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  • 36.40.+d  (210)
  • Springer  (210)
  • Essen : Verl. Glückauf
  • Krefeld : Geologischer Dienst Nordhein-Westfalen
  • 2015-2019
  • 2005-2009
  • 1990-1994  (210)
  • 1965-1969
Collection
Publisher
  • Springer  (210)
  • Essen : Verl. Glückauf
  • Krefeld : Geologischer Dienst Nordhein-Westfalen
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Year
  • 1
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    Applied physics 58 (1994), S. 57-62 
    ISSN: 1432-0649
    Keywords: 32.80.Pj ; 42.50.Vk ; 36.40.+d
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The mechanism responsible for transitions of laser-cooled trapped ions from an ordered “crystal” state to an irregular “cloud” state has been discussed controversially. A numeric and analytic study of the relative motion of two trapped ions without laser cooling is performed and compared with the results of previous simulations involving the laser. It turns out that the system without laser, in spite of its simplicity, already exhibits a non-monotonic dependence of crystal stability on trap parameters, which is linked to the presence of low-order nonlinear resonances.
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  • 2
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    The European physical journal 349 (1994), S. 219-222 
    ISSN: 1434-601X
    Keywords: 21.60.Jz ; 36.40.+d
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We discuss and compare the gross features of resonance excitations in nuclei and metal clusters. We point out the phenomenon of ”jellium scaling” which means that different materials for metal clusters all give similar resonance spectra and we discuss the various effects which determine the exact position of the resonance.
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  • 3
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    The European physical journal 29 (1994), S. 45-48 
    ISSN: 1434-6079
    Keywords: 36.40.+d ; 34.90.+q
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A direct measurement of collisionally induced fission of C 60 2+ has been performed. We have measured coincidences between various charged fragments resulting from collisions between C 60 2+ and He atoms. The measurements show that C 60 2+ not only emits C2 units but also breaks up into larger, singly charged parts. In this paper, we report on coincidences between C n + (2≦n≦9) and C m + (42≦m≦48) fragment ions.
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  • 4
    ISSN: 1434-6079
    Keywords: 34.90.+q ; 36.40.+d
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    Topics: Physics
    Notes: Abstract Various fullerene ions are generated in a standard plasma ion source from a vaporized mixture of C60/C70. Except C 60 + and C 70 + , the fullerene ions are formed by fragmentation of C60 or C70 excited by electron impact. Information on the structure and stability of the fullerene ions is obtained by studying unimolecular dissociation and collision-induced fragmentation of C 60 + , C 58 + and C 56 + in H2 and Ar target gases.
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  • 5
    ISSN: 1434-6079
    Keywords: 34.20.−b ; 36.40.+d ; 31.20.Ej
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    Topics: Physics
    Notes: Abstract A detailed study of the intrinsic consistency of the semiempirical method of P. Claverie namely, the effects of the basis set and intramolecular correlation on the multipole distributions of molecular subunits and the influence of the electronic population of each atom in the molecular subunit on its van der Waals radius, is performed on some van der Waals dimers. The validity, limits of this model and the appropriate way to use it is established. In particular, the dependence of the geometry and the interaction energy on the basis set chosen and the intramolecular correlation shows that the multipole distribution involved in the calculation of the electrostatic and polarization terms must be derived from a correlated wave function within an extended basis set. Associated to non local methods for finding stationary points, the method of P. Claverie reproduce reliably the intermolecular geometrical parameters observed for the equilibrium structures and the transition states of the dimer and trimer of acetylene. In addition, a study of the equilibrium structures of the ethylene dimer is presented, the aim of being to clarify the considerable uncertainty in their number and their geometry.
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  • 6
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    The European physical journal 30 (1994), S. 349-356 
    ISSN: 1434-6079
    Keywords: 36.40.+d ; 61.55.Hg ; 68.35.Dv
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Density functional theory is used to study the atomic and electronic structures of Na10K10Cs n clusters with up to sixty atoms. The simplifying approximation has been made of replacing the external potential of the ionic background by its spherical average about the cluster centre in the iterative process of solving the Kohn-Sham equations for each geometry tested. The search for the equilibrium geometry is performed by employing the technique of simulated annealing. We have always found segregation of Cs atoms to the surface as well as a rather neat separation of different species in different (radial) regions of the cluster. This layering effect appears to be consistent with a tendency to maximize electronegativity differences. When the cluster is big enough, Cs atoms begin to appear also inside the cluster. Those geometrical effects do not perturb the electronic magic numbers well known for pure alkali metal clusters.
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  • 7
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    The European physical journal 31 (1994), S. 85-93 
    ISSN: 1434-6079
    Keywords: 36.40.+d ; 07.75.+h ; 07.77.+p
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The cluster source we use, a low pressure, rare gas condensation cell, is capable of producing clusters containing more than 45 000 atoms or having masses exceeding 2 500 000 amu. Details of this source and the dependence of the cluster size distribution on adjustable working parameters (oven temperature, inert gas pressure, inert gas type) are discussed in this report. Measurements of the mass-dependent velocity distributions of the clusters emitted by the source are presented and compared to a simple model calculation. The clusters are mass-analyzed with a time-of-flight mass spectrometer and detected by a multi-channel plate. The dependence of the detectability of large clusters on the acceleration voltage is investigated.
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  • 8
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    The European physical journal 31 (1994), S. 187-189 
    ISSN: 1434-6079
    Keywords: 36.40.+d
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    Topics: Physics
    Notes: Abstract Collision of Ar cluster ions, Ar n + (n=3−16), with He and Ne atoms was investigated by use of mass spectroscopic techniques. The cross sections for the production of Ar n′ + (n′〈n) were measured as functions of the size of the parent cluster ion and the collision energy (0.1–10 eV in the center-of-mass frame). These results were analyzed in the scheme of hard-sphere spectator collision with RRK theory. It was concluded that the reaction proceeds via collisional excitation of the parent cluster ion and following sequential loss of the constituent Ar atoms.
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  • 9
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    The European physical journal 32 (1994), S. 125-136 
    ISSN: 1434-6079
    Keywords: 31.20.Sy ; 31.20.Rx ; 36.40.+d
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present an extension of the jellium model for alkali clusters which allows a selfconsistent determination of the jellium background in addition to the Kohn-Sham solution of the electron dynamics. It includes the volume and the surface energy of the ionic distribution. We discuss applications to the shape of clusters and to the position and width of the plasmon resonances.
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  • 10
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    The European physical journal 29 (1994), S. 67-72 
    ISSN: 1434-6079
    Keywords: 36.40.+d ; 75.60.Jp ; 76.50.+g
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The deflection of free magnetic metal clusters in a Stern-Gerlach magnetic field is studied. In particular we investigate magnetic resonance effects resulting from lattice anisotropy and cluster rotation. In analogy to small suspended particles in an oscillating magnetic field the anisotropy field fixed to the rotating atomic lattice of the cluster acts on the cluster magnetization like an rf field in NMR experiments. In our calculation we have used the Bloch equations and assumed different anisotropy field symmetries (uniaxial, cubic). A minimum in the magnetization as a function of the Stern-Gerlach field and also of the cluster size, as observed recently, is obtained under certain conditions. However, such a resonance behavior occurs only if the distribution of the rotation frequency ωrot is relatively narrow, while a broad distribution of ωrot yields an almost superparamagnetic behavior. In addition, the strength of the anisotropy field and the relaxation time are important variables which determine the magnetic behavior of the clusters.
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  • 11
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    The European physical journal 29 (1994), S. 81-83 
    ISSN: 1434-6079
    Keywords: 33.80.Eh ; 36.40.+d ; 36.90.+f
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Ionization dynamics of free C60 is investigated after single photon excitation with synchrotron radiation in the energy range between 8.9 eV and 27.6 eV. Upper limits for the time constants describing possible delayed ionization are obtained from peak shape asymmetries in time of flight spectra as a function of excitation energy. The results clearly indicate that delayed ionization is not observed after single photon excitation in contrast to the results obtained in ns-laser MPI experiments.
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  • 12
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    The European physical journal 29 (1994), S. 147-150 
    ISSN: 1434-6079
    Keywords: 36.40.+d ; 34.20.−b
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The limits of stability of small atomic clusters with angular momentum are investigated as a function of the size and charge of the aggregates. Critical angular momenta are obtained in a di-cluster picture from the balance of a cohesive surface-surface interaction and the disruptive centrifugal and electric interactions. The calculation scheme incorporates the mass and charge asymmetry degrees of freedom and it is thus particularly suited to explore cluster-cluster reaction processes. Deformation of the fragments is taken into account.
