Static and dynamic properties of a dipole chemisorbed on a metal

Abstract

The interaction of a dipole with a metal surface is of interest in at least two ways. Firstly, it can be of use as a prototypical adsorption problem. The chemisorption of some polar molecules may be described by the chemisorption of an extended dipole. Secondly, it is a starting point in modelling the properties of the solid/solution interface. In the electrochemical literature several such models have been presented for the double layer. Here one can distinguish three approaches. Models using classical electrostatics were the first to be developed. In these the metal is represented by a classical, infinitely conducting plane and the solution by point dipoles or extended dipoles /1–3/. The second approach uses methods from statistical physics, representing the metal by a hard wall and the solution by point dipoles and charges embedded in hard spheres /4–6/. In recent years this has been extended to a more realistic representation of the metal by using the jellium model to allow for the electronic spill over /7,8/. These latter works raise some questions concerning the effects of screening on the adsorbed dipoles and charges. The electronic structure of a dipole interacting with a metal surface has been calculated self-consistently for a model consisting of an extended dipole oriented perpendicular to a jellium surface /9,10/. Strong differences in the screening properties for dipoles of different orientations give rise to asymmetries in both adsorption energy and induced dipole moment. The total dipole moment (i.e. bare plus induced dipole moment) depends on the magnitude of the bare dipole moment, but is almost independent of the construction of the dipole for given bare dipole moment. Near the surface the dipole is strongly screened. Further insight into the screening properties is gained by studying the damping rate of the internal vibrational stretch mode due to electron-hole pair excitations. The size of the damping correlates with that of the induced density of states at the Fermi level and is sensitive to the construction of the bare dipole. This dependence illustrates the importance of the electronic structure close to a chemisorbed polar molecule in this context. The screening has a local character, primarily being due to compensation of the positive dipole charge by an adsorbate-induced resonance localized around it and at the negative one by repulsion of conduction electrons near it. The model should be directly extendable to polar molecules such as LiF and LiH.