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Samples of suspended gold nanoparticles in the diameter range 10 to 100 nm were subjected to a single 7 ns pulse from a 532 nm laser to determine the effect of laser power on particle size distribution, mean size, and morphology. The experimental techniques used were dynamic light scattering (DLS), depolarized dynamic light scattering (DDLS), electrospray-differential mobility analysis (ES-DMA), ultraviolet−visible absorption spectroscopy, and transmission electron microscopy (TEM). For 60 nm particles, a laser pulse of fluence 10 mJ/cm 2 was sufficient to produce observable changes. In the range 10−72 mJ/cm 2 , DLS indicated little change in mean particle size but a more than three-fold reduction in the polydispersity index (significantly tightened distribution) and a decrease in scattering intensity. TEM showed that the particles became highly spherical and that there was a growing population of particles <10 nm in size that could not be detected by DLS and ES-DMA. Fused dimers were also observed, which suggest that heated particles can interact prior to cooling. DDLS showed a decrease in scattering due to shape anisotropy with a 20 mJ/cm 2 pulse and a decrease in the diffusion time constant. At higher power, the mean particle size decreased until all particles were <10 nm in size. The threshold for observable changes decreased with increasing particle size in the range 10 to 60 nm but increased for 100 nm particles. These results will be useful for potential therapeutic applications for pulse-heated nanoparticles and demonstrate the use of a simple laser treatment for modifying and improving nanoparticle properties.

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The forward scattering of a Gaussian laser beam by a spherical particle located along the beam axis is analyzed with the generalized Lorenz-Mie theory (GLMT) and with diffraction theory. Forwardscattering and near-forward-scattering profiles from electrodynamically levitated droplets, 51.6 µm in diameter, are also presented and compared with GLMT-based predictions. The total intensity in the forward direction, formed by the superposition of the incident and the scattered fields, is found to correlate with the particle-extinction cross section, the particle diameter, and the beam width. Based on comparison with the GLMT, the diffraction solution is accurate when beam widths that are approximately greater than or equal to the particle diameter are considered and when large particles that have an extinction efficiency near the asymptotic value of 2 are considered. However, diffraction fails to describe the forward intensity for more tightly focused beams. The experimental observations, which are in good agreement with GLMT-based predictions, reveal that the total intensity profile about the forward direction is quite sensitive to particle axial position within a Gaussian beam. These finite beam effects are significant when the ratio of the beam to the particle diameter is less than approximately 5:1. For larger beam-to-particle-diameter ratios, the total field in the forward direction is dominated by the incident beam.

A state-of-the-art, rapid laser-heating technique, referred to as the laser-driven thermal reactor, was used to characterize National Institute of Standards and Technology Standard Reference Material (SRM) diesel and biodiesel fuels, as well as a prototype biodiesel fuel. Also described are the various issues associated with carrying out these measurements under different operating conditions (i.e., temperature, pressure, heating rate, and sample mass). The technique provides measurement of various relevant thermochemical characteristics; for this investigation the focus was on the sample endothermic/exothermic behavior, specific heat release rate and total specific heat release. The experimental apparatus consists of a copper sphere-shaped reactor mounted within a vacuum chamber, along with integrated optical, gas-supply, and computer-controlled data-acquisition subsystems. At the center of the reactor, the sample rests on a thermocouple. The reactor is heated from opposing sides by a near-infrared laser to achieve nearly uniform sample temperature. The change in sample temperature with time (i.e., thermogram) is recorded and compared to a baseline (no sample) thermogram obtained prior to the experiment. Then processed (using an equation for thermal energy conservation) for the thermochemical information of interest. Results indicated that the modification of the baseline is attributed to residue remaining after completion of reactions and a change in the oxide layer of the reactor sphere outer surface. Thus, the sphere must be pre-oxidized in air using the laser prior to any sample or baseline measurement. This investigation provides preliminary evaluation of SRM biodiesel fuels, with the results being consistent with distillation curve work reported in the literature.

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