F. Calvayrac

NONLINEAR ELECTRON DYNAMICS IN METAL CLUSTERS

Abstract Recent experimental developments give more and more access to cluster excitations beyond the regime of linear response. Most theoretical descriptions of the induced nonlinear electron dynamics are based on the time-dependent local density approximation (TDLDA) and related schemes. We review the present status of TDLDA calculations for metal clusters, considering formal aspects of the theory, recipes for its numerical implementation as well as a variety of applications. These applications are presented by first summarizing basic linear spectral properties of the systems under study and then introducing two mechanisms for strong excitations: collision with highly charged and fast ions, and irradiation with strong femtosecond laser pulses. We present results for observables that are relevant for experiments, including ionization, energy balance, second-harmonic generation, electron emission spectra and, last but not least, we discuss the effects of ionic motion during the electronic dynamics. On the theoretical side, we also discuss semiclassical approaches and extensions beyond TDLDA, such as self-interaction corrections and the influence of electron–electron collisions.

Grafting of diazonium salts on oxides surface: formation of aryl-O bonds on iron oxide nanoparticles

Combining ab initio modeling and 57Fe Mössbauer spectrometry, we characterized the nature of the chemical linkage of aminoalkyl arenediazonium salt on the surface of iron oxide nanoparticles. We established that it is built through a metal–oxygen–carbon bonding and not a metal–carbon one, as usually suggested and commonly observed in previously studied metal- or carbon-based surfaces.

Coulomb explosion of an cluster in a diabatic electron-ion dynamical picture

We present non-adiabatic simulations of the excitation and de-excitation of an sodium cluster irradiated by intense femtosecond lasers. Both electronic and ionic degrees of freedom are explicitly accounted for, whatever nonlinear regime is accessed. We find that the Coulomb fragmentation of the thus excited cluster depends sensitively on the laser frequency. We furthermore show that in the first 100 fs following the laser excitation, ionic degrees of freedom can safely be considered to be frozen. They then couple quickly to the electronic ones, within about another 100 fs, well before electronic excitation has fully levelled off.

Ionic structure and plasmon response in sodium clusters

The linear and nonlinear optical response of sodium clusters is studied using fully three-dimensional electronic dynamics with frozen ionic positions. The detailed ionic structure produces more spectral fragmentation than a comparable jellium model, and the fragmentation patterns come close to measured spectra, even at low temperatures. The nonlinear response shows the same trends as found in former, symmetry restricted, investigations, namely that the plasmon resonance is a very harmonic mode showing no trace of nonlinear effects as secondary harmonics or spectral diffusion.

Optical response of Na clusters on NaCl surfaces

Abstract We consider Na clusters on NaCl surfaces as examples of metal clusters attached to an insulating substrate. We concentrate on the cases Na 8 and Na 20 where three-dimensionally extended clusters compete with planar clusters and compute the optical response of ground states and isomers using the time-dependent local-density approximation. The interface interaction leads to strong deformations of the deposited clusters. Nonetheless, one sees a well-developed plasmon response which allows conclusions to be made on the underlying cluster structure (planar or three-dimensional).

Swift cluster–atom collisions: a progress report

Abstract Significant progresses in the field of swift cluster–atom collisions have been made, thanks to the development of new beam facilities. We report on experiments performed nearby the Tandem accelerator at Orsay (France) with beams of small carbon clusters C+n(n⩽5) of 2 MeV/C colliding with a single helium atom. New results concerning the ionization cross sections of the clusters are reported and compared to the predictions of the independent atom and electron model. New results concerning the fragmentation patterns of C+5 (branching ratios and dissociative energies) are presented. A strong observed multifragmentation is observed, and discussed.

On the electron dynamics in metal clusters: a Vlasov approach

We study the dynamical response of the valence electrons of sodium clusters to strong excitations through colliding particles. The electronic dynamics is described using a time-dependent local density approximation in the semiclassical approximation (Vlasov - LDA). The ionic background is modelled by a smooth jellium distribution. Fast proton - cluster collisions lead to an immediate stripping of electrons and a pronounced dipole excitation of the residual electron cloud.

Theoretical studies of vibrational spectra of [N(CH3)4]2ZnCl4−yBry compounds with y = 0, 2 and 4

The infrared and Raman spectra of [N(CH3)4]2ZnCl4−yBry, where y = 0, 2 and 4, have been analyzed with ab initio calculations of the vibrational characteristics of constitutive polyhedra, tetramethylammonium [N(CH3)4]+ and [ZnCl4−xBrx]2− (x = 0, 1, 2, 3 and 4) tetrahedra. The optimized geometries, calculated vibrational frequencies, infrared intensities and Raman activities are calculated using Hartree–Fock and density functional theory B3LYP methods with 3-21G, 6-31G(d) and 6-311G+(d,p) basis sets. Calculation of the root mean square difference δrms between the observed and calculated frequencies allows to give scaling factors and to deduce that the best agreements are obtained by B3LYP/6-311G+(d,p) for [N(CH3)4]+ and B3LYP/3-21G for [ZnCl4−xBrx]2−. The present study establishes a strongly reliable assignment of the vibrational modes of [ZnCl4−xBrx]2− tetrahedra based on comparison between experimental and ab initio calculations, both of the frequencies and the intensities of the Raman signals.

Status of APEmille

Abstract This paper presents the status of the APEmille project, which is essentially completed, as far as machine development and construction is concerned. Several large installations of APEmille are in use for physics production runs leading to many new results presented at this conference. This paper briefly summarizes the APEmille architecture, reviews the status of the installations and presents some performance figures for physics codes.

Path probability distribution of stochastic motion of non dissipative systems: a classical analog of Feynman factor of path integral

Abstract We investigate, by numerical simulation, the path probability of non dissipative mechanical systems undergoing stochastic motion. The aim is to search for the relationship between this probability and the usual mechanical action. The model of simulation is a one-dimensional particle subject to conservative force and Gaussian random displacement. The probability that a sample path between two fixed points is taken is computed from the number of particles moving along this path, an output of the simulation, divided by the total number of particles arriving at the final point. It is found that the path probability decays exponentially with increasing action of the sample paths. The decay rate increases with decreasing randomness. This result supports the existence of a classical analog of the Feynman factor in the path integral formulation of quantum mechanics for Hamiltonian systems.