OFF-RESONANCE EFFECTS IN 2-DIMENSIONAL ZERO-FIELD NUTATION NQR SPECTROSCOPY

Two-dimensional zero-field nutation NQR spectroscopy enables one to determine the quadrupole coupling constant and the asymmetry parameter in a single experiment on powder samples. The method has been originally developed for the conditions which apply to narrow-line solids so that the nutation frequency is much larger than the NQR linewidth. The irradiation must also be on-resonance. In broad-line solids, these conditions are not fulfilled, and consequently the nutation frequency changes over the NQR lineshape with the irradiation offset. The nutation lineshape varies with offset over the NQR spectrum in a complicated way. The rather simple nutation lineshapes, which can be used for the asymmetry parameter eta determination, are obtained only in the on-resonance irradiated parts of the NQR spectrum. Even there, the presence of a finite homogeneous linewidth smears and obscures the characteristic nutation lineshapes, which for narrow homogeneous linewidths exhibit singularities and shoulders. In this paper the theoretical description of the off-resonance effects in the 2D nutation NQR spectroscopy is given and the results are compared to the experiment for a broad-line proton glass Rb-0.98(NH4)(0.02) H2AsO4 at T = 77 K. (C) 1995 Academic Press, Inc.