HREELS TO IDENTIFY ELECTRONIC-STRUCTURES OF ORGANIC THIN-FILMS

The electronic structure of alpha-oligothiophene (alpha nT) thin films has been investigated for increasing chain lengths of n = 4-8 thiophe ne units with high resolution electron energy: loss spectroscopy (HREE LS) in the specular reflection geometry at a primary energy of 15 eV. The great advantage of this technique in contrast to UV/VIS absorption spectroscopy results from the fact, that the impact scattering mechan ism of HREELS makes it possible to also detect optically forbidden ele ctronic transitions. On the other hand, the electrons used as probes i n HREELS have a wavelength which is two orders of magnitudes smaller i f compared to those of photons used in UV/VIS absorption spectroscopy. Therefore individual molecules are excited by HREELS independent from each other and hence the excitation of collective excitons is not pos sible. As a result, information about the orientation of the molecules cannot be achieved with HREELS, which, however, is possible in polari zation-dependent UV/VIS spectroscopy.