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a Nanoperiodic, two-photon fluorescence properties for various Au nanoclusters as a function of cluster sizes b absorbance of Au25 versus wavelength (nm). Note the two-photon absorption cross section (TPA) in c suggests a periodic transition between cluster Au309 and particle Au976. Reprinted with permission from Ramakrishna et al. (2008). Copyright: 2009 American Chemical Society
