Magic clusters Na57 - and Na59 +

The electronic structure of positively charged sodium cluster with 59 atoms and negatively charged sodium cluster with 57 atoms (both having 58 electrons) have been studied using an ab initio molecular dynamics method. Although the geometries of both clusters are nearly spherical, the results show differences in the electronic shells.     

Evaporation and fission of hot metallic clusters and hot nuclei

Evaporation and fission of metallic clusters are described by models developed in nuclear physics which are appropriately modified. Results of calculations for evaporation rates, appearance sizes and fission times (with and without viscosity) are presented. Presented at the International Conference on “Atomic Nuclei and Metallic Clusters”, Prague, September 1–5, 1997.     

Static electric dipole polarizabilities of Na clusters

The static electric dipole polarizability of Na _N clusters with even N has been calculated in a collective, axially averaged and a three-dimensional, finite-field approach for , including the ionic structure of the clusters. The validity of a collective model for the static response of small systems is demonstrated. Our density functional calculations verify the trends and fine structure seen in a recent experiment. A pseudopotential that reproduces the experimental bulk bond length and atomic energy levels leads to a substantial increase in the calculated polarizabilities, in better agreement with experiment. We relate remaining differences in the magnitude of the theoretical and experimental polarizabilities to the finite temperature present in the experiments. Received 8 November 1999     

Photoabsorption resonances of Na20, Na21Cl and Na22Cl2 clusters

Plasma resonance profiles in the visible part of the spectrum were measured by photoabsorption spectroscopy for the clusters Na₂₀, Na₂₁Cl and Na₂₂Cl₂ in a beam. The resonance positions in Na₂₀ and Na₂₁Cl are close, suggesting that the Cl^- ion does not locate at the center of the metallic droplet and does not strongly modify the effective valence electron density. The spectrum of Na₂₂Cl₂ is noticeably dissimilar to the other two, raising the possibility of structural differences and/or incomplete charge transfer.     

Evaporation rates of droplet arrays in turbulent reacting flows

Studies of regularly ordered droplet arrays facilitate the analysis of local effects on evaporation rates. This work investigates, using Direct Numerical Simulations (DNS), the effects of droplet density and flow conditions on evaporation of kerosene droplets in inert and reactive convective environments. A novel model, coupling a mass conservative Level Set approach with the Ghost Fluid method, is used. The rates obtained from the DNS are compared to two evaporation models based on heat and mass transfer numbers commonly used for RANS methods and Large Eddy Simulations (LES). The results show that predictions of evaporation rates of dense sprays using these models has a limited success. The use of the 1/3-rule to calculate mixture properties results in underpredictions of the evaporation rates by around 20% to 50% in most of the cases studied. The models can only predict the DNS results accurately with errors lower than 2%, if the properties in the evaporation rate models are based on properties in the near field around the droplet. Further studies on the effects of turbulence on the evaporation process showed no evident correlation between the evaporation rates and the subgrid kinetic energy relating the effects of turbulence to vapour dispersion away from the droplet surface.     

Linear and non-linear response of embedded Na clusters

We investigate Na clusters embedded in Ar matrices. The surrounding Ar atoms are modeled in terms of their dynamical polarizability and the strong electron repulsion. The calibration of the model is discussed. First results for the non-linear optical response of the Na clusters are presented for the test case of Na₈ embedded in Ar ensembles of different sizes. It is shown that blue shift through core repulsion and red shift through dipole polarizability counterweight each other to the end that very little global shift is seen in the spectra. This feature persists to all excitation strengths considered. There are, however, detailed effects, such as for example the Landau fragmentation of the Mie plasmon peak. PACS 36.40.Gk; 36.40.Vz; 31.15.Ew     

Dynamics of Na clusters in picosecond laser pulses

We investigate the electronic and ionic dynamics of Na clusters under the influence of a laser pulse in the range 100 femtoseconds to picoseconds. The dynamics is described by means of the time-dependent local-density approximation coupled to ionic molecular dynamics (TDLDA-MD). Variation of pulse length allows us to explore the time scales of ionic motion in a manner similar to pump and probe experiments. Resonant enhancement of electron emission serves as a measure for the time scale of Coulomb explosion. Received: 3 July 2001 / Published online: 10 October 2001     

