From Molecular Autoionization to Thermionic Emission.

International audienceAutoionization of Rydberg states and delayed ionization are two decay processes where the excess energy in the molecule or cluster is released by ejecting a free electron. While precise spectroscopic studies of molecular autoionization are commonly performed, allowing a very detailed understanding of the ongoing processes, the study of delayed ionization in clusters is by far less well known and its description is noticeably less precise. Indeed, in complex systems such as clusters, only a statistical approach is found to be relevant as describing the decay dynamics of the system. Beyond these apparent profound differences we show that both phenomena are actually two facets of the same general phenomenon and that delayed ionization, or thermionic emission in clusters, are analogous to molecular autoionization. The transition from autoionization of Rydberg states to delayed ionization, depicted as a transition from a process entirely described within the framework of quantum mechanics to a process described only in the framework of statistical mechanics may further be foreseen as a prototype of the transition from quantum to classical dynamics

Delayed Ionization and Delayed Detachment in Molecules and Clusters

International audienceThe evolution of a molecular system excited above its ionization threshold depends on a number of parameters that include the nature of the excited states and their couplings to the various continua. The general nature of the processes governing this evolution depends also essentially on the complexity of the molecule, more precisely on its size, density of states, and strength of the couplings among the various internal degrees of freedom. In this paper we address the question of the transition between autoionization that prevails in small molecules, and delayed ionization occurring in larger molecules or clusters. This transition is illustrated by autoionization of Na-2 Rydberg states on one hand, delayed ionization in fullerene C-60, and delayed detachment in small cluster anions on the other hand. All processes are studied in the case of nanosecond laser excitation, corresponding to a rather slow deposition of the internal energy

Plasmon spectroscopy of small indium-silver clusters: Monitoring the indium shell oxidation

cited By 9International audienceOwing to the very different electrovalences of indium and silver, nanoparticles made of these elements are among the simplest examples of hybrid plasmonic systems retaining a full metallic character. The optical properties of small indium-silver clusters are investigated here for the first time in relation to their structural characterization. They are produced in the gas phase by a laser vaporization source and co-deposited in a silica matrix. The optical absorption of fresh samples is dominated by a strong surface plasmon resonance (SPR) in the near UV, in an intermediate position between those of pure elements. A combination of SPR analysis and electron microscopy imaging provides evidence for the favourable surface segregation of indium. After a prolonged exposure to ambient air and because of the silica matrix porosity, changes in the SPR reflect the spontaneous formation of a dielectric indium oxide shell around a metallic silver core. The metallic character of indium can nevertheless be recovered by annealing under a reducing atmosphere. The reversibility of these processes is directly mirrored in optical measurements through SPR shifts and broadenings as supported by multi-shell Mie theory calculations. By controlling their oxidation level, In-Ag clusters can be considered as new candidates to extend SPR spectroscopy in the UV range and model plasmonic systems consisting of a silver particle of potentially very small size, fully protected by a dielectric oxide shell. © 2014 the Owner Societies

Surface Plasmon Resonance Damping in Spheroidal Metal Particles: Quantum Confinement, Shape, and Polarization Dependences

A key parameter for optimizing nanosized optical devices involving small metal particles is the spectral width of their localized surface plasmon resonances (LSPR). In the small size range the homogeneous LSPR line width is to a large extent ruled by the spatial confinement-induced broadening contribution which, within a classical description, underlies the popular phenomenological limited mean free path model. This unavoidable contribution to the LSPR line width is basically a quantum finite-size effect rooted in the finite extent of the electronic wave functions. This broadening reflects the surface-induced decay of the coherent collective plasmon excitations into particle–hole (<i>p</i>–<i>h</i>) excitations (Landau damping), the signature of which is a size-dependent fragmented LSPR band pattern which is clearly evidenced in absorption spectra computed within the time-dependent local density approximation (TDLDA). In this work we analyze the spatial confinement-induced LSPR damping contribution in the framework on an exact Hamiltonian formalism, assuming for convenience a jellium-type ionic density. In resorting to the harmonic potential theorem (HPT), a theorem stating that in the case of a harmonic external interaction the electronic center-of-mass coordinates separate strictly from the intrinsic motions of the individual electrons, we derive a simple approximate formula allowing to (i) quantify the size dependence of the LSPR damping in spherical nanoparticles (1/<i>R</i> law, where <i>R</i> is the sphere radius) and (ii) bring to the fore the main factors ruling the confinement-induced LSPR broadening. Then the modeling is straightforwardly generalized to the case of spheroidal (prolate or oblate) metal particles. Our investigations show that the LSPR damping is expected to depend strongly on both the aspect ratio of the spheroidal particles and the polarization of the irradiating electric field, that is, on the naturetransverse or longitudinalof the collective excitation. It is found that the magnitude of the damping is tightly related to that of the LSPR frequency which rules the number of <i>p</i>–<i>h</i> excitations degenerate with the plasmon energy. Qualitative analysis suggests that the results are quite general and probably hold for other nonspherical particle shapes. In particular, in the case of elongated particles, as rods, the enlargement of the longitudinal LSPR band by the confinement effects is predicted to be much smaller than that of the transverse LSPR band

