Optical pumping and xenon NMR

Abstract

Nuclear Magnetic Resonance (NMR) spectroscopy of xenon has become an important tool for jnvestigating a wide variety of materials, especially those with high surface area. The sensitivity of its chemical shift to environment, and its chemical inertness . and adsorption properties make xenon a particularly useful NMR probe. This work discusses the application of optical pumping to enhance the sensitivity of xenon NMR " experiments, thereby allowing them to be Used in the study of systems with lower surface area.A novel method of optically-pumping 129Xe in low magnetic field below an NMR spectrometer and subsequent transfer of the gas to high magnetic field is described.NMR studies of the highly polarized gas adsorbed onto powdered samples with low to moderate surface areas are now possible. For instance, NMR studies of optically~ pumped xenon adsorbed onto polyacrylic acid show that xenon has alarge interaction with the surface. By modeling the low temperature data in terms of a sticking probability and the gas phase xenon-xenon interaction, the diffusion coefficient for xenon at the surface of the polymer is determined. The sensitivity enhancement afforded by optical pumping also allows the NMR observation of xenon thin films frozen onto the inner surfaces of different sample cells. The geometry of the thin films results in interesting line shapes that are due to the bulk magnetic susceptibility of xenon.Experiments are also descriped that combine optical pumping with optical detection for high sensitivity in low magnetic field to observe the quadrupolar evolution of 131 Xe spins at the surface of the pumping cell~. In cells with macroscopic asymmetry, a residual quadrupolar interaction causes a splitting in the 131XeNMR frequencies in bare Pyrex glass cells and cells with added hydrogen.Conventional xenon NMR experiments conducted on coadsorbed organic molecules _ in Na-Y zeolites show that the xenon chemical shift is sensitive to the identity and concentration of these guests. Clusters of xenon trapped in Na-A zeolite have interesting statistical distributions due to the finite atomic size of the xenon. The statistics can be described in the low to moderate loading regime by binary and hypergeometric distribution functions.Finally, a comprehensive review of the xenon NMR literature, including previous optical pumping studies, is provided.; ; "