DFT studies of ionic vibrations in Na clusters

We use time-dependent density functional theory coupled to molecular dynamics for ionic motion to compute the spectra of ionic vibrations in small Na clusters. Comparison with results from the distance dependent tight-binding approach shows good agreement between these two very different methods. We discuss the evolution of the spectra with cluster size and charge and the impact of ionic vibrations on the optical response. Received 23 July 2001 / Received in final form 5 July 2002 Published online 8 October 2002 RID="a" ID="a"e-mail: suraud@irsamc.ups-tlse.fr     

Na5, Na6和Na7团簇光学吸收谱的理论研究

以含时密度泛函理论为基础,结合从头赝势方法运用含时局域密度近似计算了Na5,Na6和Na7团簇的动力学极化强度,并通过傅里叶变换得出了团簇的光学吸收谱.研究表明,计算结果可以较好地再现实验谱,而且Na6和Na7团簇的结果和组态相互作用的结果符合较好.二维结构和三维结构的Na6团簇的计算结果表明,只有二维结构可出现低于2 eV以下的峰,而且二维结构光谱的计算结果与实验结果符合较好.     

Coupled plasmon and phonon dynamics in embedded Na clusters

We investigate, from a theoretical perspective, the coupled electronic and ionic/atomic dynamics of Na clusters embedded in Ar matrices. The system is described by time-dependent density-functional theory for cluster electrons and classical motion for Na⁺ ions as well as for Ar atoms. The interaction with the surrounding Ar atoms is modelled by polarization potentials plus core repulsion. We use this model to study coupled electronic and ionic/atomic motion in embedded clusters following a very short laser pulse. For excitations in the non-linear regime, we find clear signs for the coherent coupling of the Mie plasmon resonance with ionic vibrations (phonons). In addition, an incoherent line stretching is observed which can be traced back to the turning point of ionic vibrations. The coupling to the atomic motion of the surroundings leads to a slow and far reaching rearrangement of the matrix. PACS 36.40.Gk; 36.40.Vz; 31.15.EW     

Interfacial reaction rates and free energy of cubic clusters

A new formulation of interfacial reaction rates for clusters in binary alloys is presented. It accounts for the matrix structure and the topological properties of the clusters at the atomic scale. It is shown that the probabilities per unit time that a solute atom be captured or released by a cluster are functions not only of the partition function but also of a transition function. The principles of calculation of these functions are general but only the case of cubic clusters is treated here (results can be used for L₁₂ clusters in fcc matrices). Exact calculations have been done for small clusters (size<10), followed by a Monte-Carlo sampling method for intermediate sizes as a function of temperature and interaction energy (a material characteristic). Finally, it is shown that generic results can be extrapolated at higher cluster size in a large range of temperature and/or interaction energy.     

Orientation averaged angular distributions of photo-electrons from free Na clusters

We explore, from a theoretical perspective, photo-electron angular distributions (PAD) of small Na clusters using the time-dependent local-density approximation (TDLDA). We assume an isotropic ensemble of free clusters and develop, for the case of one-photon emission, analytical formulae for the orientation averaged PAD which require only six TDLDA calculations in properly chosen reference frames. All information in the averaged PAD is contained in one anisotropy parameter. This parameter varies little with system size, but is crucially influenced by the detailed ionic structure.     

Electron dynamics in Na and Pt clusters from time-dependent Hartree-Fock theory

We present a method for the numerical investigation of the electron dynamics in small metallic clusters in intense laser fields. We obtain information about collective excitations and relaxation processes in the Na ₉ ⁺ and Pt₃ clusters analyzing the power spectrum of the dipole moment within a mean-field approach. The power spectrum is computed for various laser pulse parameters as well as for the limit of an infinitely short laser pulse. Due to the basis set expansion of the wave function our method is capable to follow the dynamics not only of the whole electron cloud, but of any particular molecular orbital. Received 28 March 2002 / Received in final form 31 May 2002 Published online 24 September 2002 RID="a" ID="a"e-mail: pavlyukh@mpi-halle.de