Using High-Powered, Frequency-Narrowed Lasers For Rb/129Xe and Cs/129Xe Spin-Exchange Optical Pumping To Achieve Improved Production of Highly Spin-Polarized Xenon For Use In Magnetic Resonance Applications
Date of Award
Doctor of Philosophy
Nuclear magnetic resonance (NMR) spectroscopy has been extensively used to investigate numerous systems of interest, ranging from collections of molecules to living organisms. However, NMR suffers from one key drawback: an inherent lack of detection sensitivity, as compared to other common forms of spectroscopy. This is due to the minute nuclear magnetic moments and low nuclear spin polarization levels at thermal equilibrium (~10-5 to 10-6), and thus necessitates the use of relatively large sample volumes. One way to overcome this low detection sensitivity is to introduce a species with highly non-equilibrium nuclear spin polarization, such as `hyperpolarized' xenon-129. Hyperpolarized xenon can either be used as its own chemical sensor (due to its exquisitely sensitive chemical shift range), or the non-equilibrium polarization may be transferred from xenon to another molecule of interest (such as a protein or inclusion complex). Hyperpolarized xenon is produced through a process known as spin-exchange optical pumping (SEOP), where the angular momentum from resonant, circularly-polarized light is transferred to the electronic spins of an alkali-metal, and is subsequently transferred to the xenon nuclei through gas-phase collisions. While SEOP has been extensively characterized throughout the years, new experimental techniques and emerging technologies have considerably advanced the field in recent years, and may enable a new understanding of the underlying physics of the system. The first five chapters in this dissertation review background information and the principal motivations for this work. Chapter one reviews the basics of NMR, from the various components of the nuclear spin Hamiltonian and different spin-relaxation pathways to the reasons behind the low polarization of nuclear spins at thermal equilibrium and a few alternative methods to `boost' the NMR signal. Chapter two discusses the fundamental aspects of SEOP, including the electronic spin polarization of the alkali-metal, polarization transfer to the xenon nuclei, and different avenues for the spin polarization to be depleted. The third chapter covers the practical considerations of SEOP from the viewpoint of an experimentalist; namely, the experimental differences when using a variety of alkali metals and noble gases, as well as different SEOP apparatuses and experimental parameters. Chapter four details a variety of different light sources that may be used for SEOP; specifically, the use of laser diode arrays (LDAs) are reviewed, including LDAs that have been frequency-narrowed for more efficient light absorption by the alkali metal. The fifth background chapter covers a variety of magnetic resonance applications of hyperpolarized xenon, including molecular biosensors, specific and non-specific binding with proteins, materials studies, and in vivo applications. The sixth chapter is used as an overview of the dissertation research, which is presented in chapters seven through eleven. Chapter seven details the arrangement of the particular SEOP apparatus used in this research, as well as the experimental protocol for producing hyperpolarized xenon. The eighth chapter accounts the implementation and characterization of the first frequency-narrowed LDA used in this research, as well as an equal comparison to a traditional broadband LDA. Chapter nine introduces the use of in situ low-field NMR polarimetry, which was used to distinguish an anomalous dependence of the optimal OP cell temperature on the in-cell xenon density; the low-field set-up is also used to examine the build-up of nuclear spin polarization in the OP cell as it occurs. The tenth chapter covers the use of high power, frequency-narrowed light sources that are spectrally tunable independent of laser power; this allows for the study of changes to the optimal spectral offset as a function of in-cell xenon density, OP cell temperature, and laser power. Xenon polarization build-up curves are also studied to determine if the spectral offset of the laser affects the nuclear spin polarization dynamics within the OP cell. Finally, chapter eleven accounts the use of high power, broadband LDAs to perform SEOP in which cesium is used as the alkali metal; these results demonstrate (for the first time) that the xenon polarization generated by cesium optical pumping can surpass that of rubidium OP under conditions of high laser flux and elevated in-cell xenon densities.
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