Origin of rebounds with a restitution coefficient larger than unity in nanocluster collisions

We numerically investigate the mechanism of super rebounds for head-on collisions between nanoclusters in which the restitution coefficient is larger than unity. It is confirmed that the temperature and the entropy of the nanocluters decrease after the super rebounds by our molecular dynamics simulations. It is also found that the initial metastable structure plays a key role for the emergence of the super rebounds.Comment: 8 pages, 10 figures, to be published in Phys. Rev.

Gold dimer in neon: an absorption and fluorescence study

We report for the first time the absorption and fluorescence spectra of gold dimers in a neon matrix. The dimer absorption spectra show the A ← X transition predicted from measurements in the gas phase and not observed so far in a matrix, as well as the so-called B ← X and C ← X transitions. Fluorescence measurements on the atom reveal new emission lines at 1.97, 3.59 and 4.09eV that can be assigned to the 2P1/2 → 2D3/2, 2P1/2 → 2D5/2 and the 2P3/2 → 2D5/2 transitions. For the dimer, excitation of both A and B state results in distinct emission spectra with vibrational structur

Morphology and stability of Au nanoclusters in HOPG nanopits of well-defined depth

Gold nanoparticles with a diameter comprised between 4 and 6 nm are stabilized in nanosized pits of well defined depth in highly oriented pyrolytic graphite (HOPG). These pits are produced by creation of artificial defects, followed by etching under a controlled oxygen atmosphere. At low Au coverage, clusters are found on the edges of the hexagonal pits maximizing the contact to dangling bonds on graphite multisteps. Larger coverage results in Au beads of surprisingly well defined shape and with a constant bead density per unit length. Most remarkable is the stability of these nanostructures under ambient conditions. Temperatures as high as 650K do not alter the morphology of the gold clusters. Higher temperatures do not lead to a change of the cluster morphology but to catalytically driven etching of the HOPG substrat

Spectroscopy of PTCDA attached to rare gas samples: clusters vs. bulk matrices. I. Absorption spectroscopy

The interaction between PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) and rare gas or para-hydrogen samples is studied by means of laser-induced fluorescence excitation spectroscopy. The comparison between spectra of PTCDA embedded in a neon matrix and spectra attached to large neon clusters shows that these large organic molecules reside on the surface of the clusters when doped by the pick-up technique. PTCDA molecules can adopt different conformations when attached to argon, neon and para-hydrogen clusters which implies that the surface of such clusters has a well-defined structure and has not liquid or fluxional properties. Moreover, a precise analysis of the doping process of these clusters reveals that the mobility of large molecules on the cluster surface is quenched, preventing agglomeration and complex formation

An experimental setup combining a highly sensitive detector forreaction products with a mass-selected cluster source andalow-temperature STM for advanced nanocatalysis measurements

We report on a home-built detector for catalytic reaction measurements offering good gas isolation from the surrounding ultrahigh vacuum components, high sensitivity for reaction products and a fast response time of 10ms enabling dynamic studies correlated to reactant gas pulses. The device is mounted in ultrahigh vacuum and combined with a low-temperature scanning tunneling microscope and a source for the deposition of mass-selected clusters. This combination allows for a direct correlation between surface morphology and catalytic properties of model catalysts. The performances of the new detector are illustrated by measurements on two model systems. Thermal desorption spectroscopy of CO carried out on morphologically well characterized Pt on TiO2(110)-(1×1) reveals several desorption features, which can be attributed to different surface sites. Catalytic CO oxidation performed by alternatingly pulsing isotopic CO and O2 on a Pt film on yttria stabilized zirconia reveals the CO or O rich temperature regimes. The CO2 production rate correlated with either one of the reactants can perfectly be reproduced by a kinetic reaction model giving access to the respective adsorption energie

Morphology and stability of Au nanoclusters in HOPG nanopits of well-defined depth

