ACOSTA RODOLFO HECTOR
Congresos y reuniones científicas
Título:
NMR PULSED SPIN LOCKING EXPERIMENTS IN THERMOTROPIC LIQUID CRYSTALS
Autor/es:
R.C. ZAMAR; R. H. ACOSTA; G.A. MONTI
Lugar:
Caxambú, Brasil
Reunión:
Congreso; XXV Encontro Nacional de Fisica da Materia Condensada; 2002
Resumen:
There is a deep analogy between the behavior of a spin system in continuous external radiofrequency fields and in fields
produced by multi-pulse sequences. The sequence 900
y − − x − − x − ..., known as Ostro -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 particular sequence causes a refocusing of the transverse magnetization
after each pulse and thus a persistence of the magnetization for periods much longer than T2 . In this experiment, the long
time decay of the magnetization (rate constant T2e) and the equilibrium properties of the spin system depend strongly
on the many body nature of the dipolar spin-spin interactions and the parameters of the external fields that define a
stroboscopic resonance condition , first introduced by Provotorov et al.
Historically, multi-pulse techniques have been applied in magnetic resonance in solids, and more recently in Nuclear
Quadrupole Resonance in solids. The Ostro -Waugh sequence was applied in deuterium to thermotropic liquid crystals
and low mass polymers in order to obtain information of the molecular dynamics of the system.
Due to the high orientational molecular order, liquid crystals present residual dipolar interactions within the molecule,
that makes possible to study the dipolar spin interactions applying multi-pulse sequences.
In this work, we analyse the behavior of T2e on the pulse sequence parameters in the nematic and smectic A mesophases
of 5CB and 8CB. Our results are in agreement with previous ones obtained in solids, and confirm the predictions of the
existent theory based on the Floquet method, which extends statistical mechanical ideas to systems evolving under a
periodic time-dependent Hamiltonian.
produced by multi-pulse sequences. The sequence 900
y − − x − − x − ..., known as Ostro -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 particular sequence causes a refocusing of the transverse magnetization
after each pulse and thus a persistence of the magnetization for periods much longer than T2 . In this experiment, the long
time decay of the magnetization (rate constant T2e) and the equilibrium properties of the spin system depend strongly
on the many body nature of the dipolar spin-spin interactions and the parameters of the external fields that define a
stroboscopic resonance condition , first introduced by Provotorov et al.
Historically, multi-pulse techniques have been applied in magnetic resonance in solids, and more recently in Nuclear
Quadrupole Resonance in solids. The Ostro -Waugh sequence was applied in deuterium to thermotropic liquid crystals
and low mass polymers in order to obtain information of the molecular dynamics of the system.
Due to the high orientational molecular order, liquid crystals present residual dipolar interactions within the molecule,
that makes possible to study the dipolar spin interactions applying multi-pulse sequences.
In this work, we analyse the behavior of T2e on the pulse sequence parameters in the nematic and smectic A mesophases
of 5CB and 8CB. Our results are in agreement with previous ones obtained in solids, and confirm the predictions of the
existent theory based on the Floquet method, which extends statistical mechanical ideas to systems evolving under a
periodic time-dependent Hamiltonian.
