Surface Plasmon?s in Non Linear Response Function in Metal Optics

If one interpret the surface layer as the selvedge region of the metal half space, differing from the bulk metal only by a reduced density of free electrons, we deal with the hydrodynamic model used to discuss surface electro magnetic fields. The ?local Plasmon? excitation discussed in that context is physically the same thing as a standing plasma wave in the surface layer. Surface resonances of this type are closely related to the ?multiple surface Plasmon?s?