A model for the optical properties of oriented molecular layers

Abstract This contribution is concerned with developing procedures for extracting information of the mean orientations of transition dipole moments of molecules that make up an ordered molecular layer, e.g. a Langmuir-Blodgett (LB) film, from polarized reflectance spectra. Previous methods are based on approximations, among which is the neglect of or at best approximate account of local field effects. Thus in general the orientation of a transition moment for a molecule imbedded in an anisotropic dielectric medium does not coincide with that of a free molecule with exactly the same orientation and confirmation. In this paper the question of local field effects for such oriented molecular layers is addressed, first in a general way, then for a film made up of randomly oriented biaxial domains, each of which is regular two-dimensional array of polarizable species. On neglecting the interlayer contribution to the local field, this leads to a dichroic ratio that is proportional to the well-known function 1 2 tan 2 θ , where θ is the angle between the molecular transition moment and the surface normal, with the proportionality constant being a simple function of the spacing and polarizability of the moieties. The result provides a practical method for calculating θ that should be reasonably accurate for transition moments located on the head groups of many LB films.