Nuclear Quadrupole Resonance NQR

the resonance absorption of electromagnetic energy in a crystal owing to transitions between energy levels that are formed as a result of the interaction of nuclei that have an electric quadrupole moment with the crystal electric field. NQR is a particular case of nuclear magnetic resonance (NMR) in crystals. Pure NQR is observed in the absence of a static magnetic field.

The interaction of the quadrupole moment of a nucleus with the inhomogeneous electric field E in a crystal results in the appearance of energy states that correspond to various orientations of the nuclear spin S relative to the crystallographic axes. As in the case of NMR, a radio-frequency magnetic field induces magnetic dipole transitions between the energy states, giving rise to the resonance absorption of electromagnetic energy. Since the quadrupole interaction energy varies over a wide range as a function of the properties of the nucleus and the structure of the crystal, NQR frequencies range from hundreds of kilohertz to thousands of megahertz. The location of the energy levels does not depend on the orientation of the crystal axes with respect to an instrument, making it possible to use polycrystalline specimens. The equipment used to investigate NQR is not fundamentally different from NMR spectrometers.

In the investigation of NQR, measurements made in the absence of a static magnetic field H₀ are supplemented by measurements made in a field H₀. Depending on the relation between the energy of the nuclear quadrupole interaction with the field E and the energy of the magnetic interaction with the field H₀, a distinction is made between quadrupole splitting of NMR lines and Zeeman splitting in NQR.

NQR is used in nuclear physics to determine the quadrupole moments of nuclei. It is also used to investigate the symmetry and structure of crystals, the extent of ordering in macromolecules, and the nature of chemical bonds. Investigations of crystals are based on the relation between crystal structure and the values of gradients of the field E. In the case of NMR, crystal structure governs only the Zeeman-level perturbations that result in line broadening and splitting. However, in the case of NQR, crystal structure specifies the resonance frequencies themselves. A strong dependence of line width on the presence of crystal defects is characteristic of NQR. The measurement of the width of lines makes it possible to investigate internal stresses, the presence of impurities, and ordering in crystals.