Vibrational properties of ultrathin PTCDA films on Ag(110)

We have investigated 3,4,9,10-perylene-tetracarboxylic acid-dianhydride (PTCDA) films on the Ag(110) surface by High-resolution electron energy loss (HREEL) and Fourier transform infrared (FTIR) spectroscopy. X-ray photoemission spectroscopy, low-energy electron diffraction (LEED), and quadrupole mass spectroscopy were used to control thickness, composition, and order of the vacuum-sublimated PTCDA films. Taking advantage of the high surface sensitivity of HREELS, it is possible to clearly distinguish molecules in direct binding contact to the surface from those being in a PTCDA environment, because the spectra show differential peak shifts of the vibrational lines for the former molecules. To aid the interpretation of the spectra, discrete Fourier transform calculations of the vibrational frequencies of the free molecule were carried out. The results are in good agreement with the experimental values for the molecules in the multilayer and allow the identification of most of the experimental modes, if experimental information from off-specular HREELS measurements is taken into account. By the comparison of the calculated frequency spectrum with FTIR data it is possible to extract the effects of the intermolecular interaction on the vibrational spectra.