Physisorption and chemisorption on silver clusters.

Adsorption and coadsorption studies on free silver clusters show that nitrogen physisorbs like rare gases, whereas oxygen chemisorbs with similarities and differences to bulk silver surfaces. Silver nanoparticles activate, or even dissociate adsorbed oxygen molecules. The global electron configurations of the adsorbent and adsorbate dominate the stability at small clusters.

Enhancement of Ag cluster mobility on Ag surfaces by chloridation.

To understand the role of chlorine in the stability and the observed fragmentation of Ag dendritic nanostructures, we have studied computationally two model systems using density functional theory. The first one relates to diffusion of Ag(n) and Ag(n)Cl(m) (n = 1-4) clusters on an Ag(111) surface, and the second demonstrates interaction strength of (Ag(55))(2) dimers with and without chloridation. Based on our calculated energy barriers, Ag(n)Cl(m) clusters are more mobile than Ag(n) clusters for n = 1-4.

Adsorption of small molecules on silver clusters.

We report investigations of adsorption of N(2) and O(2) molecules on silver cluster cations. We have first revisited structures of small silver clusters based on first-principles calculations within the framework of density functional theory with hybrid functional. The 2D to 3D transition for the neutral clusters occurs from n = 6 to 7 and for cations, in agreement with experiments, from n = 4 to 5.

Hydrogen peroxide and ammonia on protonated ice clusters.

A temperature controlled source for protonated water clusters has been combined with high-resolution mass spectroscopy to study the stability pattern of ice clusters and compounds with ammonia and hydrogen peroxide depending on temperature. The stability pattern of pure protonated ice shows the two well known peaks at 21 and 28 molecules and also less pronounced structure up to n=55. Ammonia and hydrogen peroxide do not destroy this pattern but shift it by a number of water molecules.

Coarsening and pearling instabilities in silver nanofractal aggregates.

We investigate the morphological changes of 3D supported fractal aggregates generated through the deposition of silver clusters on graphite. The fractal relaxation, activated after their formation by perturbing them either by thermal annealing or by using a surfactant, as oxide molecules, carried by silver clusters in a subsequent deposition, shows evidence of two distinct fragmentation patterns. The post coarsening, driven by thermal heating, gives a broad asymmetrical distribution of fragments in agreement with a random process, whereas the entire silver fractal pearling fragmentation is driven by chemical adjunction of the surfactant.

Dynamics of polymorphic nanostructures: from growth to collapse.

The deposition of preformed clusters on surfaces offers new possibilities to build complex artificial nanostructures, the shape of which depends on the cluster size. We describe routes for generating unusual polymorphic nanoislands, which constitute unique platforms for exploring instabilities. As coverage increases, the constraints accumulated in such nanostructures induce spectacular flattening collapse processes, which are not observed when the constraints are imposed by the substrate.

Enhancement of nitrogen physisorption in coadsorption with oxygen on free, positively charged silver clusters.

The coadsorption of molecular nitrogen and oxygen on small cationic silver clusters in the gas phase is experimentally studied. The presence of oxygen enhances the adsorption probabilities of N2. This indicates a partial charge transfer out of the finite free electron reservoir of the small silver particles into the chemisorbed oxygen molecule.

Synthesis of silver molybdate clusters driven by laser-annealing.

The synthesis of silver rich molybdate clusters is achieved by laser induced chemical reaction of coadsorbed MoO(3) and O(2) molecules on free silver clusters. The reactants MoO(3) and/or O(2) molecules condensed at low temperature (77 K-175 K) on free silver clusters. Then, the silver clusters together with their adsorbed molecules are flashed either ionized with a discharge or ionized and heated by a laser.

Dissipation effects in cluster fission.

The fission of Sr(2+)(n) is studied from time-of-flight (TOF) measurements. The TOF acts both as a mass spectrometer and as a velocity dispersion analyzer. Evidence of the postfission ejection of a fast neutral atom is shown.

Oxygen and silver clusters: transition from chemisorption to oxidation.

We use the adsorption probabilities of molecular nitrogen and oxygen to study the physi- and chemisorption on small silver particles. The physisorption of nitrogen is governed by the structure of the particle surface. The sticking of oxygen additionally involves the electronic configuration of the metal cluster.

Caloric curves of small fragmenting clusters.

The kinetic energy release distribution of neutral atoms emitted from photoexcited clusters Sr(+)(n) with n=4-15, has been obtained by time-of-flight velocity dispersion. The deduced temperature is plotted as a function of the excitation energy. For small sizes n<7 a general increase is observed.

Thermal quenching of electronic shells and channel competition in cluster fission.

Experimental and theoretical studies of fission of doubly charged Li, Na, and K clusters in the low-fissility regime reveal the strong influence of electronic shell effects on the fission products. The electronic entropy controls the quenching of the shell effects and the competition between magic-fragment channels, leading to a transition from favored channels of higher mass symmetry to the asymmetric channel involving the trimer cation at elevated temperatures.

Charge transfer and dissociation in collisions of metal clusters with atoms.

We present a combined theoretical and experimental study of charge transfer and dissociation in collisions of slow Li31(2+) clusters with Cs atoms. We provide a direct quantitative comparison between theory and experiment and show that good agreement is found only when the exact experimental time of flight and initial cluster temperature are taken into account in the theoretical modeling. We demonstrate the validity of the simple physical image that consists in explaining evaporation as resulting from a collisional energy deposit due to cluster electronic excitation during charge transfer.

Instability driven fragmentation of nanoscale fractal islands.

Formation and evolution of fragmentation instabilities in fractal islands, obtained by deposition of silver clusters on graphite, are studied. The fragmentation dynamics and subsequent relaxation to the equilibrium shapes are controlled by the deposition conditions and cluster composition. Sharing common features with other materials' breakup phenomena, the fragmentation instability is governed by the length-to-width ratio of the fractal arms.