The negatively charged electrons, while not able to break free of the nucleus, seek to travel against the electric field. What follows is a phenomenon called polarization. The negative and positive charges within the dielectric are rendered neutral because there is a positive charge for every negative. A charge remains on the surface appropriate to the direction of the electric field because atoms on the surface have induced charges that remain unpaired. The following paragraph explains how surface plasmons are formed.
Light passes through a right angle prism until the interface between the metal and prism is reached.
For surface plasmons, the light that reaches the interface must be p-polarized. The p-polarized light polarizes the dielectric. The polarization is alternating because the electric field is also. The alternating electric field causes the electrons in the dielectric to oscillate, producing evanescent waves. The evanescent waves are spatially decaying fields, meaning they cannot propagate.