Free electrons and HO formed in an optical breakdown plasma are found to directly control the kinetics of [AuCl] reduction to form Au nanoparticles (AuNPs) during femtosecond laser-assisted synthesis of AuNPs. The formation rates of both free electrons and HO strongly depend on the energy and duration of the 800 nm laser pulses over the ranges of 10-2400 μJ and 30-1500 fs. By monitoring the conversion of [AuCl] to AuNPs using in situ UV-vis spectroscopy during laser irradiation, the first- and second-order rate constants in the autocatalytic rate law, k and k, were extracted and compared to the computed free electron densities and experimentally measured HO formation rates. For laser pulse energies of 600 μJ and lower at all pulse durations, the first-order rate constant, k, was found to be directly proportional to the theoretically calculated plasma volume, in which the electron density exceeds the threshold value of 1.8 × 10 cm. The second-order rate constant, k, was found to correlate with the measured HO formation rate at all pulse energies and durations, resulting in the empirical relationship k ≈ HO. We have demonstrated that the relative composition of free electrons and HO in the optical breakdown plasma may be controlled by changing the pulse energy and duration, which may make it possible to tune the size and dispersity of AuNPs and other metal nanoparticle products synthesized with femtosecond laser-based methods.

Direct, multiphoton photolysis of aqueous metal complexes is found to play an important role in the formation of nanoparticles in solution by ultrafast laser irradiation. In situ absorption spectroscopy of aqueous [AuCl4](-) reveals two mechanisms of Au(0) nucleation: (1) direct multiphoton photolysis of [AuCl4](-) and (2) radical-mediated reduction of [AuCl4](-) upon multiphoton photolysis of water. Measurement of the reaction kinetics as a function of solution pH reveals zeroth-, first-, and second-order components. The radical-mediated process is found to be zeroth-order in [AuCl4](-) under acidic conditions, where the reaction rate is limited by the production of reactive radical species from water during each laser shot. Multiphoton photolysis is found to be first order in [AuCl4](-) at all pHs, whereas the autocatalytic reaction with H2O2, the photolytic reaction product of water, is second order.

Simultaneous spatiotemporal focusing (SSTF) of femtosecond laser radiation is used to produce gold nanoparticles from aqueous [AuCl 4 ] − solutions. Multiphoton ionization and dissociation of water produces electrons and hydrogen atoms for the reduction of [AuCl 4 ] − to Au(0) during irradiation with the temporally chirped (36 ps) pulse and produces hydrogen peroxide (H 2 O 2 ) as a long-lived reducing agent which persists after irradiation is terminated. Aqueous H 2 O 2 is found to reduce [AuCl 4 ] − , remaining in solution after the laser irradiation is terminated, leading to growth and transformation of the existing Au(0) species. The highly efficient postirradiation reduction of [AuCl 4 ] − to Au(0) by H 2 O 2 is ascribed to reactions occurring on gold nanoparticle surfaces. In the absence of added surfactant, the negatively charged gold particles formed during irradiation are a complex mixture of irregularly shaped and spherical morphologies that are only metastable as aqueous dispersions. These particles become transformed into more perfectly shaped gold crystals, as the remaining [AuCl 4 ] − is reduced in the postirradiation period. The addition of polyethylene glycol (PEG 45 ) accelerates the rate of the [AuCl 4 ] − reduction during laser irradiation and directs the exclusive formation of spherical nanoparticles. Varying the concentration of PEG 45 tunes the diameter and size distribution of the Au nanoparticles formed by laser processing from 3.9 ± 0.7 to 11 ± 2.4 nm.