Two-photon dichroism studies of molecular-orientational distribution in a nematic liquid crystal

A two-photon dichroism measurement is used for the first time to deduce the second nontrivial moment $〈 P_4 〉$ of the orientational distribution of 4-cyano-4′-pentylbiphenyl (5CB) in the nematic phase. An anisotropic local-field model is developed for the data analysis. The resultant temperature variation of $〈 P_4 〉$ is in fair agreement with that deduced from Raman depolarization measurements. The negative $〈 P_4 〉$ near the nematic-isotropic transition temperature is believed to be due to a strong ordering in the local structure of the medium.