  The structural and vibrational properties of metal-organic interfaces have
been examined by means of infrared (IR) absorption spectroscopy and density
functional theory (DFT) with an approach accounting for long-range dispersive
interactions. We focus on a comparative study of the PTCDA monolayer and
bilayer on Ag(111). The equilibrium geometry at the molecule-metal interface
and the infrared spectrum of the chemisorbed monolayer of PTCDA on Ag(111) are
well described by the computations. In the bilayer structure, the presence of a
physisorbed adlayer on top of PTCDA/Ag(111) presents a challenge for DFT. As
previously described for other systems, the polarization of the substrate is
not captured correctly and results in too low energies of frontier molecular
orbitals. This results in an apparent contribution from the vibrations of
second-layer PTCDA to the IR spectrum from interfacial dynamical charge
transfer processes. After removing these peaks with artificially strong
intensity, calculated and experimental data show good agreement and the IR
spectrum can be described as the sum of the spectra of the PTCDA/Ag(111)
contact layer and a physisorbed PTCDA monolayer on top.