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  • 13
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    The European physical journal 29 (1994), S. 241-243 
    ISSN: 1434-6079
    Keywords: 21.60.Jz ; 36.40.+d ; 71.10.+x
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Numerical Hartree-Fock calculations on the2S+1 L states arising from 1s 2 1 pv and 1s 2 1p 6 1 pμ open-shell configurations of Nan-type (n≦20) jellium spheres are reported for the first time. TheL, S multiplet energy spread is substancially larger than that expected from earlier estimates of exchange interaction energies. A new way of considering cluster abundance structures in mass spectra emerges from the results.
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  • 14
    ISSN: 1434-6079
    Keywords: 36.40.+d
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    Topics: Physics
    Notes: Abstract The effects of adding molecules on the LIF at 540 nm of a barium atom at the surface of an argon cluster (average size 420) has been investigated. We showed that molecules like ethanol,n-hexane and O2 from stable complexes with ground state barium. In the case of molecules like N2, CH4 and SF6, the collisional quenching of solvated Ba(1 P) is observed. The large quenching rates obtained are interpreted by a surface mobility of the collisional partners. Moreover, we showed that this collisional quenching leads to the ejection of free Ba(3 P 1).
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  • 15
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    The European physical journal 30 (1994), S. 69-75 
    ISSN: 1434-6079
    Keywords: 36.40.+d ; 33.80.Eh ; 82.30.Nr ; 82.40.Dm
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Geometric structures and the energies for the ground and several excited electronic states of a sodium atom bound with one or two ammonia molecules are presented. All self consistent field (SCF) calculations are performed with extended basis sets. Geometry optimization and one electron properties have been performed within the SCF approximation. Excited states have been calculated with the multi-configuration SCF (MCSCF) technique. This system may be viewed as a precursor to solvation in a macroscopic system. The excited state calculations provide important information for spectroscopic studies of these complexes.
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  • 16
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    The European physical journal 32 (1994), S. 257-260 
    ISSN: 1434-6079
    Keywords: 34.20.cf ; 36.40.+d
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A new empirical potential energy function (PEF) is proposed, which is formed from pair-interactions only, and containes the many-body contributions. The PEF satisfies bulk cohesive energy and bulk stability condition. The PEF is parameterized for copper, silver, and gold elements in fcc crystal structure. The elastic constantsC 11 andC 12 and the bulk modulus of the elements are calculated, and the structural stability and energetics of microclusters containing 3 to 7 atoms of the same elements are investigated.
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  • 17
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    The European physical journal 29 (1994), S. 73-80 
    ISSN: 1434-6079
    Keywords: 36.40.+d ; 78.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have computed the electronic structure and the imaginary part of the dielectric function of the Au55 cluster using a tight-binding model which takes into account size and boundary effects. Besides the Drude behavior, we obtain anomalous absorption peaks in the infrared region due to the breakdown of the selection rule Δk=0. Taking into account surface coating effects, the onset of the interband absorption above 2.5 eV results to be red shifted, in agreement with the experimental data. As a consequence, we obtain a dramatic increase of the width of the plasmonic absorption peak, which accounts for the experimental optical spectra observed by Kreibig.
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  • 18
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    The European physical journal 31 (1994), S. 125-129 
    ISSN: 1434-6079
    Keywords: 36.40.+d ; 73.20.Mf ; 21.60.Cs
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The origin of the plasmon excitation in small metal clusters is studied within the jellium model through ab initio electronic-structure calculations based on the nuclear shell model. In the limit of infinite size, the plasmon classically represents pure harmonic motion of the center of mass of the valence electrons. It is shown that this limit is already well approximated by clusters of only eight electrons.
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  • 19
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    The European physical journal 29 (1994), S. 139-145 
    ISSN: 1434-6079
    Keywords: 36.40.+d ; 34.10.+x ; 82.20.Rp
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract In this article, the dynamics of fragmentation of highly excited van der Waals clusters are analyzed. An approach to the characterization of complex cluster fragmentation is described, based on a combination of structural and dynamical analysis of classical trajectories. In this method, the final fragmentation pattern is analyzed by a depth-first search algorithm based on graph theory, which provides a partitioning of the system into intact fragments of various sizes: monomers, dimers, trimers, and higher multimers. With knowledge of the final products, a dynamical analysis is accomplished by tracing back the time dependence of the relevant fragment energies. A transition from oscillatory to constant behavior of these energies indicates that the fragment has become energetically isolated from the parent cluster. This allows an unambiguous definition of the dissociation time of the fragment. The method is applied to the characterization of cluster fragmentation of chemically activated I2Ar 12 * formed in the association reaction I+I(Ar)12→I2+12 Ar. It is found that cluster decomposition channels, leading to the formation of at least one multi-atom fragment, play an important role in the evaporative cooling of I2, in addition to monoatomic sequential evaporation.
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  • 20
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    The European physical journal 30 (1994), S. 57-68 
    ISSN: 1434-6079
    Keywords: 36.40.+d ; 64.70.-p
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    Topics: Physics
    Notes: Abstract Using a distance-dependent tight-binding hamiltonian, we have studied the influence of the temperature on the geometries of small alkali clusters (Na4, Na8, and Na20). We have applied a Monte-Carlo thermodynamical method which consists in performing canonical samplings for various temperatures, these samplings being reexpressed in the microcanonical ensemble. This method provides thermodynamical values such as the entropy and the specific heat. Their behaviour shows one phase transition in the case of Na4 and Na8, and two phase transitions for Na20. As concerns Na4 and Na8, the transition occurs at ≈200 K, between a solid-like phase and a phase for which the geometry of these clusters oscillates between numerous shapes. In the case of Na20, the two observed phase transitions can be described as a melting of the surface atoms (at ≈200 K) preliminarily to the fluctuation of an inner icosahedron seed (at ≈300 K).
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  • 21
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    The European physical journal 30 (1994), S. 323-326 
    ISSN: 1434-6079
    Keywords: 36.40.+d ; 34.90.+q
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    Topics: Physics
    Notes: Abstract Electron capture in collisions of C 60 2+ and C 60 3+ molecular ions with atomic and molecular gases has been studied at impact energies around 100 keV. The cross-section dependence on the target-ionization potentials is discussed, and a simple over-barrier model is evoked to explain the energy dependences. The cross sections for endothermic processes are discussed in the light of a simple two-state model, and a general understanding of their behaviour is obtained.
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  • 22
    ISSN: 1434-6079
    Keywords: 36.40.+d ; 34.50.−s ; 35.20.Gs
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    Topics: Physics
    Notes: Abstract The stability of gold cluster ions Au n + (2≦n≦23) has been investigated via collision induced dissociation in a Penning trap. Threshold energies and dissociation channels have been determined. The cluster stability exhibits a pronounced odd — even alternation: Clusters with an odd number of atoms,n, are more stable than the even-numbered ones. Enhanced stabilities are found for Au 3 + , Au 9 + , and Au 19 + in accordance with the Clemenger-Nilsson and the deformed jellium model of delocalized valence electrons. Excited odd cluster ions withn≦15 predominantly decay by evaporation of dimers; all others decay by monomer evaporation. From the dissociation channels estimates of the binding energies are deduced.
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  • 23
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    The European physical journal 31 (1994), S. 95-104 
    ISSN: 1434-6079
    Keywords: 36.40.+d ; 33.10.Gx
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    Topics: Physics
    Notes: Abstract A second order perturbation approach for the evaluation of the splitting and shifting of the infrared vibrational bands of molecular clusters, based on early publications of Buckingham, is presented. The Hamiltonian of the system comprises harmonic- and anharmonic intramolecular vibration terms, as well as the intermolecular potential. The anharmonic contributions of the intramolecular force field and the intermolecular potential are treated as a perturbation. In order to extend the applicability of the approach to homogeneous molecular clusters, the formalism of degenerate perturbation theory is employed. The new approach is applied to methanol clusters from dimer to hexamer for calculating the frequency shifts of the OH stretching mode (ω1=3681.5 cm−1), the CH3 rocking mode (ω7=1074.5 cm−1), and the CO stretching mode (ω8=1033.5 cm−1). The numerical results compare favourably with experimental and previous theoretical data (except for ω1), but with a tendency to overestimate the shifts.
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  • 24
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    The European physical journal 31 (1994), S. 131-134 
    ISSN: 1434-6079
    Keywords: 61.16.Di ; 36.40.+d ; 66.30.−h
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    Topics: Physics
    Notes: Abstract Cluster-size dependence of alloying behavior in nm-sized atom clusters has been studied by transmission electron microscopy, using clusters in the Au-Cu system. It was revealed that occurrence of rapid spontaneous alloying becomes more difficult with increasing cluster size. In gold clusters of approximately 4 nm in the mean size, a rapid dissolution of copper atoms took place and homogeneously mixed Au-Cu alloy clusters were formed. In gold clusters of approximately 10 nm in the mean size, rapid alloying of copper took place only at a shell-shaped region beneath the free surface of individual clusters and pure gold was retained at the central region of clusters. In gold clusters of approximately 30 nm in the mean size, no rapid alloying of copper was induced. The ease with which spontaneous alloying takes place is discussed in terms of the lattice softening in atom clusters.
