CHARACTERIZATION OF DIRECTOR FLUCTUATIONS IN LIQUID-CRYSTALS USING THE DIPOLAR CORRELATION EFFECT OF THE STIMULATED ECHO

The purpose of this work is to show that the stimulated-echo pulse seq uence can be employed for the examination of slow collective motions i n ordered systems. In nematics of low molecular weight (4'-n-pentyl-4- cyanobiphenyl) the attenuation of the ratio of the stimulated and prim ary echo amplitudes was measured as a function of the time delay betwe en the first and the second 90 degrees pulses. The attenuation of the echo quotient as well as the atypical (compared to the isotropic phase ) oscillating behavior of the attenuation curves are described by mean s of stochastically time dependent interactions of dipolar coupled pro ton pairs. Slow director fluctuations are considered as a mechanism te nding to average these interactions. Good agreement with the experimen tal data was obtained.