I. Ultrasensitive surface NMR using parahydrogen spin labeling. II. High-resolution optical NMR of semiconductor heterostructures using larmor beat detection

• Main Author: Carson, Paul Jonathan
• Format: Others
• Published: 1997
• Online Access: https://thesis.library.caltech.edu/4709/1/Carson_pj_1997.pdf; Carson, Paul Jonathan (1997) I. Ultrasensitive surface NMR using parahydrogen spin labeling. II. High-resolution optical NMR of semiconductor heterostructures using larmor beat detection. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/8382-QF81. https://resolver.caltech.edu/CaltechETD:etd-12022003-155839 <https://resolver.caltech.edu/CaltechETD:etd-12022003-155839>

This dissertation presents two techniques for ultrasensitive high-resolution nuclear magnetic resonance. The first exploits the spin order inherent in spin-symmetry enriched H2 to achieve orders of magnitude improvement in the sensitivity of surface NMR over traditional techniques. The second uses a Larmor beat method to allow real-time optical detection of NMR transients in single epitaxial heterostructures and achieves orders of magnitude improvement in spectral resolution over previous steady-state techniques. Molecular addition of para-enriched hydrogen to sites in which the protons are magnetically inequivalent results in large nonequilibrium spin population differences, detected as enhanced NMR signals in coupled spin systems. This PASADENA technique (parahydrogen and synthesis allow dramatically enhanced nuclear alignment) has previously been applied to liquids, with the potential for improvement in sensitivity of up to four orders of magnitude. The extension of the technique to the solid state is described here. Enhanced spectra of hydrogen chemisorbed on powdered ZnO are observed, revealing a reversible binding site. The spectra show a strong dipolar coupling which is not detected without PASADENA enhancement. This indicates the selectivity of the method for molecular binding and its potential for structural and mechanistic studies. A random comparison of theoretical lineshapes with the data yields clustering of possible interproton distances around 1.8 [angstroms], and 2.3-2.5[angstroms], although other distances are not rigorously excluded. The inverse technique, in which the branching fraction of ortho and para molecules desorbed after spin evolution reports on the surface NMR spectrum, is also discussed. Next, the Larmor beat method for optical detection of NMR in III-V semiconductors is presented. The technique utilizes modulation of the circular polarization of luminescence, via a Hanle effect, at the difference in Larmor precession frequencies of two nuclear species. Order-of-magnitude improvements in both sensitivity and resolution over previous methods for studying single epitaxial structures are observed. High-resolution spectra of a GaAs heterojunction reveal weak quadrupole splittings that report on the electric field gradient at nuclear sites. The spectra also allow spin thermometry, indicating that optically pumped nuclear spin polarizations on the order of 10% are achieved with less than 5 seconds of optical nuclear polarization