The smallest free-electron sphere sustaining multipolar surface plasmon   oscillation
  We study the oscillation frequencies and radiative decay rates of surface plasmon modes of a simple-metal sphere as a function of sphere radius without any assumptions concerning the sphere size. We re-examine within the framework of classical electrodynamics the usual expectations for multipolar plasmon frequency in the so called "low radius limit" of the classical picture. 
K. Kolwas, A. Derkachova and S. Demianiuk