Off-resonance effects in ¹⁴N NQR signals from the pulsed spin-locking (PSL) and three-pulse echo sequence; A study for monoclinic TNT

In NQR detection applications signal averaging by the summation of rapidly regenerated signals from multiple pulse sequences of the pulsed spin-locking (PSL) type is often used to improve sensitivity. It is important to characterise and if possible minimise PSL sequence off-resonance effects since they can make it difficult to optimise detection performance. We illustrate this with measurements of the variation of the decay time T_2e and the amplitude of PSL signal trains with pulse spacing and excitation offset frequency for the 870 kHz ν₊ ¹⁴N NQR line of monoclinic TNT under carefully stabilised temperature conditions. We have also carried out a similar study of signals from monoclinic TNT and 1H-1,2,3-triazole generated by a three-pulse echo sequence and the results are shown to agree well with a theoretical treatment appropriate to polycrystalline NQR samples such as TNT for which spin I=1, asymmetry parameter η≠0 and T₁ 蠑 T₂. Based on this theory we derive simple models for calculating TNT PSL signal trains and hence the pulse spacing and off-resonance dependence of signal amplitude and T_2e which we compare to our experimental data. We discuss the influence of PSL echo summation on off-resonance effects in detected signal intensity and show how a phase-alternated multiple pulse sequence can be used in combination with the PSL sequence to eliminate variation in detection performance due to off-resonance effects.