Tracking the restructuring of oxidized silver–indium nanoparticles under a reducing atmosphere by environmental HRTEM

International audienc

Forward and Backward Extinction Measurements on a Single Supported Nanoparticle: Implications of the Generalized Optical Theorem

International audienc

Fano Transparency in Rounded Nanocube Dimers Induced by Gap Plasmon Coupling

International audienceHomodimers of noble metal nanocubes form model plasmonic systems where the localized plasmon resonances sustained by each particle not only hybridize but also coexist with excitations of a different nature: surface plasmon polaritons confined within the Fabry−Perot cavity delimited by facing cube surfaces (i.e., gap plasmons). Destructive interference in the strong coupling between one of these highly localized modes and the highly radiating longitudinal dipolar plasmon of the dimer is responsible for the formation of a Fano resonance profile and the opening of a spectral window of anomalous transparency for the exciting light. We report on the clear experimental evidence of this effect in the case of 50 nm silver and 160 nm gold nanocube dimers studied by spatial modulation spectroscopy at the single particle level. A numerical study based on a plasmon mode analysis leads us to unambiguously identify the main cavity mode involved in this process and especially the major role played by its symmetry. The Fano depletion dip is red-shifted when the gap size is decreasing. It is also blue-shifted and all the more pronounced that the cube edge rounding is large. Combining nanopatch antenna and plasmon hybridization descriptions, we quantify the key role of the face-to-face distance and the cube edge morphology on the spectral profile of the transparency dip. T he optical excitation of localized surface plasmon resonances (LSPR) in single metallic nanoparticles or multicomponent nanostructures is responsible for efficient resonant far-field scattering and near-field concentration of light. 1,2 Such nanoantennas offer the opportunity to manipulate light at scales much shorter than the wavelength by making the best use of the LSPR sensitivity to variations in particle shape, size, chemical composition, and dielectric environment. 3,4 The electrostatic coupling between several plasmonic subunits is an additional tool for tailoring the optical response over a wide spectral range. 5,6 In this respect, much attention is paid to analogues of Fano resonances in classical electrostatics and especially those arising from the coupling between a " dark " plasmonic mode (weak dipole moment and narrow line width) and a degenerate mode that is highly radiative over a much broader spectral range (" bright " mode, large dipole moment). On either side of the resonance, the rapid phase shift of the " dark " mode polarizability relative to that of the bright mode and the exciting field may induce constructive or destructive interference in the net far field scattering process. They result in the formation of a dip in the broad spectral band of the far field scattered light with a typical dissymmetric profile and a narrow width related to the " dark " resonance lifetime. 7−11 Fano resonances open sharp and transparent windows in the plasmonic response of metallic nanostructures. 12−14 Their high sensitivity to relative changes in the nanostructure dielectric environment can be efficiently exploited for chemical and biological sensing. 15,16 Most of the studies in this field deal with noble metal nanoantennas built from the assembly and coupling of subunits of various size, shape, and orientation: oligomers of colloidal particles (DNA assembled, 17,18 heterodimers 19) and predominantly objects engraved by electron-beam lithography. 20−22 This method offers considerable flexibility for designing the spectral response of intricate structures with an advantageous symmetry breaking. 13,23 In the far field, they act as wide ban

Environmental Plasmonic Spectroscopy of Silver–Iron Nanoparticles: Chemical Ordering under Oxidizing and Reducing Conditions

SSCI-VIDE+ECI2D+LPIInternational audienc

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