By combining experimental and theoretical approaches, we study the adsorption of pentacene on copper as a model for the coupling between aromatic molecules and metal surfaces. Our results for the interface electronic structure are not compatible with a purely physisorption picture, which is conventionally employed for such systems. Nay, we demonstrate electronic mixing between molecular orbitals and metal electronic states.

The formation of long-range ordered organometallic structures has been demonstrated for metal-phthalocyanines (MPcs) with the metal M belonging to the 3d series (M=Fe,Co,Ni,Zn) deposited on the nanostructured template Au(110) by low-energy electron diffraction. The interaction of the molecular macrocycles of the regularly spaced MPc chains in the gold channels stabilizes the planar geometry, producing interface electronic states, as observed by high-resolution ultraviolet photoelectron spectroscopy. The MPc interaction with the substrate is further strengthened for FePc and CoPc molecules, where the central metal d orbitals are directly involved in the interaction process giving rise to interaction states close to the Fermi level.

The C K-edge photoabsorption and 1s core-level photoemission of pentacene (C22H14) free molecules are experimentally measured, and calculated by self-consistent-field and static-exchange approximation ab initio methods. Six nonequivalent C atoms present in the molecule contribute to the C 1s photoemission spectrum. The complex near-edge structures of the carbon K-edge absorption spectrum present two main groups of discrete transitions between 283 and 288eV photon energy, due to absorption to π* virtual orbitals, and broader structures at higher energy, involving σ* virtual orbitals. The sharp absorption structures to the π* empty orbitals lay well below the thresholds for the C 1s ionizations, caused by strong excitonic and localization effects. We can definitely explain the C K-edge absorption spectrum as due to both final (virtual) and initial (core) orbital effects, mainly involving excitations to the two lowest-unoccupied molecular orbitals of π* symmetry, from the six chemically shifted C 1s core orbitals.