Momentum-dependent excitation processes in crystalline and amorphous filmsof conjugated oligomers

The electronic structure of periodic materials is usually described on the basis of band-structure models, in which each state is not only characteriz ed by its energy but also by the corresponding electron momentum. In this p aper we present investigations of momentum-dependent excitation processes i n a number of molecular crystals and amorphous thin films. For our studies we have chosen ladder-type quinquephenyl (5LP), distyrylbenzene (3PV), a su bstituted quinquephenylenevinylene (5PV), and a bridged quarterthienyl (4TB ). These substances are representative for several classes of conjugated or ganic materials. Their physical properties are dominated by the molecular b uilding blocks. The investigated films, however, also allow us to study dif ferences in the characteristics of crystalline (3PV and 4TB), partly amorph ous (5LP) and fully amorphous (5PV) systems. Momentum-dependent excitations are induced by inelastic electron scattering in electron-energy-loss spect roscopy (EELS) experiments. The experimental data are compared to molecule based post-Hartree-Fock quantum-chemical simulations performed with the int ermediate neglect of differential overlap (INDO) approach coupled to a conf iguration interaction (CI) technique applying the proper momentum-dependent transition matrix elements. Our results show that even in relatively small systems the molecular electronic states can be characterized by an associa ted range in momentum space. In addition, differences between inelastic ele ctron scattering spectra for low values of momentum transfer and the optica l data obtained for the crystalline samples underline the strong impact of light propagation on the absorption characteristics of highly anisotropic c rystalline materials.