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  • 25
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    The European physical journal 31 (1994), S. 191-197 
    ISSN: 1434-6079
    Keywords: 36.40.+d
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    Topics: Physics
    Notes: Abstract Highly charged metal clusters can be easily produced using the technique of multi-step photoionization. The critical sizes at which fission occurs are reported here for alkali metal clusters having charges as high as +7. An interpretation within the framework of the liquid droplet model reveals that the fission process is strongly asymmetric and thermally activated.
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  • 26
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    The European physical journal 32 (1994), S. 249-255 
    ISSN: 1434-6079
    Keywords: 31.20.Ej ; 36.40.+d
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    Topics: Physics
    Notes: Abstract Alkali halide clusters Na n F n have most frequently cuboid structures and Na n F n−1 clusters are derived from Na n F n by removing a F− centre. An excess electron localises on the site of the removed F− ion. In this paper, other kind of clusters are studied at the SCF level: i) The non-cubic structures. ii) The cubic Na n F n−1 for which the excess electron has no defined site. Due to very large size of these species (at least 27 atoms!) some equivalent fictive linear and planar geometries are studied.
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  • 27
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    Applied physics 56 (1993), S. 215-217 
    ISSN: 1432-0630
    Keywords: 71.25.Mg ; 71.25.Rk ; 71.20.−b ; 36.40.+d
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract A tight-binding method which has been previously applied to study the effect of uncorrelated orientational disorder on conduction-band properties is extended here to the case of systems with long-range order and/or short-range correlations. The density of states and conductivity are not highly sensitive to the specific short-range correlations, so long as the system is not too close to being fully ordered. Hence the strong effects of disorder found previously appear to be robust and should play an important role in the interpretation of normal-state properties of A3C60.
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  • 28
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    Applied physics 57 (1993), S. 101-104 
    ISSN: 1432-0630
    Keywords: 73.20.Mf ; 82.65.Pa ; 82.50.−m ; 36.40.+d
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Laser-induced desorption of metal atoms from the surface of small metal particles has been investigated as a function of the shape of the particles and the polarization of the incident laser light. The particles were supported on LiF, quartz or sapphire substrates. In a first set of experiments, the shape of the particles was determined by recording optical transmission spectra with s- and p-polarized light incident under an angle of typically 40° with respect to the surface normal. The metal particles turn out to be oblate, the ratio of the axes perpendicular and parallel to the substrate surface being on the order of 0.5. This ratio decreases with increasing particle size. Also, the particles change shape if the temperature is raised. In further experiments, s- and p-polarized light has been used to stimulate desorption of atoms via surface plasmon excitation. It is found that the desorption rate markedly depends on the polarization of the light. This is explained by excitation of the collective electron oscillation along different axes of the non-spherical particles.
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  • 29
    ISSN: 1434-6036
    Keywords: 36.40.+d ; 74.75.+t ; 79.60.−i
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    Topics: Physics
    Notes: Abstract The electronic structure of K-doped C60 was investigated by photoemission (PE) and X-ray absorption near-edge structure (XANES) studies at the C-1s and K-2p thresholds. In addition, information on the local K-derived partial density of states in superconducting K3C60 was obtained by resonant PE at the K-2p 1/2 threshold. The experimental observations support a complete charge transfer from K to C60 and we clearly observe a finite density of states atE F . From resonant PE, occupied states with K-p, d character could be identified in the binding-energy region from 1.5 to 8 eV below, but not directly at the Fermi level. This partial-density-of-states structure agrees well with the results of our band-structure calculations based on the local-density approximation.
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  • 30
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    The European physical journal 26 (1993), S. 141-143 
    ISSN: 1434-6079
    Keywords: 36.40.+d ; 61.50.Lt
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    Topics: Physics
    Notes: Abstract Size selected silicon clusters have been isolated in rare gas matrices and studied by optical absorption spectroscopy. The clusters were produced in a pulsed laser vaporization source, size selected with a quadrupole mass spectrometer and deposited at low energies into a cocondensed krypton matrix held at T〈20 K. A comparison of the optical spectra of ten atom wide bands (Si25-Si35, Si35-Si45 and Si45-Si55) shows the general size evolution of the optical properties. Single cluster sizes have also been isolated and show somewhat sharper spectra than the bands. The measured spectra show similarities to spectra calculated using Mie theory and bulk optical constants. Cluster-cluster agglomeration was studied by evaporating the inert gas matrix. The results suggest that the clusters agglomerate into larger particles even under the mildest "soft landing" conditions.
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  • 31
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    The European physical journal 26 (1993), S. 159-161 
    ISSN: 1434-6079
    Keywords: 36.40.+d ; 82.30.N ; 82.60
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    Topics: Physics
    Notes: Abstract A new class of molecular clusters with the composition M8C12 (M=Ti, V, Hf and Zr) has been discovered. The cation, anion and neutral species are unusually abundant and stable. In the case of the Zr-C system, evidence has also been obtained for growth through the development of multicage structures.
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  • 32
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    The European physical journal 26 (1993), S. 195-197 
    ISSN: 1434-6079
    Keywords: 36.40.+d ; 05.45.+b
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    Topics: Physics
    Notes: Abstract The structure and long-time dynamics of (LiBr) n n=4,5,8 were studied by optimization and molecular dynamics. The ring structure, double ring stack for (LiBr)8, was found to be the energy minimum. They are dynamically flexible and, particularly for (LiBr)4 and (LiBr)5, hardly isomerise into other isomers determined by optimization. (LiBr)8 is probably a promising candidate to exhibit dynamic coexistence among a number of isomers.
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  • 33
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    The European physical journal 26 (1993), S. 207-209 
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    Keywords: 36.40.+d ; 33.20. Ea ; 33.80 Gj
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    Topics: Physics
    Notes: Abstract We report the vibrational spectra of the cluster ions Cs+(CH3NH2)N, N=3–22. Bands in the 1015–1050 cm−1 region of the infrared are due to the v8 mode (CN stretch) of methylamine molecules displaying different degrees of interaction with the central ion. Monte Carlo simulations of the solvated Cs+ ion indicate that nine methylamine molecules fill the first solvation shell of Cs+ and that possible rearrangements in cluster structure occur at N=14−15. No absorptions due to bulklike methylamine molecules are observed through N=22.
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    The European physical journal 26 (1993), S. 217-219 
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    Keywords: 36.40.+d ; 33.70.Jg ; 34.10.+x
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    Topics: Physics
    Notes: Abstract We present results of semiclassical simulations of the electronic spectra and dynamics of aniline-Arn (1≤n≤3) clusters. The spectral density formalism of Mukamel [3] is used to generate the spectra from the time dependent energy difference of the S0 and S1 states of aniline solvated by the argon atoms. A repulsive Ar-N interaction is incorporated in the Hamiltonian of the S1 state; this term permits a quantitative prediction of the origin shifts of the S1〈--S0 transition (both red and blue shifts) for all the clusters studied. The temperature dependence of the spectrum of aniline-Ar2 is correlated with the underlying dynamics of this cluster.
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  • 35
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    The European physical journal 26 (1993), S. 33-35 
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    Keywords: 33.80.-b ; 36.40.+d
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    Notes: Abstract Using picosecond pump&probe technique followed by mass-selective detection, the dynamics of excited states of cold Nan=3...8-clusters in a supersonic beam — excited at λ=422nm — have been studied in time-resolved two-photon-ionization (TPI)-experiments. With growing cluster size a monotonous decrease of the decay times is observed except in the case of Na7 known to be a very symmetric cluster.
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  • 36
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    The European physical journal 26 (1993), S. 166-168 
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    Keywords: 36.40.+d ; 61.22.Pg
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    Topics: Physics
    Notes: Abstract We describe here molecular dynamics computer simulations performed to study the solvation of ions (Cl− and Na+) in water clusters. Our simulations show that the calculated structure and dynamics of the clusters is very sensitive to the potential model which is used to describe the interactions. From the comparison with thermodynamic data and data from the photoelectron spectra we conclude that in Cl−(H2O)n (n≤20) clusters the ion is located on the surface of the cluster.
