  A spectral technique is applied to evaluate the resonance frequencies of the
full retarded scattering from spherical nanoparticles. This approach allows one
to unambiguously identify the modes that are responsible of both the peaks and
the asymmetric lineshapes of the scattered power spectra. The fundamental
differences between the electromagnetic scattering properties of dielectric and
metal spheres are discussed. These results are used to investigate the
scattering from isolated silicon and silver nanospheres.
