Upgrading the productivity of nanoparticles (NPs), generated by pulsed laser ablation in liquid (PLAL), still remains challenging. Here a novel variant of PLAL was developed, where a doubled frequency Nd:YAG laser beam (532 nm, ~ 5 ns, 10 Hz) at different fluences and for different times was directed into a sealed vessel, toward the interface of the meniscus of ethanol w ith a tilted bulk metal target. Palladium, copper and silver NPs, synthesized in the performed proof of concept experiments, were mass quantified, by an inductively coupled plasma optical emission spectrometry, and characterized by ultraviolet-visible extinction spectroscopy, transmission electron microscopy and X-ray diffraction. The NPs consist of crystalline metals of a few nm size and their ablation rates and agglomeration levels depend on the employed laser fluences. The ensuing laser power-specific productivity curves for each metal, peaked at specific laser fluences, were fitted to the results of a simple model accounting for plasma absorption and heat transfer. The resulting peaked yields and concentrations were more than an order of magnitude higher than those obtained for totally immersed targets. Besides, the measured productivities showed nearly linear dependencies during time intervals up to 30 min of ablation, but became saturated at 1 h, due to accumulation of a significant number of NPs along the laser beam path, reducing the laser intensity reaching the target. This suggested approach could inspire future studies that will contribute to further developments of efficient generation of NPs with controlled characteristics.

Synthesis of alloyed copper palladium nanoparticles through lateral collimated and top focused laser irradiation of their suspended colloids in ethanol. These configurations allowed control of the extent of alloying at the nanoscale.

The defining characteristics of nanoparticles (NPs) include variation of properties as a function of size and shape, requiring their preparation with desirable dimensions and morphologies. Here, a unique approach based on pulsed laser irradiation in liquid (PLIL) was developed to reshape commercial CeO2 NPs suspended in water in an attempt to respond to these challenges. Guided by the results of Mie theory and the predictions of the heating–melting–evaporation model, the fourth harmonic of an Nd:YAG laser (266 nm, 10 Hz, ∼5 ns) was selected and used for lateral irradiation of colloidal CeO2 samples at particular laser fluences and number of laser pulses for producing size- and shape-specific crystalline NPs. Spectral and structural analyses of the NPs, prior to and following PLIL, revealed shape and crystal size alterations depending on the irradiation parameters, affecting melting, evaporation, and solidification of the particles. The change in size, shape, and agglomeration could be followed by the results of the analyses, where the first two could be also interpreted by the modeling results. This study demonstrates that controlled CeO2 crystal particles, from nano- to submicrometer diameters, could be generated by PLIL, shedding new light on the dynamics of the process.

An experimental study of the kinetics of the B z H~/ O (3P) system at room temperature, is presented.Modeling was based on a multiple-parameter fitting process to a complex kinetic mechanism. The aim of the study was to propose and evaluate a preferred set of elementary reactions which might be important for the oxidation of boron-hydrides. In this study, relative concentration-vs.-time profiles of the radicals OH and BOz were measured in a low pressure flow reactor, by the technique of laser-induced-fluorescence. A range of almost two orders of magnitude in the initial fuel to oxygen ratio was covered, while the residence times of the gases in the reactor were up to 1 s. A comprehensive kinetic mechanism was constructed from the available measured and estimated data in literature. After the fitting process and dropping all of the reactions with negligible contribution, a 46 elementary reactions mechanism was obtained. In this mechanism, 27 reactions are not measured and their rate coefficients were used as the adjustable (within reasonable limits deduced from kinetic and thermodynamic considerations) parameters of the fitting process. Good agreement was obtained between all of the measured and calculated profiles. From sensitivity analysis it was found that only a limited group of these reactions highly contributes to the calculated concentrations of OH and BOz. With only the highly contributing reactions and all the oxygenhydrogen reactions which are measured and relatively well known, a 30 elementary reactions mechanism was obtained. In this mechanism only 13 reactions are not measured and the agreement between the measured and the calculated profiles is still reasonable. To demonstrate the possible usefulness of the proposed mechanism, the rate coefficient of the reaction BzH6 + OH -BzH5 + H20 which was not measured before, was directly measured in our experimental set-up. The rate coefficient that was obtained in the direct measurement is (3.3 2 1.1) X lo1' cm3mol-ls-l, in excellent agreement with the predicted one by the multiple-parameter-fitting process, which was 2 X 10l1 cm3mol-'s-'. 0 1995 John Wiiey & Sons, Inc.

Polished silica (synthetic SiO2 glass) samples were ablated to vacuum by 266 nm laser pulses at a flounce of 110 J/cm2. The concentration of ground level atomic silicon in the plume was measured as a function of time, by laser-induced fluorescence (LIF), at distances of 5–7 mm from the ablated point. The angular velocity distribution was found to be highly asymmetric with respect to surface normal, as the LIF signal dropped abruptly at angles approaching the laser direction. The symmetric morphology of the craters that were formed by ablation indicates that the above asymmetry is not likely to be attributed to the interaction of the laser or the ablated plume with the surface. Therefore, it is suggested that the silicon atoms, ejected in the direction of the incident laser beam, were preferentially ionized via a multiphoton off-resonance absorption process. The velocity of the silicon atoms was detected in the 1–13 km/s range. Since the velocity distribution for a thermal ablation process is expected to extend down to zero, the photochemical channel is suggested to be dominant in this case. Doppler broadening, calculated from the lateral velocity distribution, was found to be consistent with the broadening of the silicon absorption spectrum.