Elucidating Strong Field Photochemical Reduction Mechanisms of Aqueous [AuCl4](-): Kinetics of Multiphoton Photolysis and Radical-Mediated Reduction.

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.