FRICTION MODEL OF PHOTOINDUCED REORIENTATION OF OPTICAL-AXIS IN PHOTO-ORIENTED LANGMUIR-BLODGETT-FILMS

A model of the Photo-induced Optical Anisotropy (POA) is proposed for molecular films consisting of ordered domains. The model describes the molecular motion in an anisotropic molecular field at different rates of collisions (friction) between, respectively, excited and ground st ate molecules and a substrate under polarized light illumination into a molecular absorption band. It is shown that if an angular distributi on function of domain molecules has a non-zero width, the different fr iction coefficients would result in domain director rotation. At low l ight intensity, these different frictions could be due to three proces ses: i) change in molecular conformation, ii) change in molecular inte raction and iii) local recrystallisation The domain director rotation is considered as a result of the conservation of the angular momentum for the whole system, which includes the substrate. A new method, base d on double illumination of photo-oriented films, is proposed to obser ve the light-induced optical axis rotation. Experimental results conce rning photo-induced rotation of the optical axis in photo-oriented Lan gmuir-Blodgett films are presented and explained by the model. The ang ular dependence of the photo-induced angular torque is confirmed. The existence of an angular threshold for out-of-plane rotation (theta-rot ation) is demonstrated and qualitatively explained by anchoring of mol ecular layers with the substrate.