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Advances in Pulsed Zero-Field NMR

  • Author(s): Sjolander, Tobias Fredrik
  • Advisor(s): Pines, Alexander
  • et al.

Nuclear Magnetic Resonance (NMR) is ubiquitous as a tool for investigating the structure of both molecules and materials. In most cases the best results are obtained by placing the sample under study in the largest possible magnetic field, since this maximizes both the sensitivity and the resolution of the experiment. This fact underlies the momentous engineering effor that has gone into developing ever larger super-conducting magnets for use in NMR experiments. However, there are many applications of NMR where the sheer

size, not to mention cost, of a high-field instrument can not be accommodated. Therefore there has been a sustained research effort aimed at developing small mobile NMR systems that are not based on super-conducting magnets.

NMR performed in zero-field, using magnetic shielding to screen out the ambient field of the earth, and detected using vapor-cell magnetometers, is one such approach. In zero-field NMR the signal originates entirely in the spin-spin coupling part of the Hamiltonian and chemically informative spectra are obtained even in the absence of chemical-shifts. The very large absolute field homogeneity obtainable at zero-field ensures that the spectral resolution is even better than in high-field spectrometers. Perhaps even more interestingly, the different geometry that results from not having a preferred axis imposed on the system enables experiments that are not possible in the presence of magnetic fields. Examples include the ability to distinguish enantiomers based on the phase of an NMR signal.

This work extends directly detected zero-field NMR beyond pulse-acquire type experiments. New methods for spin control and excitation, as well as two-dimensional spectroscopy and various decoupling techniques are introduced and demonstrated experimentally. For completeness this dissertation also contains a review of zero-field J-spectroscopy, as well as a detailed guide to the relevant instrumentation. It is my hope that the techniques presented herein will find use in future NMR experiments performed in the regime of zero and ultralow field, whether the application is portable chemical analysis, fundamental physics, or anything in between.

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