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  • 37
    ISSN: 1434-6079
    Keywords: 31.10.+z ; 36.40.+d ; 33.45.+x ; 33.60.-q
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    Topics: Physics
    Notes: Abstract We discuss the electronic structure, bonding and physical properties of the gold cluster compound Au55(PPh3)12Cl6. Results from our experimental measurements, including EXAFS, specific heat, Mössbauer, UV-visible and photoelectron spectroscopy, are combined with those of other work to form a consistent physical picture of the system. The bonding in Au55(PPh3)12Cl6 is much more delocalised and non-directional than in smaller gold cluster molecules. The Au55 cluster exhibits a substantial degree of metallic bonding, while displaying some of the characteristics of a discrete energy level spectrum.
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  • 38
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    The European physical journal 26 (1993), S. 178-180 
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    Keywords: 36.40.+d
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    Topics: Physics
    Notes: Abstract The (KCl)32 cluster is used as a model system to study the possibilities for clusters to exhibit amorphous or glassy solid forms. The problem has two aspects: first, whether the potential surface of the cluster supports a myriad of locally stable, disordered structures, the ensemble of which would constitute the glassy state, and second, whether an ensemble of amorphous clusters can be prepared under laboratory conditions. Molecular dynamics studies give an emphaticyes to the first issue, and an equally emphaticno to the second, for cooling rates up to 1012 K/s, a thousand-fold faster than the fastest rates yet reported. However, if the long-range Coulomb interaction of the ions is replaced by a shielded Coulomb (Debye or Yukawa) potential, the secondary minima are sufficiently stabilized and the saddles, sufficiently high, that disordered equilibrium structures can be reached by cooling at fast, but still conceivably attainable rates. The implication is that while alkali halide clusters probably cannot form glasses, binary clusters with shorter-range forces, such as those of II–VI and III–V compounds, probably can form glasses. The highly disordered structures of (KCl)32 are perhaps the most disordered forms yet seen for solid matter.
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  • 39
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    The European physical journal 26 (1993), S. 192-194 
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    Keywords: 31.20.Tz ; 31.50.+w ; 33.10.-u ; 36.40.+d
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    Topics: Physics
    Notes: Abstract Stable ground state structures for neutral and charged Li n H, Li n H2 and Na n F m (n↛m) have been determined usingab —initio methods accounting for electronic correlation effects. The consequences of replacing alkali-atom by foreign-atom or -atoms have been studied. The similarities and differences with respect to topologies of the most stable Li n and Na n clusters will be pointed out. The ionization potentials and spectroscopic patterns are compared with available experimental data. This allows for the geometrical and spectroscopic assignments as well as for a study of the influence of localized versus non localized bonding on the ground and excited state properties of mixed clusters.
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  • 40
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    The European physical journal 26 (1993), S. 270-272 
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    Keywords: 36.40.+d ; 33.20.Kf ; 35.20.Pa
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    Topics: Physics
    Notes: Abstract The B state of Na3 is often cited as an example of fractional quantization of the pseudorotational motion on the lower adiabatic surface of theE×e Jahn-Teller system. Recently, an alternative interpretation of the experimental results was given which is based on a pseudo-Jahn-Teller treatment and implies integer quantization. We present rotationally resolved spectra of a number of vibronic bands of the Na3 B-X system and believe that they can only be explained in terms of integer quantum numbersj of the vibronic angular momentum.
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  • 41
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    The European physical journal 26 (1993), S. 301-303 
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    Keywords: 36.40.+d ; 75.50.Cc
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    Topics: Physics
    Notes: Abstract We use a variation of the Stern-Gerlach experiment to study the magnetic behavior of transition metal clusters. We report measurements of the magnetic properties of nickel clusters as a function of cluster size, vibrational temperature, and applied magnetic field. Results for these nickel clusters resemble those previously published for cobalt clusters in that superparamagnetic behavior is observed. As is the case for cobalt clusters, nickel clusters are observed to have magnetic moments per atom that are greater than the bulk value.
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  • 42
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    The European physical journal 26 (1993), S. 325-327 
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    Keywords: numbers 75.50Bb ; 36.40.+d
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    Topics: Physics
    Notes: Abstract Magnetic properties of small iron clusters in a supersonic molecular beam are investigated. The magnetization is probed as function of magnetic field, temperature and cluster size. Temperatures are controlled by changing the source temperature (100 K to 500 K) and the expansion conditions. The clusters also may be heated in flight with light from a pulsed laser. Hot clusters are found to be superparamagnetic however cold clusters are not but show strongly reduced magnetization which furthermore is non-linear with the applied field. Experimentally it is found that the anomalies are related to the cluster rotations. We also address a controversy between our earlier findings [1] and those by Bucher et al. [2], and demonstrate that their temperature determinations and consequent conclusions are incorrect.
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  • 43
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    The European physical journal 26 (1993), S. 352-354 
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    Keywords: .64.70.Fx ; 36.40.+d ; 35.20.Vf
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    Topics: Physics
    Notes: Abstract In this paper, we report properties of selenium clusters produced by vapor condensation technique. Impact electronic ionization is performed on clusters in the size range from 2 to 36 atoms. The measured ionization potentials exhibit small oscillation corresponding to the wiggles observed on the mass distribution. An attempt to connect these experimental observations with the geometrical structure of the molecules is made in the discussion.
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  • 44
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    The European physical journal 26 (1993), S. 394-396 
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    Keywords: 36.40.+d
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    Notes: Abstract Noble gas atoms trapped in the intracrystalline cavities of zeolites may form clusters. A classical-mechanical isoenergetic molecular dynamics simulation is performed to simulate the dynamical behavior of noble gas clusters in zeolite cavities. To implement the simulation, a model is adopted of a homogeneous spherical cavity with Morse interaction between the noble gas atoms and cavity walls. The results for Ar6 clusters indicate that the noble gas clusters in the cavity undergo the same solid/liquid phase changes as in free space, and, at high enough energies, a rapid exchange between atoms adsorbed on the inner surface and thosein the interior of the cavity. Mathematical quenching is used to investigated the multidimensional potential surface of Ar clusters in the cavity.
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  • 45
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    The European physical journal 26 (1993), S. 391-393 
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    Keywords: 36.40.+d ; 05.45.+b
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    Notes: Abstract We have computed the local Kolmogorov entropy of molecular dynamics trajectory segments near the potential energy saddles of model Ar3 and Ar5 clusters. In the case of Ar3 clusters bound with a Lennard-Jones potential, the local Kolmogorov entropy of the cluster is significantly smaller in the saddle region than in other areas of the potential surface. This behavior indicates an increase in the degree of nearly quasiperiodic motion near the Ar3 saddle due to the partial decoupling of the cluster's vibrational modes there. Lennard-Jones Ar5 clusters do not exhibit similar behavior, but Ar5 clusters bound with a short-range Morse potential do. This suggests that the “regularizing” effect of saddle regions is strongly dependent on the shape of the energy surface near the saddle. From these observations, we can determine which features of the saddle are most important in this respect; the flatness of the saddle region seems to be one such feature.
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  • 46
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    The European physical journal 26 (1993), S. 95-97 
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    Keywords: 36.40.+d ; 33.20.Kf
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    Topics: Physics
    Notes: Abstract The dipole surface plasmon forK N + clusters is analyzed using the RPA sum-rule technique within a semiclassical Density Functional Theory and the spherical jellium model. The theoretical frequencies are blue shifted as compared to the experimental ones. The discrepancies between theory and experiment are reduced when considering non-local energy contributions in the density functional and phenomenologically including atomic lattice effects by means of an electron effective mass and a static dielectric constant.
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  • 47
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    The European physical journal 26 (1993), S. 73-75 
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    Keywords: 61.14.−x ; 36.40.+d
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    Notes: Abstract Calculations of the dynamical Debye-Scherrer electron diffraction pattern for ultrafine gold and silver particles have been performed using the multislice method. Two cluster sizes, 31 and 55 Å in diameter (923 and 5083 atoms, respectively), of both f.c.c. and icosahedral structures were used, at incident voltages of 40 kV and 100 kV.
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  • 48
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    The European physical journal 26 (1993), S. 92-94 
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    Keywords: 36.40.+d ; 03.65.Sq ; 33.80.Eh ; 35.20.Vf
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    Topics: Physics
    Notes: Abstract Neutral aluminum clusters have been produced by laser vaporization technique, ionized by a low-power near-threshold laser light and detected using standard TOF spectrometric methods. Ionization potentials have been deduced in the low size range. In the large size range 250〈N〈1400 the patterns of the mass spectra exhibit a regular and continuous oscillation, originating from size-dependent ionization threshold effects. The period, constant on a Ne 1/3 scale (Ne=3N is the number of valence electrons), is approximately two times shorter than the one observed in alkali experiments. This feature is analyzed in terms of shell structure.
