PHOTOENHANCED VAN-DER-WAALS ATTRACTIVE FORCE OF SMALL METALLIC PARTICLES

The phenomenon of photoinduced coagulation of Au nanocolloids is theoretically studied as due to the photoenhanced van der Waals attractive force. The dielectric function of metals as a function of size of a particle, external field frequency, and the momentum of wavevector derived from a local density functional approach is used to give the polarizability of metal small particles. It is found that a giant electric dipole moment, the magnitude of which directly reflects the size of a particle, is induced by an illumination of the photon with a Mie resonance frequency. From the polarizability of particles and the induced electric dipole thus derived, the interparticle interaction energy is given as a function of the size of a particle. Thus, the effective Hamaker constant (macroscopic interparticle attraction energy) of small particles under illumination is several orders of magnitude higher than in dark conditions, giving rise to instantaneous coagulation of nanometer-sized particles in solution. This phenomenon is suggested to be a characteristic of the mesoscopic scale.