Work functions, ionization potentials, and in between: Scaling relations based on the image-charge model

We reexamine a model in which the ionization energy of a metal particle is associated with the work done by the image-charge force in moving the electron from infinity to a small cutoff distance just outside the surface. We show that this model can be compactly, and productively, employed to study the size dependence of electron removal energies over the range encompassing bulk surfaces, finite clusters, and individual atoms. It accounts in a straightforward manner for the empirically known correlation between the atomic ionization potential (IP) and the metal work function (WF), IP/WF∼2. We formulate simple expressions for the model parameters, requiring only a single property (the atomic polarizability or the nearest-neighbor distance) as input. Without any additional adjustable parameters, the model yields both the IP and WF within ∼10% for all metallic elements, simulates the concentration dependence of the WF of regular binary bulk alloys, and matches the size evolution of the IP of finite metal clusters for a large fraction of the experimental data. The parametrization takes advantage of a remarkably constant numerical correlation between the nearest-neighbor distance in a crystal, the cube root of the atomic polarizability, and the image-force cutoff length. The paper also includes an analytical derivation of the relation of the outer radius of a cluster of close-packed spheres to its geometric structure.