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  • 49
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    The European physical journal 26 (1993), S. 110-112 
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    Keywords: 36.40.+d ; 21.60.Cs
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    Topics: Physics
    Notes: Abstract Under experimental conditions which do not favor delocalization of valence electrons alkali atoms remain neutral and are taken as atomic fermions. Thus, two neutral alkali component microclusters are treated by a two-fermion shell model producing two sets of magic numbers containing many common numbers.
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  • 50
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    The European physical journal 26 (1993), S. 113-114 
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    Keywords: 36.40.+d ; 82.80.Ms
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    Notes: Abstract The present paper supports coexistence of electronic shells and shells of ion cores for large alkali clusters.
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    The European physical journal 26 (1993), S. 146-149 
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    Keywords: 36.40.+d ; 33.60.Cv ; 07.77.+p
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    Topics: Physics
    Notes: Abstract Bismuth cluster anions are produced in a new type of source (PACIS). For Bismuth the intensity is estimated to be 10 times higher than that of a laser vaporization source. UV-photoelectron spectroscopy serves to study the electronic level structure of the cluster anions. In the photoelectron spectra of Bi 2 − , Bi 3 − and Bi 4 − the main transitions agree with earlier results of LIF investigations on the neutral species as well as with recent theoretical and experimental data. Here we present a first extension to higher photon energies.
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    The European physical journal 26 (1993), S. 15-17 
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    Keywords: 68.35.Bs ; 36.40.+d ; 35.20.Bm
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    Topics: Physics
    Notes: Abstract Rigorous analytical formulae for mean nearest-neighbour coordination numbers in clusters as a function of cluster size have been derived for a range of geometries: the tetrahedron, octahedron, cuboctahedron, icosahedron and bcc rhombic dodecahedron. Formulae for outer-neighbour coordination numbers are also reported, including a complete analysis of interatomic distances and mean coordination numbers in icosahedra. The formulae will find application in studies of electronic structure and interpretation of EXAFS data.
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    The European physical journal 26 (1993), S. 180-182 
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    Keywords: 36.40.+d ; 33.80.Eh
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    Topics: Physics
    Notes: Abstract Electronically excited states of magnesium-water cluster ions, Mg+(H2O) n ,n=1–5, are studied by photodissociation after mass selection. The observed photodissociation spectra are assigned to the2P–2S type transitions localized on the Mg+ ion with the aid of ab initio CI calculations. In addition to evaporation of water molecules, photoinduced intracluster reaction to produce MgOH+(H2O) n is found to occur efficiently, with pronounced size dependence. The intriguing features observed in the mass spectrum of nascent cluster ions are discussed in relation to the stepwise solvation of this reaction.
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    The European physical journal 26 (1993), S. 213-215 
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    Keywords: 31.20.Di ; 31.20.Gm ; 36.40.+d
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    Topics: Physics
    Notes: Abstract In this paper we present the theoretical results obtained for (NaCl) n clusters withn≤19. The calculations were performed using the ab-initio Perturbed-Ion (PI) model. That model was first developed for the study of ionic crystals and we have adapted it to study clusters. Within the PI method we can determine the total energy of the cluster as a function of the position of the atoms in the cluster and minimizing the total energy with respect to the positions of the atoms we can obtain the ground state geometry and other related properties. The results obtained for the equilibrium geometries are in good agreement with theoretical calculations using pair potentials. The study of the relative stabilities of clusters with different numbers of molecules show that the clusters are specially stable for n=4, 6, 9, 12, 15, 16 and 18 molecules, in good agreement with experimetal results.
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    The European physical journal 26 (1993), S. 237-239 
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    Keywords: 36.40.+d ; 31.20.Ej ; 31.20.Tz
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    Topics: Physics
    Notes: Abstract In this paper, we report the results ofab initio molecular orbital calculations on cube-like (MgO)n clusters (n=1−13). We determine the relative stability of the clusters by analyzing binding energies and dissociation energies involved in the evaporation of a neutral MgO monomer. The calculated magic numbers are in very good agreement with the abundance maxima observed in the mass spectra. We also explore the size-dependence of structural, energetic and electronic properties of the clusters, finding different rates to reach the corresponding bulk limit.
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    The European physical journal 26 (1993), S. 255-257 
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    Keywords: 36.40.+d ; 31.90.+s ; 71.45.Nt
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    Topics: Physics
    Notes: Abstract A local-density approximation calculation is performed on various M8C12 structures, including Ti8C12, V8C12, VTi7C12 and ScTi7C12. It is found that the magic numbers observed during the experimental discovery of these species does not depend on the details of the electronic structure, but rather can be described as a chemical system derived from ethylene.
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    The European physical journal 26 (1993), S. 282-284 
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    Keywords: 36.40.+d ; 71.45.−d ; 78.65.−s
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    Topics: Physics
    Notes: Abstract We discuss the collective dipolar resonances of theC 60 molecule described by a spherically symmetric shell. The shell is modeled by a step in the radial direction. We present results for π and π+σ plasmons in good quantitative agreement with recent experiments. New features, like a monopole mode of oscillation, are predicted for theC 60 molecule.
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    The European physical journal 26 (1993), S. 300-304 
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    Keywords: 36.40.+d ; 34.50.-s
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    Topics: Physics
    Notes: Abstract This article reviews recent work in Los Angeles on elementary processes in fullerene vapors. The production of fullerene molecules typically involves extreme high-temperature conditions and processes which are poorly understood at date [1–3]. Once generated, these molecules may represent the most stable molecules known [4,5]. In a recent work [C. Yeretzian et al., Nature 359, 44 (1992)] we presented clear evidence for coalescence reactions between fullerene molecules. Mass spectrometric measurements on hot, dense vapors of small fullerenes (C60 and C70) reveal the formation of stable higher fullerenes which are multiples of the initial masses. These processes are shown to occur in the gas-phase rather than in the solid film and their dependences on laser fluence and He-gas pressure are investigated. Three distinct reactions are proposed—coalescence, emission and capture—to account for the observed distributions at higher fullerene sizes. Specifically, the heat of coalescence is released through emission of small, even-numbered fragments which, in a very dense vapor, are efficiently captured by other coalesced fullerenes. These findings have implications for the long-time stability of the fullerene vapor, and for the mechanism of fullerene formation and growth, and may open new ways to the synthesis of selected higher fullerenes and encapsulation compounds.
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  • 59
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    Keywords: 07.77.+p ; 34 ; 36.40.+d
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    Topics: Physics
    Notes: Abstract The hydrogen cluster accelerator existing at Institut de Physique Nucléaire de Lyon (IPN Lyon) has been upgraded by adding a Variable Energy Post-accelerator of RFQ type (VE-RFQ). This operation has been performed in the frame of a collaboration between KfK Karlsruhe, IAP Frankfurt and IPN Lyon. The facility has been designed to deliver beams of mass selected Hn+ clusters, n chosen between 3 and 49, in the energy range 65–100 keV/u. For the first time, hydrogen clusters have been accelerated at energies as high as 2 MeV. This facility opens new fields for experiments which will greatly benefit from a velocity range never available until now for such exotic projectiles.
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    The European physical journal 28 (1993), S. 67-72 
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    Keywords: 36.40.+d ; 71.28.+d
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    Notes: Abstract Rare earth clusters of europium, thullium and ytterbium were generated by gas aggregation technique and probed by photoionization mass spectrometry. Their relative intensities in mass spectra have shown that their stabilities are governed by compact geometrical structures. The addition of oxygen gas in the nucleation region was used to produce the reactive nucleation. Several stages of oxidation were observed as a function of oxygen pressure up to saturation. For the maximal degree of oxidation the observed oxide ion compositions enable one to follow the valence of metal atom in its oxide as cluster size increases. This exhibits a divalent to trivalent valence change with cluster size. Moreover it emerges from the data that the divalent to trivalent transition for Tm, Yb, and Eu occurs at different size values.
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    The European physical journal 25 (1993), S. 261-265 
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    Keywords: 36.40.+d ; 79.20.Kz
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    Topics: Physics
    Notes: Abstract Gold clusters with diameters from 2 to 10 nm are prepared by evaporation on mica substrates. They are investigated with low energy electron loss spectroscopy in the reflected beam and characterised in a transmission electron microscope. The energy loss spectra show a broadening of the plasma peak with decreasing particle size. The plasma frequency shifts to higher energies. The size dependence of the half width and of the plasma frequency is compared to known models. The results support the quantum box model of Genzel et al.
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  • 62
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    Notes: Abstract The optical absorption spectra of small niobium clusters have been determined over the wavelength range 260 – 740 nm by photodetaching Krypton atoms from the corresponding neutral van der Waals, vdW, complexes, NbnKrm, n=5–15, m=1–3. Cross sections for small gold clusters were determined by photodetachment experiments oncharged vdW complexes [AunXem]+, m=1, 2. The absorption cross sections are observed to increase monotonically with decreasing wavelength. At the long wavelength end of the range, the cross section is practically independent of the cluster nuclearity, n; whereas, at the short wavelength end of the range, the cross section increases monotonically with n.
