Observation and Analysis of Incoherent Second-Harmonic Generation in Gold Nanoclusters with Six Atoms

We report the first-hyperpolarizability, beta(-2 omega;omega,omega), of gold nanoclusters Au-6(GSH)(2)(MPA)(2) having six gold atoms capped by 3-mercaptopropionic acid and glutathione (GSH) that is unprecedented. Here, we used the concentration of 2.1 X 10(16) nanoclusters/mL in experiments to determine beta(-2 omega;omega,omega), applying the hyper-Rayleigh scattering technique by using a 1064 nm laser and analyzing the scattered light at 532 nm. The measured hyperpolarizability is found to be beta(-2 omega;omega,omega) = 760 X 10(-30) esu. The static hyperpolarizability, beta(0) = 300 X 10(-30) esu, was determined by using a two-level model approximation. The large beta(-2 omega;omega,omega) is attributed to the quantum confinement effect and the geometry of the nanoclusters that have no inversion symmetry. Computer calculations based on the density functional theory method were performed, and the numerical results present the same order of magnitude than the experimental values.