Molecular motion influence on the NQR spin-lock relaxation

G. FAHRRER; R. H. ACOSTA; D. J. PUSIOL

Minas Gerais, Brasil

Congreso; XXIV Encontro Nacional de Fisica da Materia Condensada; 2001

Wider use of Nitrogen-14 nuclear quadrupole reonance (NQR) as a research tool has long been hampered by the relatively poor signal-to-noise ratio obtained from typical samples. In addition, generally long spin-lattice relaxation times have prevented efficient signal averaging. A modified Carr-Purcell sequence applied on resonance to a pure quadrupole resonance transition in NaNO_2 evokes in response a train of spin echoes which persists for seconds even though the spin-spin relaxation time, T_2, measured is only 3.4 x 10^3sec. The unexpected persistance of the echo train for times much greater than the T2 occurs when T_2, where \tau is the time spacing between the first two pulses of the excitation sequence, and when the Carr-Purcell sequence is modified in the follwing two ways: all the pulses are 90^0 pulses and the first pulse is pahse-shifted by 90^0 with respect to all others.This effective relaxation time, T_{2,eff}, has a strong dependence with \tau.In this work we study the temperature dependence of the spin-lock response for sodiun nitrite.  The dependence of T_{2,eff} on  for the \nu_{-} line of NaNO_2 at 770 K, at this temperature T_1=80sec. and T_2=3ms, fits very well by the relation T_{2,eff}^{-5}, as predicted in the theory. We meassured the dependence of T_{2,eff} at 3000K (T_1= 370ms and T_2 = 3.7ms). The relation obtained, T_{2,eff}^{-2} cannot be explained by the actual theoretical model for NQR spin-lock.  Complete meassurements of the dependence of the spin locked effective relaxation time dependence with  as a function of temperature will be presented for sodium nitrite and the discrimination of different molecular mechanisms influence will be discussed.