Radiative defects and optical response in oriented para-hexaphenyl films

The optical properties of an ordered para-hexaphenyl film, grown with molec ules lying parallel to the substrate, are investigated. The absorption spec trum exhibits two peaks at 3.1(L) and 4.35(H) eV, featuring a different deg ree of polarization. The photoluminescence emission and the absorption at 3 .1 eV are highly polarized. The presence of the L peak is strongly related to the growth conditions, and is favored at high deposition rates and low s ubstrate temperatures. Upon 4.66 eV ultraviolet irradiation in air, the pho toluminescence intensity and the absorption at 3.1 eV are reduced. Converse ly, transitions at higher energies are almost unaffected by irradiation, Ex perimental data show that the L peak and emission arise from a crystal defe ct. (C) 1999 Elsevier Science B.V. All rights reserved.