HREELS studies on the electronic structure of oligothiophene films

High resolution electron energy loss spectra (HREELS) were recorded for qua terthiophene (4T) films and compared to those of quinquethiophene (5T) and sexithiophene (6T). Excitonic gap was evaluated to be 2.64+/-0.02 eV. A sec ond threshold appearing in HREELS but not in optical spectra, is assigned t o excitation of charge transfer levels with thresholds of 3.60+/-0.02, 3.24 +/-0.02 and 2.97+/-0.02 eV for 4T, 5T and 6T respectively. Losses between 1 .0 and 2.3 eV in 4T and 5T are assigned to S-0 --> T-n transitions. Resonan ce mechanisms are responsible for induced excitations in the oligothiophene film surfaces.