Metal ion reductions by femtosecond laser pulses with micro-Joule energy and their efficiencies

•This paper describes a part of primary mechanisms of metal ion reductions leading to metal nano-particle (NP) formation by femtosecond laser pulses, even in the laser intensity range of micro-Joule/pulse, where the intensities were around the supercontinuum (SC) generation threshold.•The new findings are the reduction efficiencies of the metal ions 10−3 per incident photon, and major absorbing species of the laser energy was water.•We estimate that the solvated electrons act as a reducing agent and that direct multi-photon dissociation to neutral atoms was unlikely.

Chemical reactions of metal ions in solution were studied using 800 nm, 90 femtosecond laser pulses with a laser intensity range beginning at sub-micro-Joules/pulse, where the intensities were at or just below supercontinuum generation. Products were observed as surface resonance absorptions assignable to Ag, Au nanoparticles (NPs) and light scattering, indicating the formation of Pd NPs. The consumption efficiencies of metal ions from the Au and Pd ions were estimated to be 10−3 per incident photon. The major absorbing species of the laser energy was water, which led to the formation of solvated electrons that acted as a reducing agent of metal ions, while direct multi-photon dissociation to neutral atoms was unlikely. Metal ion Fe3+and Yb3+ systems are also discussed, where Fe3+ to Fe2+ was ascribed to two-photon absorption and Yb3+ to Yb2+ to a reduction followed by solvent ionization.