Momentum dependent excitation processes in organic materials

We have developed a model to describe momentum dependent excitation processes in conjugated organic materials based on quantum-chemical techniques including electron-electron correlation effects. The results of our simulations do agree very well with electron energy-loss experiments. We have also been able to show the equivalence between quantum-chemical and solid state physics approaches yielding a band structure like behavior of finite size molecules with a sufficient number of repeat units. This explains the possibility of describing conjugated polymers via band-structure calculations, in spite of the fact that only finite conjugation lengths can be achieved.