quadrupolarecho

Quadrupole echo is a nuclear magnetic resonance (NMR) technique used to study electric quadrupole interactions in nuclei with spin I greater than 1/2. It produces an echo signal that arises from partial rephasing of quadrupolar-induced dephasing, enabling observation of signals that would otherwise be broadened beyond detection in solids.

In solids, the electric quadrupole moment of a nucleus interacts with the electric field gradient at the site, leading to rapid dephasing of transverse magnetization and broad spectral features. A quadrupole echo sequence applies radiofrequency pulses in a controlled way to refocus this dephasing, yielding an echo at a characteristic time after the pulses. The resulting signal is sensitive to the quadrupolar interaction and provides information about the local environment of the nucleus.

Common implementations use two or more RF pulses with specific phase relations and delays. The echo amplitude and shape depend on the quadrupole coupling constant, C_Q, defined as e^2 q Q / h (where q is the principal component of the electric field gradient and Q is the nuclear quadrupole moment), and on the asymmetry parameter η of the EFG. The technique is often used with solid-state NMR to extract C_Q and η and to gain insight into the symmetry and orientation of the local environment.

Applications include characterizing sites in minerals, ceramics, catalysts, and polymers, where quadrupolar interactions are prominent. Quadrupole echo experiments can be performed with and without magic-angle spinning, and are commonly complemented by other solid-state NMR methods to fully characterize quadrupolar nuclei.

See also: Nuclear magnetic resonance, Quadrupole interaction, Electric field gradient, Central transition, Spin echo, Solid-state NMR.