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Repetitive experiments of one or two-pulse sequences in NQR of spins I=3/2: Liouville space, steady-state, Ernst angle and optimum signal

Abstract : In NMR, the repetition of pulse sequences with a recycle time that does not allow the spin system to completely relax back to equilibrium is a well known and often used method to increase the signal to noise ratio at given total measuring time. For isolated spins I=1/2, the steady-state of a train of strictly identical pulse sequences separated by free evolution periods of same duration is described by the well known Ernst-Anderson model, and the optimum pulse angle is given by the Ernst angle. We showed recently that equivalent formula, but with super-operators in the Liouville space, can be obtained for general spins I. In this article, this formalism is generalized to pure NQR of spins I=3/2, and applied to calculate the signal resulting from single and solid-echo sequences, in the limit when the recycle time T>5T2q, where T2q is the transverse (coherence) quadrupolar relaxation time. In particular, we show that powder samples have a behaviour that is very close to NMR of spins I=1/2. For instance, the generalized Ernst angle βM that maximizes the signal amplitude for a single pulse train is well described by the simple formula cos(1.52βM)≈exp(−T/T1q), whatever the quadrupolar asymmetry parameter η, T1q being the longitudinal (population) quadrupolar relaxation time. Moreover, a simplified NMR-like formula that describes the overall behaviour of nutation curves is proposed, and it is shown that the signal to noise ratio (SNR) at given experimental time is exactly the same as in NMR of spins I=1/2 as a function of recycle time, when properly normalized. Some theoretical predictions for the single pulse and solid-echo sequence were compared to experiments, and validated, by performing 35Cl pure NQR experiment on chloranil (C6Cl4O2 tetrachloro-1,4-benzoquinone) powder.
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Submitted on : Wednesday, May 16, 2018 - 10:13:22 AM
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Christophe Odin. Repetitive experiments of one or two-pulse sequences in NQR of spins I=3/2: Liouville space, steady-state, Ernst angle and optimum signal. Solid State Nuclear Magnetic Resonance, Elsevier, 2017, 85, pp.25-33. ⟨10.1016/j.ssnmr.2017.04.004⟩. ⟨hal-01580454⟩

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