Differences in vibronic and electronic excitations of PTCDA on Ag(111) and Ag(110)

We have investigated ultrahigh vacuum (UHV)-sublimated films of the model molecular semiconductor 3,4,9,10-perylene-tetracarboxylicacid-dianhydride (PTCDA) on Ag(110) and Ag(111) surfaces by high resolution electron energy loss spectroscopy (HREELS) in the loss energy ranges of both vibrational and electronic transitions for film thicknesses from submonolayers up to multilayers. Marked differences are observed not only between (sub)monolayer and multilayers but also between the monolayers on Ag(111) and Ag(110) which exhibit different layer-structures. With the aid of density-functional-calculations (DFT) of the vibrational modes details of the intermolecular and the molecule-substrate interaction can be derived. Electronic transitions similar to those found by optical spectroscopy are detectable only for molecules beyond the monolayer. In the (sub)monolayer range the electronic transition signature is drastically changed, showing much narrower gaps which, moreover, are different for the two surfaces.