Raman depolarization ratios and order parameters of a nematic liquid crystal

The depolarization ratios of the Raman vibrational bands from the terminal C≡N and the central C–C biphenyl link of an oriented nematic liquid crystal [p-n-pentyl-p′-cyanobiphenyl (5CB)] have been measured. The orientational order parameters 〈P2〉 and 〈P4〉 were calculated. The parameter 〈P4〉 remained far less than the mean-field theory prediction although a systematic discrepancy between the order parameters obtained from the two bands was noted. The Raman tensor of C≡N band was found not to be uniaxial, contrary to the previous assumption, and a strong association of molecules is suggested. Problems associated with this technique are discussed.