Calculation of the electrical-surface (image-plane) position for aluminium

For the material aluminium, there now exist good quantum-mechanical calculations of the charge distribution at various crystal faces, under conditions where an external field exists above the surface. These calculations take the atomic structure of the crystal face properly into account. A result of these calculations is the zero-field position of the electrical surface (image plane) for the face, as defined by the parameter `repulsion distanceʹ. The position of the electrical surface can also be calculated from a classical array model, by making use of a tabulated value for the free-space polarisability of an aluminium atom. Repulsion distances calculated by the two methods are very similar. This lends further credence to the classical model as a simple picture of the physics of a charged surface, in which the surface atoms in the outermost layer are seen as polarisable dipoles.