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Colloidal gold nanoparticles having an average diameter of 8 nm were prepared by laser ablation at 1064 nm of a gold metal plate in an aqueous solution of sodium dodecyl sulfate (SDS) and were subjected to fragmentation under irradiation of a pulsed laser at 532 nm. Gold clusters exhibiting no significant optical absorption in the visible wavelength region were produced in a solution, together with nanoparticles. UV−visible optical absorption spectroscopy revealed that the gold clusters grow gradually through attachment to the nanoparticles and through mutual aggregation. The growth processes depend crucially on the concentration of SDS in the aqueous solution of nanoparticles.

A continuous liquid flow in a vacuum (liquid beam) of an aqueous solution of phenol was irradiated with an IR laser at 2.92 μm, and species from the liquid beam surface were ionized by a UV laser at 266 nm and mass-analyzed by a time-of-flight mass spectrometer. The mass spectra showed that the product ions were solvated phenol cluster ions, Phenol⁺ (H2O)m (m = 0−4). The mechanism of the cluster ion formation was investigated by measuring the abundances of the product ions as a function of the IR laser power, the UV laser power, and the delay time between an IR laser pulse and the subsequent UV laser pulse. It was concluded that phenol and its hydrated clusters are isolated within several microseconds from the liquid beam due to selective excitation of the solvent water molecules under irradiation of an IR laser having a fluence of less than 1100 mJ pulse^-1 cm^-2.

Gold nanoparticles having an average diameter of ~8 nm were prepared in an aqueous solution of sodium dodecyl sulfate by laser ablation at 1064 nm of a gold metal plate and were irradiated by a laser at 532 nm for size-reduction of the gold nanoparticles produced by laser ablation. The diameters of the gold nanoparticles thus produced were measured directly by electron microscopy, while the average diameter was estimated from the optical absorption spectrum of the solution containing the nanoparticles with the aid of the Drude theory of a conducting droplet. The average diameter of the nanoparticles was found to decrease toward the smallest possible diameter as the laser shot increases; in this condition, the resulting nanoparticles have comparable diameters. In addition, the smallest possible diameter was found to decrease with the laser fluence. It was also shown that aggregation of the nanoparticles is not negligible when the laser fluence is high. In conclusion, nanoparticles with an average diameter was pulverized into smaller nanoparticles with a desired average diameter and a narrow distribution by a proper selection of the laser fluence and the laser shots. The mechanism of the particle pulverization was proposed.

Gold nanoparticles were produced by laser ablation of a gold metal plate in an aqueous solution of sodium dodecyl sulfate. The absorption spectrum of the gold nanoparticles was essentially same as that of gold nanoparticles chemically prepared in a solution. The size distribution of the nanoparticles thus produced was measured by an electron microscope and was found to shift to a smaller size with an increase in surfactant concentration. This behavior is explained in terms of the dynamic formation model. Dependence of the nanoparticle abundance on surfactant concentration in the solution shows that stable gold nanoparticles tend to be formed as the surfactant concentration exceeds 10^-5 M. The gold nanoparticles having diameters larger than 5 nm were pulverized into those having diameters of 1−5 nm by a 532-nm laser.

Silver nanoparticles were produced by laser ablation of a metal silver plate in an aqueous solution of sodium dodecyl sulfate, C12H25OSO3Na. The absorption spectrum of the silver nanoparticles is found to be essentially the same as that of silver nanoparticles chemically prepared in a solution. The size distribution of the nanoparticles measured by an electron microscope shifts to a smaller size with increase in the concentration of sodium dodecyl sulfate and with a decrease in the irradiation laser power. These findings are explained by a scheme that the nanoparticles are formed via rapid formation of an embryonic silver particle and a consecutive slow particle growth in competition with termination of the growth due to SDS coating on the particle.

