SAMs (self-assembled monolayers) of alkanethiols functionalized by R-terthienyl (3T-SAMs) and R-quaterthienyl (4T-SAMs) were formed on a gold surface presenting (111) terraces and studied by highresolution electron energy loss spectroscopy (HREELS). The angular width of the elastic peak leads to a domain size of ca. 90 Å, comparable to that of the Au(111) terraces of the substrate. Enhancement of the spectral resolution by using a new generation spectrometer enables the improvement of the spectral analysis in the energy range of the vibrational losses. A detailed analysis of the differential cross sections of the different vibrational peaks indicates the nature of their mechanisms: impact mechanisms show that the extreme surface of the monolayers is mainly constituted by thienyl moieties with the thienyl density in 3T-SAMs higher than that in 4T-SAMs; dipolar mechanisms show that molecules are standing up slightly tilted on the substrate; a resonance via a negative ion formation was detected around 4.5 eV. On the other hand, the electronic excitation domain leads to excitonic gap values of 2.62 ( 0.08 and 2.90 ( 0.08 eV for 4T-SAMs and 3T-SAMs, respectively. A sharp feature located at 0.28 ( 0.08 above the electronic threshold was detected in the 3T-SAM spectra. This feature, also appearing in the spectra of the R-oligothiophene evaporated films, is here assigned to a vibronic excitation. Its intensity is always related to the organization degree of the SAMs. An additional band without equivalence in the optical spectra appearing in the 4T-SAM and 3T-SAM HREELS spectra with thresholds at 3.58 ( 0.08 and 4.15 ( 0.08 eV, respectively, is assigned to the electronic gap. The values of the excitonic and electronic gaps are well correlated with those found in the corresponding spectra of evaporated R-quaterthiophene, R-quinquethiophene, and R-sexithiophene films formerly studied. These results lead to the conclusion that thienyl functionalities present strong intermolecular interactions.
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