ELECTRONIC-TRANSITIONS IN ALPHA-OLIGOTHIOPHENE THIN-FILMS - COMPARISON OF ULTRAVIOLET-VISIBLE ABSORPTION-SPECTROSCOPY AND HIGH-RESOLUTION ELECTRON-ENERGY-LOSS SPECTROSCOPY INVESTIGATIONS

Vapor deposited thin films of a series of alpha-oligothiophenes are investigated comparatively with polarized ultraviolet/visible absorption spectroscopy (UV/VIS) and by high resolution electron energy loss spectroscopy (HREELS) in specular reflection geometry. The complementary selection rules of these methods allow an assignment of the observed absorption and loss bands according to a Huckel molecular orbital model. By plotting the transition energies of corresponding bands of different members of the homologous series vs the reciprocal of the number of rings, the development of the one-dimensional ''pi-band-structure'' with an increasing number of rings could be followed. The extrapolation to infinite chain length leads to the electronic properties of an ideal (defect free) polythiophene. Furthermore, characteristic differences were observed in the results obtained from the two methods. The orientation of the molecules in thin films is only detectable with UV/VIS spectroscopy. It is most pronounced for alpha-quinquethiophene. On the other hand, HREELS gives information about the position of optical parity forbidden electronic transitions.