Modification of Ionic Silver Clusters Under the Influence of a High Power CW Laser Beam H. R. khalesifard A. Nahal and M. Payami Institute for Advanced studies in Basic Sciences Zanjan, IRAN

Contents Introduction Techniques for studying and producing metallic clusters Ag + -Na + ion-exchange in glass matrix Laser annealing Luminescence of laser annealed samples Optical absorption spectra of laser annealed samples Experiments results Stabilized jellium model and photo-fragmentation.

Introduction Composite glasses consisting of small metal particles nm in diameter embedded in a dielectric are optically nonlinear materials that are attractive for optical switching. Optical absorption spectroscopy is one of the most used techniques for the investigation of colloidal systems formed by such particles.

Techniques to Study the Cluster Responses Linear Responses Absorption Elastic scattering Fluorescence Luminescence Inelastic scattering (Brillouin, Smekal- Raman) Nonlinear Responses Second harmonic generation (SHG) Multiple-wave mixing Nonlinear Kerr effect

Destructive methods Ionization Evaporation of atoms, dimers, … Fission of clusters in to fragments

Preparation of Metal Clusters In beamsOn surfaceIn matrices gas aggregationAtom deposition with subsequent surface deposition Co-evaporation of atoms with matrix material (matrix isolation0 Field emission (liquid metal source) Cluster deposition from cluster beam Diffusion in/into matrix (ion-exchange) Spraying techniquesnanocrystalline material by densification Photoreduction (photographic material) SputteringSpraying technique into liquid matrix Arc dischargeCo-evaporation of clusters with matrix material Laser vaporization, ablationPressing into porous matrix

Ag + -Na + Ion-Exchange Glass Salt Melt (NO 3 ) - Ag + Na + O 2- Si 4+

The temperature of the molten salt should be less than the glass transition temperature. The silver ions inside the glass matrix gathers together to form the ionic silver clusters. The Mie resonances appearing in the optical absorption spectra of these sample are the evidence of formation of ionic clusters. Usually the ionic clusters con be reduced to the atomic ones by annealing the ion-exchanged samples in hydrogen atmosphere in temperatures lower than the ion-exchange temperature. Surface plasmun resonances at this stage appear in the optical absorption spectra.

Laser Annealing The high power Ar + laser beam changes the surface and index profile of the sample in the interaction region.

The laser beam induces a considerable anisotropy in the interaction region

A standard set-up for observing the luminescence of the interaction region

The luminescence of the sample in the interaction region can be an evidence of the clusters motion in that region

Absorption spectrum Laser annealingAnnealing in H 2 atmosphere

Exposing the sample with laser beam in two regime] 1.Fast regime: up t0 67 W/cm 2 1n less than 1 min 2.Slow regime: up to 67W/cm 2 in 1hr

Diffusion limited aggregation process and the induced anisotropy

A dark background photograph of the interaction area

Experiments Results Comparison of absorption spectra after laser annealing and annealing in hydrogen atmosphere helps us to conclude that the photo-fragmentation of the ionic clusters by the laser beam can produce neutral clusters which are smaller in size compared to ionic ones. At least two groups of clusters are exist in the interaction region. 1.Nanometer sized clusters which are responsible for the surface plasmun and Mie resonances in the absorption spectra. 2.Micrometer sized clusters which are positioned in the circumference of the interaction region and are observable with usual optical microscopes. Micrographs and absorption spectra of the interaction region at s and p polarizations show that the evolution of the atomic clusters is a diffusion limited aggregation process. We hope that to obtain some information about the motion of clusters during the interaction by observing the luminescence of the interaction region

Stabilized Jellium model and Photo-Fragmentation If we consider the following possible channels for the photo-fragmentation of the clusters: We have used the stabilized jellium model and density functional theory to show that for clusters with N>50, the evaporation processes are dominent. Evaporation Fission