Zeeman perturbed spin echo FT-NQR spectroscopy

An approach for the evaluation of the asymmetry parameter, η, of the electric field gradient tensor for spin I = 3 2 systems from Zeeman NQR spectroscopy of polycrystalline samples is presented. The method relies on Fourier transformation of the second half of the Zeeman-perturbed Hahn spin-echo signal, generated via a two-pulse excitation sequence. Suitable phase cycling of the RF pulses of the spin-echo sequence has been derived and implemented, for the first time in NQR spectroscopy, to retain only the echo component in the detected signal. The RF pulse phase-cycling scheme employed in this work permits the extraction of the parameters of interest with improved sensitivity. A numerical procedure for simulating the powder Zeeman spin-echo FT NQR spectra is also given. A comparison of the theoretically simulated spin-echo spectrum with that recorded experimentally is shown to permit the evaluation of η. The method has been successfully demonstrated for the two 35Cl sites in 2,4,6-trichloro-1,3,5-triazine ("cyanuric chloride").