Characterization of the electronic excitations in Alq3 using electron energy-loss spectroscopy

We have studied the electronic excitations of tris-(8-hydroxyquinoline)-aluminium(III) (Alq(3)) thin films using electron energy-loss spectroscopy in transmission. This allowed us to derive an effective dielectric function of such films in a large energy range. Moreover, an analysis of the momentum dependence of the lowest lying excitation allowed us to gain information on its localization. We show that this excitation does not disperse, i.e., is localized in the condensed phase. In contrast to many other molecular organic semiconductors, its spectral weight does not follow the behavior of a simple dipole-allowed electronic transition.