Observation of echoes in reorientation processes of nematic liquid crystals

Fast electrically driven reorientation processes of thermotropic nematic liquid crystals were investigated by means of 1H NMR. The reorientation time is determined by the electric field and can be varied between several hundred milliseconds and less than 50 micros. Although the reorientation of the nematic director is limited to an angular range of 90 degrees, echoes occur for reorientation times below 4 ms. It turned out that the separation of the sidebands is up to two orders of magnitude higher than it would be expected in MAS experiments. This behaviour can be understood in analogy to MAS by means of a time-dependent dipolar Hamiltonian; an exact description of the echo position is given for the heteronuclear dipolar interaction. The homogeneous sample rotation of the MAS experiment must be replaced by a more complex term which describes the director reorientation. Due to the non-linearity of this term, the separation of the sidebands is no longer proportional to the frequency of the sample rotation but can reach up to 20 times the maximum director frequency. The experimental results are presented together with a theoretical interpretation.