For the first time, we show that a beam of electrons can change the optical properties of a nanoparticle film through the mechanism of controlling the structural phase coexistence in the nanoparticles. This study is motivated by a desire to understand the exciting physics of phase equilibria in metallic nanoparticles and clusters, and is stimulated by interest in the properties of metallic nanoparticles relating to their potential role in future highly integrated photonic devices, as the active elements of waveguiding and switching structures addressed by an electron beam