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    The European physical journal 26 (1993), S. 56-58 
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    Keywords: 68.35.Md ; 36.40.+d ; 64.60.-i ; 62.50.+p
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    Topics: Physics
    Notes: Abstract High pressure optical absorption spectra are presented for CdSe nanocrystals as a function of size. The spectra show a transition to a high pressure Rock Salt type phase at pressure greatly elevated from the bulk. The size dependence of the transition pressure can be explained in part by an increased surface tension in the Rock Salt phase relative to the low pressure tetrahedral phase.
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    The European physical journal 26 (1993), S. 51-55 
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    Keywords: 71.10.+x ; 36.40.+d
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    Topics: Physics
    Notes: Abstract We present a simulation method to determine from first principles the structure of low symmetry atomic systems. Our method is based on Langevin molecular dynamics and quantum mechanical interactions derived fromab initio pseudopotential calculations. The molecular dynamics time step with this approach can be one to two orders of magnitude larger than in the Car-Parrinello method, compensating for the time required for self-consistency at each step. Moreover, because the simulation is constrained to reside on the Born-Oppenheimer surface, this method can be used for insulating as well as metallic and charged systems. Application will be made to small silicon clusters.
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    The European physical journal 26 (1993), S. 105-109 
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    Keywords: 36.40.+d
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    Notes: Abstract This invited review attempts to draw together recent advances in the structural characterisation of clusters and our theoretical understanding of dynamics, especially coexistence phenomena. It is now possible to characterise the potential energy surface of a small cluster in great detail, both in terms of local minima and transition states. A selection of results is collected includingab initio calculations on main group ligated clusters and a wide variety of systems bound by model analytic potentials. Useful comparisons may be made between the rearrangement mechanisms supported by the various potential energy surfaces. Furthermore, knowledge of transition states enables us to explain the results of dynamical simulations in great detail, and make comparisons with thermodynamic models. For larger systems, however, the number of stationary points is daunting, yet progress is still possible in terms of the underlying potential energy surface using the harmonic superposition approximation.
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    The European physical journal 26 (1993), S. 131-134 
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    Keywords: 33.80.-b ; 36.40.+d
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    Topics: Physics
    Notes: Abstract The real-time dynamics of Nan (n=3–21) cluster multiphoton ionization and fragmentation has been studied in beam experiments applying femtosecond pump-probe techniques in combination with ion and electron spectroscopy. Three dimensional wave packet motions in the trimer Na3 ground state X and excited state B have been observed. We report the first study of cluster properties (energy, bandwidth and lifetime of intermediate resonancesNa n * ) with femtosecond laser pulses. The observation of four absorption resonances for the cluster Na8 with different energy widths and different decay patterns is more difficult to interpret by surface plasmon like resonances than by molecular structure and dynamics. Time-resolved fragmentation of cluster ions Na n + indicates that direct photo-induced fragmentation processes are more important at short times than the statistical unimolecular decay.
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    The European physical journal 26 (1993), S. 204-206 
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    Keywords: 36.40.+d ; 81.10.Bk ; 35.20.Wg
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    Topics: Physics
    Notes: Abstract Using a CO2 laser we have desorbed LiOH and NaOH from a solid target into an expanding inert gas jet pulse. Subsequently the beam was ionized by photons from a UV laser. Surprisingly, we observed in mass spectra metal water clusters and metal-hydride water clusters. For the metals M=Li, Na we find that the [M(H2O)n]+ peaks are dominant for small clusters, while for large clusters (n〉20) the [MH(H2O)n]+ peaks are dominant. This indicates that the clathrate (H2O)20 may play an important role in the formation of metallo-water clusters.
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  • 68
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    The European physical journal 26 (1993), S. 226-228 
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    Keywords: 79.20.Rf ; 34.50.Lf ; 79.20.Nc ; 81.60.Cp ; 36.40.+d
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    Topics: Physics
    Notes: Abstract We review the processes which have been observed from collisions between alkali-halide clusters and solid surfaces. Soft impact of nanocrystalline NanF n−1 + clusters against solid surfaces causes them to cleave along the lowest energy (100) plane. At higher collision energies (Ei〉1 eV/atom), an evaporative cascade occurs which is characteristic of a transformation of the nanocrystal to a molten state. Efficient F− transfer from the cluster to the surface can occur for the larger clusters (〉60 atoms) scattering from Si(111), in direct competition with the cleaving channel at low energies. In this regime, strong bonds can form between the F− and silicon surface. The reaction probability increases with cluster size indicating that an impact-initiated shock wave is needed to enhance the reactive process.
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  • 69
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    The European physical journal 26 (1993), S. 264-266 
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    Keywords: 36.40.+d ; 31.90.+s
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    Topics: Physics
    Notes: Abstract We have investigated symmetrical and asymmetrical fissions of positively charged silver clusters by a shell correction method. Contour plots of the total electronic energy as a function of deformation parameters have been obtained. They show complicated topographies arising from the shell correction.
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  • 70
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    Keywords: 32.80.Fb ; 36.40.+d ; 33.20.Kf
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    Topics: Physics
    Notes: Abstract Photo-absorption cross-sections for charged sodium clusters (14 to 48 atoms) have been measured for photon energies from 2.0 eV to 3.5 eV. The spectra are dominated by surface plasma oscillations of the valence electrons exhausting 70–100% of the dipole sum rule. The mean resonance energy of ≃2.75 eV is nearly independent of cluster size. A splitting of the resonance peaks is observed for non-“magic” clusters and discussed in terms of a deformation picture involving prolate and oblate shapes.
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  • 71
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    The European physical journal 26 (1993), S. 284-286 
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    Keywords: 73.20.Mf ; 36.40.+d ; 31.20.Sy
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    Topics: Physics
    Notes: Abstract We study the origin of the discrepancy between the photoabsorption cross sections of small jellium spheres calculated by the time dependent local density approximation (TDLDA) and experiments for small metallic clusters. We have specifically studied Na 21 + . We conclude that both non-local exchange-correlation effects beyond the LDA and geometrical effects beyond the jellium approximation should be taken in the same calculation. We also present local and non-local calculations for Na n − (n=19, 91 and 197) within the framework of the jellium model. The large anions show a fragmentation of the plasmon due to its interference with the ionization threshold. This feature is absent in the TDLDA results.
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  • 72
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    The European physical journal 26 (1993), S. 296-300 
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    Keywords: 36.40.+d ; 71.10.+x
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    Topics: Physics
    Notes: Abstract Energetics and structures of small aluminum-lithium clusters were investigated using structure-minimization and dynamic simulated annealing on the Born-Oppenheimer surface, calculated via local-spin-density functional method. A transition from atomic-based characteristics forn≤5 to a perturbed delocalized electronic cluster-shell behavior forn≥6, containing an AiLi5 “core”, is suggested.
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  • 73
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    Keywords: 31.20.Tz ; 31.50.+w ; 36.40.+d
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    Notes: Abstract The ground state geometries of small neutral Ag n (n=2–9) and charged Ag n ± (n=2–9) clusters have been determined in the framework of the SCF procedure employing a relativistic pseudopotential accounting for core-valence correlation effects (RECP-CVC). Similarities and differences between neutral and charged clusters have been found. Large scale CI for 5s electrons only has been carried out for determining stabilities, ionization potentials (IP) and vertical detachment energies (VDE) of anions. A comparison between predicted and measured observables allows for the tentative structural assignments. In addition, the low lying energies of excited states for the neutral species at the anionic geometries have been calculated to account fully for geometrical and spectroscopic assignment to the photodetachment measurements.
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  • 74
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    The European physical journal 26 (1993), S. 8-12 
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    Keywords: 36.40.+d
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    Notes: Abstract The change of the cohesive energy and the optical absorption spectra of small singly sized Hg clusters is discussed. The cohesive energy allows one to determine the different regimes of chemical bonding. The optical spectra show an abrupt transition to a collective, plasmon-like absorption as a function of increasing cluster size. The position of the one plasmon peak is: 1) independent of the charge state, 2) nearly independent of cluster size, and 3) agrees with that of the classical Mie plasmon calculated from the experimental dielectric constants. The width of the plasmon peaks is discussed. A strong influence of electronic correlations on the cluster size dependence of the oscillator strength is observed.
