Mechanism of laser-induced size reduction of gold nanoparticles as studied by single and double laser pulse excitation

Gold nanoparticles with an average diameter of approximate to 11 nm were prepared by laser ablation of a gold metal plate in an SDS aqueous solution. An 80-mu m microdroplet of the solution in diameter was ejected in the atmosphere from a microdroplet nozzle. Structural changes of the nanoparticles in the microdroplet, after they are irradiated with a focused single nanosecond laser pulse at the wavelength of 532 nm, were studied by transmission electron microscopy (TEM) and optical absorption spectroscopy. It was revealed that the gold nanoparticles are fragmented into the smaller particles. In order to investigate the dynamics of fragmentation, the nanoparticles were irradiated with delayed double laser pulses, both of which have an identical wavelength and pulse energy (532 nm, 30 mJ). The average diameter of the product fragments was smallest when the two laser pulses simultaneously irradiated the nanoparticles. The diameter increased with an increase in the delay time from the first to the second laser pulse. The delay-time dependence of the particle size indicates that the fragmentation of the gold nanoparticles is caused by the Coulomb explosion of the multiply charged nanoparticles.