Spectrometer for studying NMR and relaxation in the doubly rotating frame

Abstract

A spectrometer is described that ensures observation of NMR and relaxation in the effective field Hc2 acting in the doubly rotating frame (DRF). Unlike conventional NMR spectrometers, the presented apparatus allows the investigation of nuclear spin dynamics and relaxation under conditions of the specially transformed effective multispin dipole-dipole interaction Hamiltonian not studied up to now. The setup is a modified version of the previously described spectrometer for direct recording of NMR in the effective field He1 in the singly rotating frame (RF), all its operating functions being retained. The new version of the apparatus is designed for the study of NMR and, especially, spin relaxation of1H and19F nuclei in solids under the magic-angle conditions in both the RF and the DRF. It enables one to obtain NMR spectra and longitudinal and transverse nuclear relaxation kinetics in the DRF field He2 directly in their final form. In all operating modes, the apparatus is coherent with respect to the NMR frequency in the laboratory frame. In transient modes, it is also coherent with respect to the NMR frequencies in both the RF and the DRF. In all cases the NMR signals are recorded continuously during a single-scan experiment, as a rule not exceeding 1 s. The sensitivity and resolution capability of the spectrometer are superior to those of the starting one. In particular modes, its sensitivity is comparable with that of conventional NMR spectrometers. The apparatus enables one to investigate in detail slow molecular motions in solids with rates ≅γHe2 ∽ 103−105 s−1 and ≅γHe1 ∽ 105−107 s−1; various motion parameters, including multiparticle correlations as well as a motion mechanism, can be extracted from the experimental data.