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  • 75
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    The European physical journal 26 (1993), S. 17-22 
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    Keywords: 31.20.Tz ; 31.50.+w ; 36.40.+d
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    Topics: Physics
    Notes: Abstract The contribution of quantum chemicalab — initio studies of optical response in small metal clusters towards understanding of their specific electronic and structural properties has been presented. The role of cluster size, geometry, number of valence electrons and chemical composition has been pointed out. It has been shown that an appropriate many-electron description of excited states for stable cluster structures allows for the quantum molecular interpretation of the absorption spectra and other optical probles. The nature of excitations responsible for characteristic spectroscopic patterns has been discussed.
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  • 76
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    The European physical journal 26 (1993), S. 373-376 
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    Keywords: 36.40.+d
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    Notes: Abstract The paper presents an analytic model which is based on approximation of the catchment basins in cluster potential energy surface by structures of ash-tray type. In order to see how the quantities to be specified relate to those obtained by means of numerical computation, predictions for the Ar13 caloric curve are made and they are compared with computer simulation data.
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  • 77
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    Keywords: 36.40.+d ; 65.40.−f ; 76.80.+y
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    Topics: Physics
    Notes: Abstract There are 5 different Au-sites in the Au55 core of Au55(PPh3)12Cl6. The 13 inner atoms are almost identical to that of bulk gold. At the surface the attachment of the ligands is responsible for further differentiation. We have calculated the low temperature specific heat using the Mössbauer determined electrical quadrupolar splitting of the surface atoms. Due to too long a quadrupolar relaxation time, no such contribution is seen in the zero field results (down to 0.06 K) of Goll et al., Z. Phys. D 20, 329 (1991). The latter zero and in-field data are interpreted as the contribution of an electronic spin, hyperfine coupled to a nuclear spin 3/2. The effective abundance is about one spin per ten clusters.
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  • 78
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    The European physical journal 26 (1993), S. 131-133 
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    Keywords: 36.40.+d ; 31.20.Sy ; 03.65.Sq
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    Topics: Physics
    Notes: Abstract Starting from the Hartree-Fock exchange energy density of a spin saturated system and using the Density Matrix Expansion we have built a non local Energy Density Functional without problems of divergence in the large gradient limit. We have applied this functional to the study of ground state properties and the dipole excitation energies of some metallic clusters in the quantal and semiclassical approaches.
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  • 79
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    The European physical journal 26 (1993), S. 143-145 
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    Keywords: 36.40.+d ; 31.20.Sy ; 71.30.+h
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    Topics: Physics
    Notes: Abstract Electronic structures of mercury clusters are studied by the self-consistent calculations. It is found that an icosahedral Hg13 cluster is characterized by the atomic nature with closed electronic shell configuration while a significant amount of thespd-hybridization is observed for a Hg19 cluster with a double-icosahedral shape. The electronic structure of a Hg12Ag cluster is also studied with a special focus on the electronic shell structure.
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  • 80
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    The European physical journal 26 (1993), S. 137-139 
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    Keywords: 36.40.+d ; 35.20.Vf ; 33.80.Eh
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    Topics: Physics
    Notes: Abstract Laser photoionization experiments have been performed on a large size range of indium and thallium clusters. The metal clusters were produced by laser vaporization technique and analyzed after laser ionization by standard time-of-flight mass spectrometry. For the indium clusters, individual ionization potentials (IP) are deduced for N≤132. Abrupt decreases in the IP values are observed, which correspond to the openings of new electronic shell as predicted by the spherical jellium model. For larger indium clusters, the unresolved mass spectra present small but reproducible oscillations. Interpretations in terms of either electronic shell structure or cluster geometry remain undecided for the moment. Our results on thallium clusters are less abundant and only qualitative. Nevertheless, they show that thallium behaves more like a monovalent element than like a trivalent one.
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  • 81
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    The European physical journal 26 (1993), S. 288-290 
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    Keywords: 36.40.+d ; 33.80.Eh ; 35.20.Wg
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    Topics: Physics
    Notes: Abstract We investigate the time dependence of carbon cluster ions, formed via thermionic emission from photoexcited fullerenes (C60 and C70). By pulsing the extraction field, we are able to observe delayed ions formed as late as 100 µs after excitation at 532 nm, 355 nm, or 266 nm. All even-sized clusters in the range 36 ≤ n ≤ 70 undergo thermionic emission.
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  • 82
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    The European physical journal 26 (1993), S. 297-299 
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    Keywords: 36.40.+d ; 33.60.Cv
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    Notes: Abstract The electronic structure of an isolated Ca@C60 molecule was probed by photoelectron spectroscopy (PES) of Ca@C 60 − , generated by laser desorption of a preformed Ca@C60/fullerene thin film. The PES spectrum of Ca@C60 was found to be similar to that of C 60 − , except that Ca@C60 has an electron affinity of 3.0 eV, about 0.3 eV higher than that of C60. The spectrum suggests that Ca atom donates its two 4s electrons to the C60 t lu lowest unoccupied molecular orbital. Thus, the interaction between the central Ca atom and C60 is quite ionic, and Ca@C60 can essentially be expressed as Ca2+@C 60 2− .
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  • 83
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    The European physical journal 26 (1993), S. 311-313 
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    Keywords: 36.40.+d ; 35.20.Wg
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    Notes: Abstract Using a pulsed CO2 laser we have desorbed C60 clusters from a solid target into an expanding He jet, coupled to a time-of-flight mass spectrometer. We observed self ionization of the excited clusters with a time delay of 120 ±20 µs. This is large compared to previously reported delays of up to 15 µs after laser excitation with visible or ultraviolet light. We suggest that the delayed ionization is caused by structural isomers of hot C60 clusters differing from the truncated-icosahedron structure.
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  • 84
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    The European physical journal 26 (1993), S. 174-176 
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    Keywords: 36.40.+d
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    Topics: Physics
    Notes: Abstract Cluster beams of high intensity as resulting from condensation in adiabatic nozzle expansions of pure gases or vapors can be used either for thin film deposition or for substrate erosion, depending on the cluster impact energy. For deposition, high-intensity cluster beams of zinc and of silver have been obtained for the first time from pure metal vapor nozzle expansions. The clusters contain several thousand atoms of zinc, or several hundred atoms of silver, respectively. Reflecting thin films have been deposited without ionizing and accelerating the cluster beams. For purposes of erosion, ionized clusters have to be accelerated to impact speeds larger than about 10 km/s in order to get surface erosion via shock wave induced crater formation. Crater dimensions of some nanometers are estimated, providing a limit for the spatial resolution of cluster impact lithography (CIL).
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    The European physical journal 26 (1993), S. 177-179 
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    Keywords: 36.40.+d ; 33.80.Eh ; 33.20.Lg
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    Topics: Physics
    Notes: Abstract Clusters of aluminum atoms solvated with water molecules, Al(H2O) n , have been studied by one-photon ionization and time-of-flight mass spectroscopy. Characteristic feature is observed in the photoionization mass spectrum: The intensities of the product Al+(H2O) n ions are about one order of magnitude larger forn≤4 than forn≥5. The ionization potentials are determined forn up to 4. The obtained results are discussed in connection with the recent theoretical works. The photodissociation spectroscopy has also been applied to the Al+(H2O) n ions withn=1−10. The observed spectra are ascribed to the transition localized on the Al+ ion on the analogy of the results of Mg+(H2O) n photodissociation. The dissociation dynamics is also discussed from the fragmentation pattern with the aid of the recent theoretical work on the structure and stability of these ions.
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    The European physical journal 26 (1993), S. 264-266 
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    Keywords: 36.40.+d ; 33.10.−n ; 34.20.cf
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    Topics: Physics
    Notes: Abstract We present the results of a first-principles calculation for the geometrical and electronic structure of C78 and C84 fullerene isomers. Our study confirms that for C84 two isomers withD 2 andD 2d symmetries are the most energetically favorable, and for C78 aC 2v isomer is the ground-state configuration. Results from the present calculation are compared with recent experimental observations.
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  • 87
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    The European physical journal 26 (1993), S. 8-11 
    ISSN: 1434-6079
    Keywords: 61.10.Lx ; 61.60.+m ; 36.40.+d
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    Topics: Physics
    Notes: Abstract Gold L3-edge EXAFS measurements at 80 K on Au55(PPh3)12Cl6 confirm that the Au-Au distances in this amorphous metal cluster compound are significantly shorter than in bulk gold. The nearest-neighbour Au-Au distances are all equal within experimental uncertainty. Outer-shell Au-Au distances have also been resolved. The results are consistent with the cuboctahedral structure originally proposed for this cluster, but not the polyicosahedral one recently suggested. Very similar results have been obtained from the sulphonated water-soluble derivative Au55(PPh2C6H4SO3Na)12Cl6. In contrast, EXAFS of Au11{PPh2(p-ClC6H4)}7I3 has clearly resolved the two nearest-neighbour Au-Au distances associated with its icosahedral structure. Palladium K-edge EXAFS has been used to study the cluster Pd561(phen)36O200. The Pd-Pd distance is nearly equal to that in bulk palladium. The results show a cubic close-packed cluster structure for this material, in contrast to the icosahedral structure reported for Pd561(phen)60(OAc)180.
