Within the framework of the time-dependent local-density approximation and the self-interaction correction, we have estimated the photoionization cross-section and the photoelectron angular asymmetry parameter of C 60 . The latter exhibits large variation around the surface plasmon resonance of the π electrons near 20 eV. Based on a semiclassical electrodynamics model, we show that this behaviour is generic for any surface plasmon resonance in the continuum and is a signature of the correlated motion of the electrons. It is related to the change of sign of the real part of the dynamical polarizability at resonance. Possible experiments to investigate this new plasmon signature are discussed.