ENHANCED INFORMATION RECOVERY IN 2D ON-RESONANCE AND OFF-RESONANCE NUTATION NQR USING THE MAXIMUM-ENTROPY METHOD

Two-dimensional zero-field nutation NQR spectroscopy has been used to determine the full quadrupolar tensor of spin -3/2 nuclei in serveral molecular crystals containing the Cl-35 and As-75 nuclei. The problems of reconstructing 2D-nutation NQR spectra using conventional methods and the advantages of using implementation of the maximum entropy meth od (MEM) are analyzed. It is shown that the replacement of conventiona l Fourier transform by an alternative data processing by MEM in 2D NQR spectroscopy leads to sensitivity improvement, reduction of instrumen tal artefacts and truncation errors, shortened data acquisition times and suppression of noise, while at the same time increasing the resolu tion. The effects of off-resonance irradiation in nutation experiments are demonstrated both experimentally and theoretically. It is shown t hat off-resonance nutation spectroscopy is a useful extension of the c onventional on-resonance experiments, thus facilitating the determinat ion of asymmetry parameters in multiple spectrum. The theoretical desc ription of the off-resonance effects in 2D nutation NQR spectroscopy i s given, and general exact formulas for the asymmetry parameter are ob tained. In off-resonance conditions, the resolution of the nutation NQ R spectrum decreases with the spectrometer offset. However, an enhance d resolution can be achieved by using the maximum entropy method in 2D -data reconstruction.