Surface analysis of oligothiophene films using HREELS: Molecular orientation effects

The surface of evaporated films of alpha-oligothiophenes-quaterthiophene (4 T), quinquethiophene (5T) and sexithiophene (6T)-was studied by high-resolu tion electron energy loss spectroscopy (HREELS), extending the common vibra tional domain of energy loss to electronic excitation and secondary emissio n. 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 gra phite substrates induce films where molecules lie flat on the surface, wher eas gold substrates generate a molecular orientation close to perpendicular ity. Using these two differently oriented samples, we demonstrate the exist ence of two relaxation channels for the electron surface interaction: elect ronic excitation and ionisation, HREELS spectra recorded with incident elec tron energy between 2 and 7 eV reveal that secondary electron emission orig inating from ionisations is always more efficient for escape directions per pendicular to the plane of the molecule. This is here associated with the p i orbital origin of this emission. Copyright (C) 2000 John Wiley & Sons, Lt d.