A study of the spin–echo spin-locking effect in multi-pulse sequences in 14 N nuclear quadrupole resonance V.T. Mikhaltsevitch * Research Group, QRSciences Limited, 8-10 Hamilton Street, Cannington, WA 6107, Australia Received 18 November 2004; revised 10 March 2005 Available online 24 May 2005 Abstract Experimental evidence of observing a rather unusual spin-locking spin echo (SLSE) effect in the fields of two multi-pulse sequences (u 0 ) x  (s  u x  2s  u x  2s  u x  2s  u x  s) n and (u 0 ) x  (s  u x  2s  u y  s) n in 14 N nuclear quadrupole resonance is presented. It was demonstrated that the SLSE effect is observed only in the even pulse intervals of both sequences. All experiments were carried out at room temperature on a powder sample of NaNO 2 . A theoretical description of the effect is given. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Nuclear quadrupole resonance; Multi-pulse sequence; Spin-locking; Spin–echo; Solid echo 1. Introduction The discovery of the multi-pulse spin-locking spin– echo (SLSE) effect in the nuclear quadrupole resonance (NQR) spectroscopy [1] greatly stimulated the develop- ment of this area of spectroscopy. The possibility pre- sented by the SLSE effect to accumulate multiple signals radically increased the range of compounds that could be studied as it permitted to detect directly even the weak signals in nitrogen-containing substances. The practical significance of this was recently confirmed by a whole new range of NQR applications [2–6]. Therefore, the great interest of researchers in the SLSE effect reflected in numerous publications of both experimental and theoretical nature [1,7–18], is justified. Until now the SLSE effect in NQR has been observed experimentally in the fields of only three multi-pulse sequences: MW-4 [1] ðu 0 Þ y ðs  u x  sÞ n ; ð1Þ MW-2 [7] ðu 0 Þ x ðs  u x  2s  u x  sÞ n ð2Þ and [18] ðu 0 Þ p=4 ðs  u x  2s  u y  2s  u x  2s  u y  sÞ n : ð3Þ (u 0 and u are the flip angles of the preparatory pulse and the other pulses of the sequence, respectively). In the case of the quadrature detection the signals ob- served after all the three sequences do not display any significant differences. This becomes obvious if we con- sider the equations for the spin system magnetisation obtained for each of the three sequences [14,18] which vary only by the value of the resonance offset of the effective field of the sequence. This communication presents data on a rather unu- sual SLSE effect that is generated by sequences of the following type: ðu 0 Þ / ðs  u x  2s  u x  2s  u x  2s  u x  sÞ n ð4Þ 1090-7807/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jmr.2005.03.011 * Fax: +61 8 9351 9522. E-mail address: vassili@qrsciences.com. www.elsevier.com/locate/jmr Journal of Magnetic Resonance 177 (2005) 173–183