Application of extended-time-excitation technique to NQR spectroscopy : an experimental investigation of Zeeman-perturbed spin-echo-envelope modulations

The extended-time-excitation (ETE) technique has been successfully adapted to NQR spectroscopy to obtain Zeeman-perturbed spin-echo-envelope modulation (ZSEEM) patterns from 35Cl(I = 3 2) nuclei in powdered samples of KClO3 and SbCl3. Soft-pulse excitation has been employed for an extended period followed by a hard π refocusing pulse. The ZSEEM signals follow immediately after the refocusing pulse and last for a period exactly corresponding to the duration of the soft pulse (τ0). Optimal power required for the low-level soft pulse, in order to obtain undistorted ZSEEM patterns, has been experimentally established. A comparison of the results from ETE experiments with those obtained from the conventional two-pulse echo method, where the echo amplitude is plotted as a function of pulse separation τ, shows that the ZSEEM patterns obtained from both these methods agree well in the larger τ region. However, it is seen in the ETE results that the initial Zeeman spin-echo modulations immediately following the π 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 compounds with shorter T*2.