Nuclear Quadrupole Resonance*

A resonance phenomenon which is closely related to NMR is nuclear quadrupole resonance (NQR), which was discovered in 1950 by Dehmelt and Kruger. All nuclei with a spin > 1 possess in addition to the magnetic moment, an electric quadrupole moment eQ, which measures the deviation of the distribution of the positive charge in the nucleus from spherical symmetry. While e is the elementary charge, Q = r/(3cos# l)pdr has the dimension cm and is defined to be positive for a cigar-shaped nucleus and negative for a disc-shaped nucleus (p is the charge density per volume, r is the distance of the volume element dr from the origin, and 0 is the angle between the radius vector and the axis of quantization of the spins). In non-cubic crystals and in almost all molecules the nuclei are situated in an inhomogeneous electric field q which is given by the second derivative of the electric potential in a given direction, for instance, the direction z of a chemical bond, qg£ = 5 V/z = V%,Z^ nucleus with a quadrupole moment can orient itself only in certain discrete angles with respect to the field gradient, each of which corresponds to a discrete energy value. The quadrupole coupling constant eqQ is the quantity which is measured directly in an NQR experiment. If the electric field gradient is not axially symmetric with respect to the z-axis, the deviation is given by the asymmetry parameter rf = (Vyy ^XX) ^7JL In NQR magnetic dipole transitions are induced, in analogy to NMR, by a magnetic radiofrequency field which fulfills the resonance condition hv = AEQ between energy levels E, the