The dynamic IR spectra of electrically modulated liquid crystalline 5CB (4-pentyl-4'-cyanobiphenyl) are determined to be due to the director reorientation. In the absence of dc fields, the IR response is mainly at two times the modulation frequency. IR signals at the modulation frequency may be induced by application of additional dc fields. These signals at the modulation frequency are caused by the creation of polar orientation due to the static field. No difference in dynamic response has been found between the rigid aromatic core and the pentyl chain of 5CB. Expressions are given for the in-phase and in-quadrature signals at the modulation frequency and at twice its value. It is shown that the frequency-domain dynamic IR experiment mainly probes the transition range between the uniform planar to the homeotropic regions in the sample, The different probing of the sample by time-domain experiments is briefly discussed.

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