Abstract
The surface of evaporated films of α-oligothiophenes—quaterthiophene (4T), quinquethiophene (5T) and sexithiophene (6T)—was studied by high-resolution electron energy loss spectroscopy (HREELS), extending the common vibrational domain of energy loss to electronic excitation and secondary emission. Vibrational spectra show that the molecular orientation is not the same on graphite and gold substrates. HREELS results corroborate those obtained by reflection–absorption infrared spectroscopy (RAIRS), confirming that graphite substrates induce films where molecules lie flat on the surface, whereas gold substrates generate a molecular orientation close to perpendicularity. Using these two differently oriented samples, we demonstrate the existence of two relaxation channels for the electron surface interaction: electronic excitation and ionisation. HREELS spectra recorded with incident electron energy between 2 and 7 eV reveal that secondary electron emission originating from ionisations is always more efficient for escape directions perpendicular to the plane of the molecule. This is here associated with the π orbital origin of this emission. Copyright © 2000 John Wiley & Sons, Ltd.
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