New Methods for Studying the Orientation Order of Uniaxial Molecular Films on the Base of Optical Data

Abstract
The components ε_(1,2)j(ω) of the dielectric function ε_j(ω) = ε_1j(ω) + iε_2j(ω) for uniaxial molecular film in the region of an isolated absorption band of the light polarized along (j = ||) and across (j = ⊥) the film optical axis were considered. The connection of the components with the orientation order parameter S of the dipole moments of molecular transitions corresponding to a given absorption band was established. New methods for determining S are developed. They are confirmed for the organic semiconductor PTCDA film of nanoscale thickness with the known dependences ε_(1,2)j(ω) in the transparency and low-frequency electron absorption regions. The effect of the orientation order and anisotropy of dynamic dipole-dipole intermolecular interactions (local-field effects) on the maxima position of the ε_2j(ω) bands was shown.

Keywords: molecular thin films, organic semiconductors, PTCDA, orientation order, local-field effects