Silver nanoparticles were produced by laser ablation of a metal silver plate in aqueous solutions of surfactants, CnH2n+1SO4Na (n = 8, 10, 12, 16). The nanoparticles thus produced were characterized by electron microscopy and UV−visible absorption spectroscopy. The abundances of the nanoparticles before and after centrifugation were measured as a function of the surfactant concentration. The concentration dependence of the abundance implies that the surfactant coverage and the charge state on the nanoparticle surface are closely related to the stability of the nanoparticles in the solutions. The nanoparticles tend to be aggregated when the coverage is less than unity, while they are very stable when the surface is covered with a double layer of the surfactant molecules.

A continuous liquid flow of a calcium chloride (CaCl2) solution in ethanol (EtOH) in a vacuum (a liquid beam) was irradiated with a 266 nm laser, and ions ejected from the surface following multiphoton ionization via the CTTS (charge transfer to solvent) band of Cl^- were observed by a time-of-flight mass spectrometer. A variety of core ions (Ca⁺, CaOEt⁺, CaOH⁺, CaCl⁺, H⁺, etc.) are formed by reactions involving Ca²⁺, solvated electrons, and solvent molecules after the CTTS excitation by the laser irradiation and are ejected into vacuum with several accompanying alcohol molecules. The proposed mechanism is verified by the change of the ion intensity with introduction of an electron scavenger, CHCl3, in the solution. The cluster ion, Ca⁺(EtOH)m, remains intact for m < 3, while it dissociates into CaOEt⁺(EtOH)m-1 for m ≥ 3. This size-dependent dissociation is simply explained by the energetics.

An aniline solution in 1-propanol, or a pure ethanol liquid, was introduced into vacuum as a continuous liquid flow (liquid beam) and was irradiated with a 266-nm laser. Ions were produced in the liquid beam by multiphoton absorption and partially ejected into the vacuum. The abundance of ions remaining inside the liquid beam and that ejected in the gas phase were measured simultaneously by using an inductive detector and a time-of-flight mass spectrometer, respectively, as a function of irradiation-laser power. The result is explained by the Coulomb ejection model for the ion ejection from the liquid surface. The essential feature changes slightly with the rate of the ion diffusion with respect to that of the ion ejection.

Solute resorcinol molecules were isolated from a liquid beam of an aqueous resorcinol solution by employing selective vibrational excitation of solvent molecules in the solution under irradiation of an infrared (IR) laser. Resorcinol molecules and their clusters with solvent water molecules isolated in the gas phase were ionized by multiphoton excitation under irradiation of an ultraviolet (UV) laser, and ions thus produced were detected by mass spectrometry. The dependence of the ion intensities on the delay time from the IR to the UV excitation indicates that the neutral species are isolated in the gas phase through two different processes; one has a short characteristic time and the other a long characteristic time.

A liquid beam (continuous liquid flow in a vacuum) of a calcium iodide (CaI2) solution in ethanol (EtOH) was irradiated with a 220 nm laser, and ions ejected from the surface following multiphoton ionization were observed by a time-of-flight mass spectrometer. Cluster ions, CaOEt⁺(EtOH)m (m = 2−9), CaI⁺(EtOH)m (m = 1−7), and H⁺(EtOH)m (m = 3−5) were found to be produced. The measurement of the velocities of the product cluster ions indicates that two mechanisms are operative; ions formed on the liquid surface are ejected with accompanying solvent molecules, and ions generated by Coulomb explosion of a divalent cluster ion, Ca²⁺(EtOH)m, are repelled with gaining a sizable translational energy.

Colloidal gold nanoparticles with a broad size distribution were prepared by laser ablation of a gold metal plate in an aqueous solution of sodium dodecyl sulfate (SDS) and were fragmented under irradiation of a 532 nm laser at different SDS concentrations and laser fluences. Gold nanoparticles with a desired average size (1.7−5.5 nm in diameter) were prepared by tuning properly the surfactant concentration and the laser fluence. The concentration of SDS was found to be higher than a critical micelle concentration so as to gain a significant size reduction and to obtain stable final products. Laser ablation in combination with the laser-induced size control provides a versatile full physical preparation method of size-selected gold nanoparticles.