  Assuming that the resonant surface plasmons on a spherical nanoparticle is
formed by standing waves of two counter-propagating surface plasmon waves along
the surface, by using Mie theory simulation, we find that the dispersions of
surface plasmon resonant modes supported by silver nanospheres match that of
the surface plasmons on a semi-infinite medium-silver interface very well. This
suggests that the resonant surface plasmons of a metal nanosphere can be
treated as a propagating surface plasmon wave.
