I. Quantum-mechanical chemical exchange. II. NMR of semiconductors

Author: Kurur, Narayanan Damodaran

Year: 1992

Degree: Dissertation (Ph.D.)

Advisor: Weitekamp, Daniel P.

Committee Members: Zewail, Ahmed H.; Campbell, Judith L.

Option: Chemistry

DOI: 10.7907/0TQK-G720

Abstract

Part I. Quantum-Mechanical Chemical Exchange:

The requirement that the total wavefunction of a system be antisymmetric to the exchange of identical fermions manifests itself as a scalar coupling between the spin angular momenta. In a class of transition metal trihydrides, this effect is observed as multiplet structure in the liquid state NMR spectrum due to proton tunnelling. Numerical fits to the temperature dependence of these couplings are described as the ensemble averaged tunnel splitting over thermally occupied rovibrational states. The accepted concept of stochastic average of the coupling is questioned on theoretical grounds and an alternative prescription given which differs significantly in its numerical predictions. It is also shown how the fluctuations of the tunnel splitting due to dissipative coupling to the lattice contribute to the chemical rate processes that incoherently exchange nuclei between different sites and the coherent exchange effect. A master equation procedure for evaluating such rates numerically is presented.

Part II. NMR of Semiconductors:

NMR is an inherently low-sensitive technique. Moreover, unless special procedures are employed the structural information obtainable from an NMR line in the solid-state is also limited. Both these problems are addressed here. A method is proposed for the high resolution solid-state NMR of nuclei around paramagnetic defects which uses multiple pulse techniques to selectively average spin couplings and extract structural information. It is shown by numerical simulations how the method could resolve a long-standing controversy on the assignment of hyperfine tensors to silicon sites around a phosphorous dopant. Experimental results on a novel time-domain optical nuclear magnetic resonance method are obtained on gallium arsenide. Comparison with an earlier continuous-wave variant of the technique show improved absolute sensitivity and relative sensitivity to low-abundance sites near an optically relevant defect.

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