Spin Physics
Long-lived signals in solids
The NMR spectra of solid-state samples are typically broadened due to strong anisotropic interactions (quadrupolar coupling, chemical shift anisotropy, dipolar couplings). Methods such as magic-angle spinning (MAS) and both hetero- and homonuclear decoupling sequences have been developed to improve resolution. Recently, Khitrin discovered long-lived (hence narrow) signals in solids after application of a low-power radiofrequency (RF) pulse followed by a spin echo or pi-pulse. It was argued that the long-lived signal was a new type of spin echo. Our work demonstrated that the signal was not a new type of echo but was a result of the RF pulse acting like a strong perturbation for transitions between nearly degenerate states as shown to the left (the delay acts similar to the delayed-acquisition described in the Improving Resolution section). We continue to investigate the connection between the long-lived state and long-lived signals under pi-pulse trains or other multiple-pulse sequences, the effects of MAS on this signal, and whether a hole can be burned in dipolar coupled solids.
NonTraditional Controls of Spin Dynamics
Our group also has developed methods for controlling spin dynamics based on what we'd call "non-traditional" controls (traditional controls would be based on chemical shifts, couplings, etc.). We have developed sequences called pathway selective pulses that can excite spin systems only if certain evolution pathways are present (see Improving Resolution) and a sequence called Multiple-Quantum DANTE that excites spins based upon multiple-quantum transition frequency. Our group has also developed methods to selectively suppress a species' magnetization based upon transverse relaxation times (T1 and T2), and also sequences that suppress magnetization based upon the self-diffusion coefficient (see right). We continue to investigate applications of these unusual controls, and also in developing new methods (excitation based on exchange rate constants, cross-relaxation rates, etc.).
