Dielectric function and infrared absorption of small metal particles

Using known quantum mechanical models of electrons in a box, a smoothed dielectric function is derived for an envelope taken over the discrete dipole transitions of a small metal particle. This dielectric function deviates strongly from the Drude function at low frequencies (IR) while it merges into the latter at high frequencies (visible and UV). The function is used then to show that it is not the electric but the magnetic dipole absorption due to eddy currents, contained already in the Mie scattering theory, which reproduces the small particle dielectric function in the infrared. The possibilities and limitations of experimental confirmation are discussed.