  We study the electromagnetic field scattered by a metallic nanoparticle with
dispersive material parameters in a resonant regime. We consider the particle
placed in a homogeneous medium in a low-frequency regime. We define modes for
the non-Hermitian problem as perturbations of electrostatic modes, and obtain a
modal approximation of the scattered field in the frequency domain. The poles
of the expansion correspond to the eigenvalues of a singular boundary integral
operator and are shown to lie in a bounded region near the origin of the
lower-half complex plane. Finally, we show that this modal representation gives
a very good approximation of the field in the time domain. We present numerical
simulations in two dimensions to corroborate our results.
