Nanosecond pulsed laser ablation of gold with an excitation wavelength of 532 nm was conducted in supercritical CO2 to generate gold nanoparticles, which were then investigated by scanning electron microscopy and small-angle X-ray scattering, and their extinction spectra and simulated extinction spectra were studied. Both the morphology and amount of gold nanoparticles changed significantly with changes in the density of supercritical CO2 during laser ablation. In a gaslike density, a network structure consisting of nanonecklaces was the major product, whereas in a liquidlike density, large nanospheres with an average diameter (⟨D⟩) of 500 nm were produced. After absorption of multiphoton of excitation light, the gold nanonecklaces and large nanospheres were generated by the fragmentation and solidification, respectively, of liquid gold droplets with ⟨D⟩ = 500 nm. The amount of both products changed according to the branching ratio, which determined whether the liquid gold droplets followed the fragmentation or solidification channel. The local structure of supercritical CO2 in the vicinity of the gold nanoparticles determined the preferred reaction channel. A significant change in the branching ratio occurred near the density ρr = 0.7, where both the enhancement of the local density of supercritical CO2 and the degree of solvation of fluid molecules around the gold nanoparticles reached a maximum. To the best of our knowledge, this is the first study to observe the density dependence of morphological changes in gold nanoparticles fabricated by laser ablation in a supercritical fluid and the local structure of the supercritical fluid that determines the morphology and amount of nanoparticles.

We study the mixing schemes or the molecular processes occurring in aqueous acetonitrile (ACN) and acetone (ACT) by near-infrared spectroscopy (NIR). Both solutions (any other aqueous solutions) are not free from strong and complex intermolecular interactions. To tackle such a many-body problem, we first use the concept of the excess molar absorptivity, epsilonE, which is a function of solute mole fraction in addition to that of wavenumber, nu. The plots of epsilonE calculated from NIR spectra for both aqueous solutions against nu showed two clearly separated bands at 5020 and 5230 cm(-1); the former showed negative and the latter positive peaks. At zero and unity mole fractions of solute, epsilonE is identically zero independent of nu. Similar to the thermodynamic excess functions, both negative and positive bands grow in size from zero to the minimum (or the maximum) and back to zero, as the mole fraction varies from 0 to 1. Since the negative band's nu-locus coincides with the NIR spectrum of ice, and the positive with that of liquid H(2)O, we suggest that on addition of solute the "ice-likeness" decreases and the "liquid-likeness" increases, reminiscent of the two-mixture model for liquid H(2)O. The modes of these variations, however, are qualitatively different between ACN-H(2)O and ACT-H(2)O. The former ACN is known to act as a hydrophobe and ACT as a hydrophile from our previous thermodynamic studies. To see the difference more clearly, we introduced and calculated the excess partial molar absorptivity of ACN and ACT, epsilon(E)(N) and epsilon(E)(T), respectively. The mole fraction dependences of epsilon(E)(N) and epsilon(E)(T) show qualitatively different behavior and are consistent with the detailed mixing schemes elucidated by our earlier differential thermodynamic studies. Furthermore, we found in the H(2)O-rich region that the effect of hydrophobic ACN is acted on the negative band at 5020 cm(-1), while that of hydrophilic ACT is on the positive high-energy band. Thus, the present method of analysis adds more detailed insight into the difference between a hydrophobe and a hydrophile in their effects on H(2)O.