Gold nanoparticles with a diameter comprised between 4 and 6 nm are stabilized in nanosized pits of well defined depth in highly oriented pyrolytic graphite (HOPG). These pits are produced by creation of artificial defects, followed by etching under a controlled oxygen atmosphere. At low Au coverage, clusters are found on the edges of the hexagonal pits maximizing the contact to dangling bonds on graphite multisteps. Larger coverage results in Au beads of surprisingly well defined shape and with a constant bead density per unit length. Most remarkable is the stability of these nanostructures under ambient conditions. Temperatures as high as 650 K do not alter the morphology of the gold clusters. Higher temperatures do not lead to a change of the cluster morphology but to catalytically driven etching of the HOPG substrate

The prolate-to-oblate shape transition of phospholipid vesicles in response to frequency variation of an AC electric field can be explained by the dielectric anisotropy of a phospholipid bilayer

The external electric field deforms flaccid phospholipid vesicles into spheroidal bodies, with the rotational axis aligned with its direction. Deformation is frequency dependent: in the low frequency range (~ 1 kHz), the deformation is typically prolate, while increasing the frequency to the 10 kHz range changes the deformation to oblate. We attempt to explain this behaviour with a theoretical model, based on the minimization of the total free energy of the vesicle. The energy terms taken into account include the membrane bending energy and the energy of the electric field. The latter is calculated from the electric field via the Maxwell stress tensor, where the membrane is modelled as anisotropic lossy dielectric. Vesicle deformation in response to varying frequency is calculated numerically. Using a series expansion, we also derive a simplified expression for the deformation, which retains the frequency dependence of the exact expression and may provide a better substitute for the series expansion used by Winterhalter and Helfrich, which was found to be valid only in the limit of low frequencies. The model with the anisotropic membrane permittivity imposes two constraints on the values of material constants: tangential component of dielectric permittivity tensor of the phospholipid membrane must exceed its radial component by approximately a factor of 3; and the membrane conductivity has to be relatively high, approximately one tenth of the conductivity of the external aqueous medium.Comment: 17 pages, 6 figures; accepted for publication in J. Phys.: Condens. Matte

Surface mobility of Ag on Pd(100) measured by specular helium scattering

We study the deposition and the very first steps of nucleation and growth of Ag on Pd(100) with thermal energy atom scattering. This technique is a very sensitive and nonperturbing probe to surface point defects, which permits an in situ and in-time monitoring of the deposition. The intention of this paper is to give a detailed description of the approach used in our work. The form of the specularly reflected helium signal as a function of coverage and surface temperature is compared to a theoretical curve, which is computed by solving a system of rate equations that describe the formation and destruction of clusters during the deposition process. The analysis of the experimental data gives two main results. The diffusion parameters (activation barrier E/sub d/=0.37+or-0.03 eV and preexponential factor nu /sub 0/=8*10/sup 9/ s/sup -1/) have been extracted for the system Ag on Pd(100). We find furthermore that all silver atoms impinging on a zone of 6.1 AA around an adatom on the surface are captured by it at surface temperatures well below the onset of thermally activated mobility. The origin of this phenomenon is discussed and tentatively assigned to a combined effect of transient and neighbor driven mobility

Fluorescence and excitation spectra of Ag4 in an argon matrix

We report the fluorescence and excitation spectra of size selected Ag4 deposited in an argon matrix. The main fluorescence peak for Ag4 is observed at 458 nm. The excitation spectrum for this emission is in good agreement with recent theoretical calculations and photodepletion experiments

Ag8 Fluorescence in Argon

The fluorescence of Ag8 in an argon matrix and in argon droplets is reported. This is the first unambiguous assignment of the fluorescence of a metal cluster larger than the tetramer, indicating that the excited state lifetime is longer than previously thought. It is discussed as a possible result of a matrix cage effect. The excitation spectrum is compared with two-photon-ionization measurements of Ag8 in helium droplets and to known absorption data. The agreement is excellent. We propose that the excited states relax rapidly through vibrational coupling to a long-lived state, from which the fluorescence occurs