The optical absorption of ultrathin organic molecular films: the thickness dependence of the absorption line position

Thin films of copper phthalocyanine and of a perylene derivative have been deposited on NaCl and quartz glass. The effective thickness of these films ranged between approximately one monolayer and 50 nm. The films were prepared by thermal evaporation in ultrahigh vacuum. For ultrathin layers, we detected a blue-shift of the optical absorption bands of about 400 wavenumbers with respect to those of the corresponding bulk material. This shift is nearly the same for phthalocyanine and the perylene derivative. The shape of the absorption pattern is also thickness dependent. Additionally, model calculations have been performed to estimate the classical local field contribution to these shifting effects. The classical contribution was found to be of the order of the experimentally observed effect.