Dynamical Effects of the Dipolar Field Inhomogeneities in High-Resolution NMR: Spectral Clustering and Instabilities

In high-resolution NMR in liquids, for “large” nuclear spin magnetization (e.g., water at equilibrium in high field, optically polarized ¹²⁹Xe, etc.), the interplay of spatial inhomogeneities in the dipolar field with corresponding inhomogeneities in magnetization has unexpected dynamical effects. Starting from equilibrium, the response to a small pulse no longer directly reflects the weak inhomogeneities of external field (“spectral clustering”), and the instabilities which develop after large pulses lead to macroscopically turbulent spin motion. These effects are discussed using various models, and by reference to experimental results of Nacher and co-workers.