Quantum size effects of charge-transfer excitonsin nonpolar molecular organic thin films

We use a quantum-mechanical model to analyze charge-transfer (CT) excitons in closely packed, nonpolar organic molecular crystal thin films grown by the ultrahigh-vacuum process of organic molecular beam deposition. The exciton Hamiltonian includes both polarization effects and the periodic pseudopotential of the crystal. This model takes into account the very large anisotropy characteristic of many organic materials such as the archetype molecular crystal, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and PTCDA-based multilayers. Using a single-exciton Hamiltonian, we quantitatively model experimental electroabsorption data and the absorption spectral shifts observed in ultrathin organic multilayers or ``multiple quantum wells.'' The data analyzed from several such experiments give independent and consistent estimations of the anisotropic effective mass tensor and exciton radii for PTCDA along different crystal axes. This treatment is general, and is found to extend to other CT and Wannier exciton systems found in many interesting, nonpolar organic molecular and inorganic semiconductor crystals such GaAs, suggesting similar physical origins for Wannier and CT excitons in a wide range of materials.