NMR PULSED SPIN LOCKING EXPERIMENTS IN THERMOTROPIC LIQUID CRYSTALS

R. H. ACOSTA; R.C. ZAMAR; G.A. MONTI

Praga, República Checa

Congreso; 16th European Experimental Nuclear Magnetic Resonance Conference; 2002

The sequence 900y – (t-qx - t)N known as the Ostroff-Waugh sequence is the simplest many pulse experiment which can be used as a satisfactory model in theoretical analyses of the magnetization under the action of many-pulse experiments. This sequence causes a refocusing of the transverse magnetization after each pulse and thus a persistence of the magnetization  for periods much longer than T2. The long time decay of the magnetization (rate constant T2e) depends strongly on the many body nature of the dipolar spin-spin interactions and on the parameters of the external fields that define a stroboscopic resonance condition . Due to the high orientational molecular order, liquid crystals present residual dipolar interactions within the molecule, that makes it possible to study the dipolar spin interactions applying multipulse sequences.      In this work we analyze the behavior of T2e as a function of the time scale defined by the spacing between the RF pulses in the nematic phase of thermotropic liquid crystal 5CB and in the nematic and smectic A phase of 8CB. Our results are in agreement with the behavior predicted by the existent theory [3] based on the Floquet method, which extends statistical mechanical ideas to systems evolving under a periodic time dependent Hamiltonian. We find a deviation from this behavior when the time scale defined by the action of the pulse sequence is of the same order as the one defined by the local fields.