Adsorbate interactions at organic/metal interfaces

The vibrational and electronic structure of 2-12˚A thick films of pyromellitic dianhydride (PMDA) adsorbed on clean and H2O pre-covered Cu(111) surfaces have been measured at 110K and 295K using high resolution electron energy loss spectroscopy (HREELS) and surface Raman spectroscopy. On both surfaces at low temperature, the adsorption geometry favors a flatlying orientation. Adsorption at 295K results in an upright orientation that is consistent with previous accounts in the literature. The adsorption at 110K leads to the now well documented ring-opening reaction of one of the anhydride groups producing a carboxylate and carbonyl species, whereas, at room temperature CO is lost to the gas phase. The presence of pre-adsorbed H2O at 110K results in a hydrogen bonded complex with the carboxylate group, dramatically altering the electronic structure of the PMDA film. Surface Raman scattering experiments of the H2O-PMDA co-adsorbate complex exhibit resonant scattering behavior that is attributed to the so-called “first layer” or “chemical” SERS effect due to dynamical charge transfer excitations on “atomically smooth” Cu(111) at 110K