We calculate the optical attractive forces that occur between 30-nm Au or Ag nanocrystals when irradiated at visible wavelengths. These forces show resonances at dipolar plasmon wavelengths, similar to resonances in the near-field electromagnetic intensities. At MW/cm 2 intensities, optical forces can be stronger than van der Waals forces and could be used to organize metallic particles. We also suggest that photonucleation of organic crystals from supersaturated liquid solutions may be caused by optical forces.nanoparticles ͉ photonucleation ͉ bispherical coordinates T his article investigates the possibility that optical forces might be used to organize metallic nanocrystals. It also considers whether optical forces might modify the precise geometry of junctions between metallic nanocrystals. Optical forces derive from the electromagnetic-field intensification that occurs near Ag and Au particles at certain resonance wavelengths. This effect is a local mode of the electromagnetic field: the dipolar surface plasmon. The particle acts as a microscopic antenna, increasing absorption and re-emission of light by nearby molecules and causing the surface-enhanced Raman effect (SERS) (1-5). This enhancement is especially interesting for two particles separated by just a few nanometers. The metallic electron excited-state polarization concentrates at the junction (6), producing a ''hot spot'' of enormous electromagnetic enhancement (7), sufficient to allow the observation of single-molecule Raman spectra (8-10).This article considers the optical forces present between two such closely spaced r ϭ 30-nm metallic particles. The internal electronic polarization in the metal that creates the enhanced local field in the junction also creates force between the particles. This force can be simply understood. Consider two identical particles (or molecules) separated by a distance z. In an electromagnetic field E ៝ , each particle will develop an ac dipole p ៝ ϭ ␣⅐E ៝ oscillating coherently with respect to E ៝ . (In general, such electronic polarization optical dipoles P are the source of Rayleigh scattering, the optical Kerr effect, and trapping of single glass beads in focused laser beams.) Neglecting retardation, the two dipoles are in phase with each other. If the light polarization is along z, then the dipoles are head to tail and the interaction is attractive. With perpendicular polarization, the interaction is repulsive. The dipole-dipole-interaction force scales linearly with the light intensity E 2 and falls off as z Ϫ4 . In the dark, the normal van der Waals force falls off as z Ϫ7 . When the particles are far apart, only the dipole-dipole optical interaction need be considered. When they are separated by a distance of the same magnitude as their size, their mutual polarization is strong, and higher-order moments are important. In this article we consider all separations z for two classical spheres of wavelength-dependent, arbitrary complex dielectric coefficient.A single polarizable particle in a uniform plane-wave fi...

Cavity ring-down spectroscopy is applied to the liquid phase by placing the target solution directly into the optical cavity. We demonstrate that solutions in the cavity can be stirred and more importantly monitored in a flow. We report a minimum detectable absorption of 10(-6) cm(-1) for a range of organic solvents. This detection limit corresponds to picomolar concentrations for strong absorbers.

The kinetics of methylene blue reduction by ascorbic acid in acetonitrile was investigated by cavity ring-down spectroscopy. Because of our high sensitivity we were able to use very low concentrations (1-10 nM) of the dye. Under these conditions, we observed a second-order loss of dye as well as a competing back reaction with dissolved oxygen. The use of an inexpensive diode laser and a relatively simple setup should make ultratrace kinetic studies more accessible.

Cavity ring-down spectroscopy (CRDS) is an extremely sensitive absorption technique that has been applied primarily to gas samples, which are characterized by having narrow absorption features. Recently, CRDS has also been applied to liquid samples, which have broad absorption features. The use of small inexpensive diode lasers as light sources for liquid samples is demonstrated. The low cost coupled with the ease and technical straightforwardness of application gives this technique wide appeal.

The steady-state reaction of NO with H2 and NH3 on Pt(100) is investigated over a temperature range of 340−570 K, and the rotational and vibrational states of the N2 product are probed by resonance enhanced multiphoton ionization. For NO + NH3 the N2 reaction product leaves the surface rotationally and vibrationally excited, whereas for NO + H2 the desorbing N2 is rotationally equilibrated with the surface and no vibrational excitation can be detected. We conclude that although these two surface reactions both yield N2, the actual reaction mechanisms must be different.