Optical Properties of a Particle above a Dielectric Interface: Cross Sections, Benchmark Calculations, and Analysis of the Intrinsic Substrate Effects

We show that the optical properties of a particle above a plane dielectric interface differ dramatically from those of the same particle embedded in a homogeneous matrix. Calculations for gold and silver spheres have been carried out in using the exact multipole expansion method, providing thus benchmark results for testing the accuracy of the available numerical methods. For silver spheres, the dependence of the extinction cross-section has been studied in detail as a function of the parameters characterizing the particle/interface system, namely, the radius of the sphere, the particle-surface distance, and the dielectric index of the substrate, as well as those characterizing the light excitation, that is, the angle of incidence and the polarization. Throughout this study we have separated the effects arising from the inhomogeneity of the applied field (interference between the incoming and reflected plane waves) from the intrinsic substrate effects resulting from the interaction with the induced surface charges on the surface. These last effects are, in the present formalism, encoded in the reflected scattered field impinging on the particle. For particles close to the interface, a rich multipolar plasmonic structure is observed, which can be described in the frame of a hybridization scheme similar to that developed for dealing with layered particles or dimers. Comparison with approximate models is also provided

UV Spectroscopy of DNA Duplex and Quadruplex Structures in the Gas Phase

UV absorption spectroscopy is one of the most widely used methods to monitor nucleic acid folding in solution, but the absorption readout is the weighted average contribution of all species present in solution. Mass spectrometry, on the other hand, is able to separate constituents of the solution based on their mass, but methods to probe the structure of each constituent are needed. Here, we explored whether gas-phase UV spectroscopy can give an indication of DNA folding in ions isolated by electrospray mass spectrometry. Model DNA single strands, duplexes, and G-quadruplexes were extracted from solution by electrospray; the anions were stored in a quadrupole ion trap and irradiated by a tunable laser to obtain the UV action spectra of each complex. We found that the duplex and quadruplex spectra are significantly different from the spectra of single strands, thereby suggesting that electronic spectroscopy can be used to probe the DNA gas-phase structure and obtain information about the intrinsic properties of high-order DNA structure

Photo-Oxidation of Individual Silver Nanoparticles: A Real-Time Tracking of Optical and Morphological Changes

Absolute extinction measurements on individual silver nanoparticles under illumination show a steady evolution of their localized surface plasmon resonance. Their progressive transformation during light exposure and the influence of various parameters such as the nature of stabilizers, the local environment (oxygen rate), the spectral range of the incident light, and the shape of the nanoparticle (spheres or nanocubes) have been carefully investigated in correlation with transmission electron microscopy imaging. A combination of optics and electron microscopy gives evidence that photoaging mainly consists of the progressive formation of an oxide shell around a metallic silver core during light illumination. Moreover, in the case of nanocubes, the metallic core not only decreases in volume but also changes morphologically since edges and corners are rounded off during the photo-oxidation process. The generalized Mie theory and finite element method, used to calculate the optical extinction cross-section of core/shell Ag@Ag_<i>x</i>O nanoparticles, well account for the observed time evolutions of the absolute extinction spectra of the silver nanospheres and nanocubes. Furthermore, the calculated electromagnetic field at the nanocube surface, enhanced on edges and corners, can explain the higher efficiency of the photo-oxidation on edges and corners and the rounding increase under illumination

Hydrogen-Induced Adsorption of Carbon Monoxide on the Gold Dimer Cation: A Joint Experimental and DFT Investigation

It is demonstrated, using tandem mass spectrometry and radio frequency ion trap, that the adsorption of a H atom on the gold dimer cation, Au₂H⁺, prevents its dissociation and allows for adsorption of CO. Reaction kinetics are measured by employing a radio frequency ion trap, where Au₂⁺ and CO interact for a given reaction time. The effect of a hydrogen atom is evaluated by comparing reaction rate constants measured for Au₂⁺ and Au₂H⁺. The theoretical results for the adsorption of CO molecules and their reaction characteristics with Au₂⁺ and Au₂H⁺ are found to agree with the experimental findings. The joint investigations provide insights into hydrogen atom adsorption effects and consequent reaction mechanisms

Plasmon Spectroscopy and Chemical Structure of Small Bimetallic Cu_{(1–<i>x</i>)}Ag_<i>x</i> Clusters

The optical properties of small Cu–Ag bimetallic clusters have been experimentally and theoretically investigated in relation to their chemical structure analyzed by high resolution transmission electron microscopy (HRTEM). Cu _{(1–<i>x</i>)}Ag_<i>x</i> clusters of about 5 nm in diameter are produced in a laser vaporization source with a well-defined stoichiometry (<i>x</i> = 0, 25, 50, 75, and 100%) and dispersed in an alumina matrix. Absorption spectra are dominated by a broad and strong surface plasmon resonance whose shape and location are dependent on both cluster composition and sample aging. Detailed modeling and systematic calculations of the optical response of pure and oxidized mixed clusters of various chemical structures have been carried out in the framework of classical and semiquantal formalisms. Optical and HRTEM measurements combined with theoretical predictions lead to the conclusion that these bimetallic clusters are not alloyed at the atomic scale but rather present a segregation of chemical phases. Most likely, they adopt a Cu@Ag core–shell configuration. Moreover, the nanoparticle oxidation process is consistent with the formation of a copper oxide layer by dragging out inner copper atoms to the cluster surface