Effects of hard but finite pi pulses: From uncontrolled coherence flow to extreme line-narrowing and MRI of solids.

摘要

This doctoral dissertation presents a detailed NMR study of the surprisingly large effects arising from the non-zero duration of strong pi pulses in spin-½ dipolar solids, and the development of new technique for spin coherence control based on our understanding of the finite pulse effects.;When multiple phase-coherent pulses are applied, NMR experiments of various dipolar solids have shown results that conflict with the conventional expectations set by the delta-pulse approximation, even when the pulses are unusually strong. One of the most dramatic results is the observation of either a long-lived echo train, or a fast decay of echoes, depending on the phase of pulses. This phenomenon is referred to as the Pulse Sequence Sensitivity (PSS) in this thesis. With the help of simulations, we demonstrated the importance of the non-zero duration of pulses for this effect. Simulations with N spins (4≤ N≤8) and an inflated dipolar coupling strength can reproduce the PSS observed in experiments. Further simulations with the snapshots of density matrix also indicate that the internal structure of pi pulses (the system's internal Hamiltonian under pi pulses) opens up extra coherence pathways that contribute to the long-lived echo tail in simulations.;Using Average Hamiltonian theory, the leading correction terms arising from the non-zero duration of pulses were identified and their important roles are discussed. Using the zeroth and first-order average Hamiltonian terms, a new class of spin echoes were designed and demonstrated in experiments. The good agreement between our theoretical predictions and the experimental observations indicated that the tiny difference between hard pi pulses and their delta-pulse approximation could be used as a new way for coherence control. Using this new technique, new approaches to extreme line-narrowing (the linewidth a Silicon sample was reduced by a factor of nearly 70,000) and magnetic resonance imaging of solids are presented.