Raman Analysis of Ordered Organic Monolayers on Metal Surfaces

In this paper organic/metal interfaces, especially PTCDA/Ag(111), are analyzed by Raman spectroscopy. Inelastic light scattering allows, in contrast to other methods, the detection of in-plane vibrational modes of PTCDA. The first PTCDA monolayer (ML) shows distinct modification in the vibrational modes due to the direct contact to the Ag surface. The most pronounced effect was observed for the 1310 cm^—1 mode. First principles calculations of the corresponding displacement pattern indicate, that the contact to the Ag surface most likely takes place at the center of the molecule. Furthermore, peak doublets (Davydov split) are observed at thin PTCDA layers (4 ML). We deduced their Raman selection rules, which allow to identify the A- and B-symmetry components. Upon annealing (300–450 K) crystalline reordering was detected by analysis of these peak doublets.