Measurement of the tensor-asymmetry parameter of the electric-field gradient for spin-3/2 nuclei in polycrystalline specimens via Zeeman FT NQR spectroscopy

A knowledge of vQ alone is insufficient to independently determine the two structural parameters of interest, namely the nuclear quadrupole coupling constant e*qQ and the electric-field-gradient tensor-asymmetry parameter 1. Different approaches, with varying degrees of utility, have been presented in the literature (2-12) for obtaining these parameters. In situations where a highly homogeneous RF field of appreciable strength is available over the entire sample volume, Harbison et al. ( 11, 12) have recently demonstrated the elucidation of 4 from an analysis of the two-dimensional zero-field nutation NQR spectral data. Alternatively, when the inhomogeneous NQR linewidth is not very large, a simpler one-dimensional approach for the extraction of 17 involves the application of a Zeeman perturbation to lift the degeneracy of the quadrupole energy levels and a subsequent analysis of the Zeeman spectra (2-9). The signal-to-noise ratios encountered in the Zeeman NQR investigations of polycrystalline materials are often very low and signal averaging is a prerequisite for obtaining highfidelity spectral lineshape. Because of the higher sensitivity and ease with which longterm signal averaging can be effected, transient rather than steady-state techniques are obviously the method of choice for 7 determination. However, in the analysis of the Zeeman FT NQR spectra, one must take into consideration factors such as the receiver dead time and the strength of the RF field in relation to the magnitude of the Zeeman field and the resonance offset.