APPLICATION OF EXTENDED-TIME-EXCITATION TECHNIQUE TO NQR SPECTROSCOPY- AN EXPERIMENTAL INVESTIGATION OF ZEEMAN-PERTURBED SPIN-ECHO-ENVELOPE MODULATION

The extended-time-excitation (ETE) technique has been successfully ada pted to NQR spectroscopy to obtain Zeeman-perturbed spin-echo-envelope modulation (ZSEEM) patterns from Cl-35(I = 3/2) nuclei in powdered sa mples of KClO3 and SbCl3. Soft-pulse excitation has been employed for an extended period followed by a hard pi refocusing pulse. The ZSEEM s ignals follow immediately after the refocusing pulse and last for a pe riod exactly corresponding to the duration of the soft pulse (tau(0)). Optimal power required for the low-level soft pulse, in order to obta in undistorted ZSEEM patterns, has been experimentally established. A comparison of the results from ETE experiments with those obtained fro m the conventional two-pulse echo method, where the echo amplitude is plotted as a function of pulse separation tau, shows that the ZSEEM pa tterns obtained from both these methods agree well in the larger tau r egion. However, it is seen in the ETE results that the initial Zeeman spin-echo modulations immediately following the pi pulse are masked by an intense oscillatory signal which decays with a time constant T-2. The ETE method therefore appears to be particularly suited to compoun ds with shorter T-2. (C) 1994 Academic Press, Inc.