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  • 88
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    The European physical journal 27 (1993), S. 193-197 
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    Keywords: 36.40.+d ; 71.45.Gm
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    Topics: Physics
    Notes: Abstract We study collective excitations of π electrons in the fullerene molecule, by using the sum rule approach and linear response theory. The results for the excitation spectrum are discussed in relation to experimental data and to other theoretical approaches.
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  • 89
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    Keywords: 81.30.-t ; 61.14.-x ; 36.40.+d
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    Notes: Abstract Solid-liquid transition of fine tin particles having diameter of 2–10 nm is studied in-situ by high-resolution transmission electron microscopy under a ultra-high vacuum condition. Melting temperature is confirmed to decrease with the decrease of particle diameter. The particles less than the critical size, 2r c⋍5 nm, are found to have a specific phase between the solid and the liquid phase. The particle in this “pseudo-crystalline” phase contains crystalline embryos in it. Particles larger than the critical size have sharp liquid-solid transition, which completed within the time resolution of our microscope observation, 33 ms upon heating or cooling process. Large solid particles have Wulff's polyhedron, while particles around the critical diameter have rather spherical shape. Structural anomaly at the critical size occurs all over the outer most surface layer slightly below the melting temperature. Origin of the “pseudo-crystalline” phase and surface pre-melting phenomena are discussed.
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    The European physical journal 28 (1993), S. 61-65 
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    Keywords: 02.70.Lq ; 36.40.+d
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    Topics: Physics
    Notes: Abstract We have successfully identified stable configurations of both rare-gas and NaCl clusters with a new optimization procedure. An initial cluster configuration is prepared in a so-called shoot-and-stay process. Its total energy is then minimized with respect to the atomic coordinates. To prevent the system from being locked in local minima, the step size of each move is chosen as the width of the energy well at a higher level. As the system evolves, the global minimum is contained in the volume bounded by the decreasing value of step sizes. We have also carried out the optimization of NaCl clusters by the simulated annealing technique, for comparison. The results show that for such heterogeneous systems, the latter method cannot always find the global minimum, because of large energy gaps between different catchment regions in phase space.
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    The European physical journal 25 (1993), S. 253-259 
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    Keywords: 36.40.+d ; 32.80.pj ; 42.50.vk
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    Topics: Physics
    Notes: Abstract The ordered structures of two-species ions stored in a Paul trap are simulated by minimizing the total effective potential energy of the ion system. It is shown that the homo-charged two-species ion clusters display separating and mixing structures for different mass and charge ratios. The hetero-charged ion clusters consisting of one light negatively (positively) charged ion and three to six heavy positively (negatively) ions are found to exist stably, and the stability conditions obtained through the perturbative analysis indicate that the ion cluster consisting of more heavy ions has a looser limitation on mass and charge ratios of two-species ions.
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    The European physical journal 26 (1993), S. 246-248 
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    Keywords: 36.40.+d ; 75.60.Jp
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    Topics: Physics
    Notes: Abstract The magnetization of an ensemble of free magnetic metal clusters in an inhomogeneous external magnet field is calculated. In particular we have investigated the effects of the combined lattice anisotropy and cluster rotation on the magnetic properties. If weak anisotropy is present, almost superparamagnetic behavior is obtained. For stronger anisotropies deviations from this are calculated as a consequence of spin resonance due to the anisotropy field and the cluster rotation. This was proposed recently by de Heer et al. to explain his experimental data as generally expected, since a rotating cluster in a static magnetic field should behave similarly than a nonrotating one in an oscillating magnetic field. The magnetization depends also sensitively on the relaxation times.
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    The European physical journal 26 (1993), S. 249-251 
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    Keywords: 36.40.+d ; 61.16.Di
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    Topics: Physics
    Notes: Abstract The atomic force microscope has been used in the attractive (non-contact) force mode to produce images of individual nanometer-size clusters pre-formed in the gas phase and deposited on a wide variety of atomically-flat substrates. Using this technique, it is now possible to reliably image pre-formed clusters in their as-deposited positions. Studies of nanometer-size Au clusters supported on highly oriented pyrolitic graphite clearly show how the clusters are distributed across the scanned region. Cluster coverages inferred from atomic force studies are compared to those obtained from TEM studies of amorphous carbon grids simultaneously exposed to the same cluster beam.
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    The European physical journal 26 (1993), S. 276-280 
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    Keywords: 33.80.−b ; 36.40.+d
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    Topics: Physics
    Notes: Abstract The dynamics of excited states of cold Na3-clusters in a supersonic beam have been studied in time-resolved two-photon-ionization (TPI)-experiments, using picosecond pump&probe technique followed by mass-selective detection. In case of the electronically excited B-state for the first time the pseudorotational motion could be recorded as a temporal sequence. The time constant of pseudorotation was found to be 3ps. For the D-state, known to predissociate very rapidly, it was possible to directly ionize this excited electronic state. Decay times of the TPI-signal were estimated to be in the order of 2.7ps providing direct information of the fragmentation probability. Simultaneously the temporal evolution of the fragment signal Na 2 + was detected.
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    The European physical journal 26 (1993), S. 13-16 
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    Keywords: 36.40.+d ; 34.20.Gj ; 34.40.+n ; 34.70.+e
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    Topics: Physics
    Notes: Abstract We have measured absolute integral cross sections for low-energy collisions of atoms and molecules with neutral sodium clusters over a wide cluster size range (n=2–40). The cross sections are exceptionally large, reaching values of thousands of square angstroms. Consequently, the scattering involves long-range interactions. The van der Waals force, acting either alone (Nan+N2) or in concert with the inelastic charge-transfer “harpooning” channel (Nan+Cl2, Nan+O2) can describe the measurements. Using interaction parameters taken from spectroscopic studies of alkali clusters, we find very good agreement with the data. This provides a point of contact between beam scattering experiments and studies of cluster electromagnetic response properties.
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    The European physical journal 26 (1993), S. 337-339 
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    Keywords: 36.40.+d ; 35.20.Wg ; 82.65.−i ; 68.45.Ax
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    Topics: Physics
    Notes: Abstract In order to modelize the CO dissociation observed on palladium particles, by means of Static Secondary Ion Mass Spectrometry, theoretical investigations were performed on small Pd clusters by using a local approximation of the density functional theory. Several types of defects were studied concerning their ability to dissociate the CO molecule.
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    The European physical journal 26 (1993), S. 343-345 
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    Keywords: 36.40.+d
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    Notes: Abstract Absolute cross-sections for photodetachment of negative carbon clusters are reported for C n − (n=3,...,8). These measurements are made using different types of ion sources, which create different isomers. These new results indicate that various negative and neutral isomers exist, some with electron affinities as low as ∼1 eV.
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    The European physical journal 26 (1993), S. 340-342 
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    Keywords: 36.40.+d
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    Notes: Abstract The combination of laser photodetachment of C 4 − and the Coulomb Explosion Imaging method was applied for the investigation of the structure of several C4 isomers and was correlated with their measured electron affinities.
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    The European physical journal 26 (1993), S. 349-351 
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    Keywords: 36.40.+d ; 31.20.Sy ; 61.55.Dc
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    Topics: Physics
    Notes: Abstract The combination of density functional (DF) theory with molecular dynamics (MD) allows one to simulate dynamical processes in solids, clusters and molecules without requiring a parameterization of the forces in the system. The method can also be used with the strategy of “simulated annealing” to determine structural isomers. We demonstrate the capabilities of the scheme in the case of the structure of neutral and charged phosphorus clusters, and discuss relationship between these structures and those of bulk systems (crystalline and amorphous). The results show clearly the tendency of phosphorus structures to have threefold coordination as well as “tubular” structures similar to that found in Hittorf's phosphorus (H-P). We discuss the interplay between electronic and geometric structure.
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  • 100
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 26 (1993), S. 355-357 
    ISSN: 1434-6079
    Keywords: 36.40.+d ; 73.20.Dx ; 63.20.Dj
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We use low temperature (10K) optical hole-burning and fluorescence line narrowing spectroscopy to investigate the electronic properties of CdSe nanocrystallites (quantum dots) as a function of crystallite diameter (20–80Å). We discuss how the homogeneous linewidth of the HOMO-LUMO transition, the energy shift between the absorbing and emitting state, and the LO phonon frequency vary with nanocrystallite size.
    Type of Medium: Electronic Resource
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