A surface plasma wave (SPW), propagating along a metal surface, embedded with regularly arranged nanoparticles, undergoes surface enhanced Raman scattering from molecules adsorbed over the particles. The enhancement in the scattered signal depends on the shape and dielectric constant of the metallic particles. The scattered signal can be detected above the metal surface as a space wave if the particles have a periodic arrangement with wave number q such that ω R>(k S− q) c, where ω R is the frequency of the scattered signal and k S is the wave number of SPW.