Vibrational properties of ultrathin PTCDA films on Ag(110)

We have investigated 3,4,9,10-perylene-tetracarboxylic acid-dianhydride (PT CDA) films on the Ag(110) surface by High-resolution electron energy loss ( HREEL) and Fourier transform infrared (FTIR) spectroscopy. X-ray photoemiss ion spectroscopy, low-energy electron diffraction (LEED), and quadrupole ma ss spectroscopy were used to control thickness, composition, and order of t he vacuum-sublimated PTCDA films. Taking advantage of the high surface sens itivity of HREELS, it is possible to clearly distinguish molecules in direc t binding contact to the surface from those being in a PTCDA environment, b ecause the spectra show differential peak shifts of the vibrational lines f or 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 exper imental values for the molecules in the multilayer and allow the identifica tion of most of the experimental modes, if experimental information from of f-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.