After a brief description of a transient grating experiment, we discuss the detection mechanisms involved in such an experiment. We give the equations of motion for a liquid of anisotropic molecules, which include the relaxation effects typical of the supercooled phase and the sources at the origin of the three effects (thermal, electrostrictive and birefringent) producing the transient grating. Two experimental examples are given, one in which the molecular anisotropy is negligible and the second, in which the effect coming from the molecular anisotropy represents up to twenty percent of the totat signal diffracted by the transient grating.
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