The quasi-band-structure description of conjugated oligomers

We have investigated momentum dependent excitations in conjugated organic o ligomer materials. Theoretical simulations based on the intermediate neglec t of differential overlap (INDO) approach coupled to a single configuration interaction (SCI) technique considering the proper transition matrix eleme nts for inelastic electron scattering are compared to experimental electron energy-loss spectroscopy (EELS) investigations. In this work we concentrat e on sexiphenyl (6P), sexithienyl (6T) and beta-carotene. 6P and 6T are wid ely used in a number of electronic and optoelectronic devices and together with beta-carotene they can be regarded as model substances for the three b ig classes of conjugated polymer materials, namely phenylenes, thiophenes a nd acetylenes. Both experimental and theoretical results underline the vali dity of the so called quasi-band-structure model for the description of per iodic molecules with only a finite number of repeat units. We also highligh t the influence of chain orientation on EELS spectra and discuss the reason s for certain deviations between molecule-based simulations and the excitat ion processes occurring in the actual molecular crystals upon inelastic ele ctron scattering.