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

Two-dimensional zero-field nutation NQR spectroscopy enables one to de termine the quadrupole coupling constant and the asymmetry parameter i n a single experiment on powder samples. The method has been originall y developed for the conditions which apply to narrow-line solids so th at the nutation frequency is much larger than the NQR linewidth. The i rradiation must also be on-resonance. In broad-line solids, these cond itions are not fulfilled, and consequently the nutation frequency chan ges over the NQR lineshape with the irradiation offset. The nutation l ineshape varies with offset over the NQR spectrum in a complicated way . The rather simple nutation lineshapes, which can be used for the asy mmetry parameter eta determination, are obtained only in the on-resona nce 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 s ingularities and shoulders. In this paper the theoretical description of the off-resonance effects in the 2D nutation NQR spectroscopy is gi ven and the results are compared to the experiment for a broad-line pr oton glass Rb-0.98(NH4)(0.02) H2AsO4 at T = 77 K. (C) 1995 Academic Pr ess, Inc.