Interaction of an Aromatic Molecule with a Surface
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INTERACTION OF AN AROMATIC MOLECULE WITH A SURFACE G. VIDALIa, M.KARIMIb aphysics Department, Syracuse University, Syracuse, N.Y.13244-1130 bPhysics Department, Utica College, Utica, N.Y. 13502
ABSTRACT The interaction between an aromatic molecule and the graphite basal plane is constructed using a Lennard-Jones 6-12 potential. The corrugation of the potential across the surface, the binding energies and vibrational frequencies of the aromatic molecules on the basal plane have been calculated and compared with experimental data. We have also calculated the interaction between a rare-gas atom and an aromatic molecule using two independent models, one based on the pairwise sum of the Lennard-Jones potential and the other on the Effective Medium Theory (EMT) for the repulsive part and a Van der Waals dispersion term for the attractive part. Our results are then compared with available experimental data. Introduction Considerable attention has been devoted lately to the study of the interaction of an aromatic molecule and a graphite surface [1-4]. Different scientific communities are interested in the study of this problem, since the interaction is relevant in processes occurring in coal gasification as well as in the interstellar medium (polyclyclic aromatic hydrocarbon molecules interacting with graphitic dust grains) [1]. Many experimental studies have been done on the interaction (adsorption/desorption, decomposition etc.) of aromatic complexes on carbon or graphitic surfaces. However, most of them were done under poorly characterized surface conditions. Until recently, not much serious theoretical effort has been devoted to the understanding of the interaction at a microscopic level. There are some common features in the interaction of aromatic molecules with surfaces of metals and semimetals: (1) The adsorption of a monolayer of benzene on graphite yields two distinct commensurate phases which differ from one another by a rigid rotation with respect to the graphite lattice [4]; (2) Benzene lies flat on the surface; (3) The overlayer is "smooth"; and (4) vibrational modes (within the benzene molecule) are not greatly influenced by adsorption [5]. In this paper, we would like to report preliminary calculations for benzene and coronene adsorbed on graphite; we also report on the interaction of Van der Waals complexes formed by the adsorption of rare gases on a free standing benzene or coronene molecule. This work is part of an on-going theoretical and experimental effort to understand the interaction of aromatic molecules with surfaces. We chose to calculate the interaction of benzene and coronene with the basal plane of graphite for the following reasons: (1) it is a system of interest to the astrochemical and physico-chemical communities, as explained above; (2) these molecules don't react with graphite; and (3) there are available pair potentials for C-C and C-H interactions [3, 6, 7]. A more technical reason for doing this calculation was the desire to explore the possibility to extend our new model (based esse
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