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  • Articles  (251)
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  • 1
    Electronic Resource
    Electronic Resource
    Experimental study of enhanced emission of the laser-ablated plume in backing gas (1994)
    Springer
    Applied physics 58 (1994), S. 143-148 
    Details
    ISSN: 1432-0649
    Keywords: 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The effects of background gases on the optical emission of the excimer-laser-ablated plume from a brass target have been studied experimentally. It is found that the plume emission can be enhanced significantly in a proper gas ambient. In hydrogen, the highest peak intensity is detected, and in argon, there is a distinctive difference in the pressure-dependent emission between in He and in the other three gases, Ar, N2 and H2. Moreover, the monitored line peak intensity remains unchanged in Ar and N2 and increases in H2 within a distance above the target surface; but in He, the observed peak intensity decreases with distance like in vacuum. Furthermore, the emissions of several more atomic lines of Cu and Zn atoms from the plume are also found to be enhanced in the same manner in gas ambient. Some physical processes involved in the plume expansion and the possible mechanisms for the enhanced emission of the plume in backing gas are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01082349
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  • 2
    Electronic Resource
    Electronic Resource
    Atomic and molecular impurities in4He clusters (1994)
    Springer
    The European physical journal 29 (1994), S. 61-66 
    Details
    ISSN: 1434-6079
    Keywords: 36.40 ; 67.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A density functional theory is used to predict the binding energy of atomic and molecular impurities (Ne, Ar, Kr, Xe, Li, Na, K, Rb, Cs, and SF6) in the center of4He clusters, in the limit of zero temperature and for zero angular momentum states. The size dependence of the binding energy, from small clusters to the bulk liquid limit, is investigated. The behaviour of the4He density near the impurity is also studied.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01437166
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  • 3
    Electronic Resource
    Electronic Resource
    Orientation dependence in collision induced electronic relaxation studied through van der Waals complexes with isomeric structures (1994)
    Springer
    The European physical journal 31 (1994), S. 105-115 
    Details
    ISSN: 1434-6079
    Keywords: 36.40 ; 34.30 ; 34.20.G
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Weakly bound molecular complexes with more than one well-defined structures provide us with an unique opportunity to investigate dynamic processes induced by intermolecular interactions with specific orientations. The relative orientation of the two interacting molecules or atoms is defined by the complex structure. The effect of the orientation in the spin changing collisions glyoxal(S1)+Ar → glyoxal(T1)+Ar and acetylene (S1)+Ar → acetylene(T)+Ar have been studied by measuring the intersystem crossing (ISC) rates of the glyoxal(S1)·Ar and acetylene(S1)·Ar complexes with different isomeric structures. Results show that there is a strong orientation dependence in the ISC of glyoxal(S1) induced by interaction with the Ar atom: the Ar atom positioned in the molecular plane is much more effective than in the out-of-plane position in inducing the S1 → T1 transition of glyoxal. On the other hand, studies of acetylene(S1)·Ar complexes indicate that the Ar-induced ISC rates are nearly identical for the in-plane and out-of-plane positions. Orientation dependence in the collision induced vibrational relaxation process C2H2(S1,v i )+Ar → C2H2(S1,v f 〈v i )+Ar is also studied by measuring the vibrational predissociation rates of the acetylene(S1)·Ar complex isomers. The results indicate that collisions of C2H2(S1,v 3=3, 4) with Ar at two orthogonal orientations are equally effective in causing vibrational relaxation of C2H2.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01426585
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  • 4
    Electronic Resource
    Electronic Resource
    Effects of the cluster surface on the electronic shell structure: faceting, roughness and softness (1994)
    Springer
    The European physical journal 31 (1994), S. 253-258 
    Details
    ISSN: 1434-6079
    Keywords: 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Several simple models have been used to study the effects of the surface on the electronic shell structure in metal clusters. The main results are as follows: The icosahedral clusters have the same electronic shell structure as the sphere up to about 1000 atoms. The surface roughness causes the distribution of the level spacings to be a Wigner distribution. By varying the softness of the potential we can obtain potentials where the simplest classical orbits are the ‘five-point star’ or even the ‘three-point star’.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01445003
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  • 5
    Electronic Resource
    Electronic Resource
    Fission barriers for Na N 2+ cluster dissociation (1994)
    Springer
    The European physical journal 31 (1994), S. 275-277 
    Details
    ISSN: 1434-6079
    Keywords: 36.40 ; 31.15 ; 31.20.S ; 31.90
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The competition experimentally observed between asymmetric fission and neutral monomer evaporation as dissociation channels of excited doubly charged sodium clusters has been investigated by means of an axially symmetric, fully selfconsistent Kohn-Sham method. Ellipsoidal equilibrium configurations for parent and daughter clusters have been considered using a deformed jellium model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01445006
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  • 6
    Electronic Resource
    Electronic Resource
    Dipole excitation of Na clusters with a non-local energy density functional (1994)
    Springer
    The European physical journal 31 (1994), S. 283-286 
    Details
    ISSN: 1434-6079
    Keywords: 36.40 ; 31.20 ; 71.45
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The elementary dipole excitations of the ionized clusters Na 9 + , Na 21 + and Na 41 + are investigated by solving the equations of the Random-Phase Approximation. The ground and excited states are described using the jellium model for the ionic background and a non-local energy density functional for the valence electrons. Non-local effects are specifically analyzed. The excitation energies thus obtained approach better than those of the Local Density Approximation both the full Hartree-Fock and the experimental results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01445008
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  • 7
    Electronic Resource
    Electronic Resource
    Photoionization studies of transition metal clusters: Ionization potentials of ScnO (n=5−36) (1994)
    Springer
    The European physical journal 31 (1994), S. 199-203 
    Details
    ISSN: 1434-6079
    Keywords: 36.40 ; 22.80.E
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The photoionization efficiency (PIE) spectra and ionization potentials are reported for scandium cluster monoxides (Sc n O,n=5−36). As found for other transition metal clusters, strong dependence of ionization potential on cluster size is found for small clusters, with the ionization potentials of larger clusters decreasing relatively smoothly with increasing size. The IPs are 0.6–0.8 eV lower than that predicted by conducting spherical droplet model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01437836
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  • 8
    Electronic Resource
    Electronic Resource
    Odd-even staggering in simple models of metal clusters (1994)
    Springer
    The European physical journal 31 (1994), S. 259-267 
    Details
    ISSN: 1434-6079
    Keywords: 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The odd-even staggering of free-electron metal clusters is studied using several simple models: Noninter-acting electrons in a rectangular box, triaxial harmonic oscillator, and Hückel model. Finite temperature effects are studied using the Monte Carlo method. All the models show qualitatively similar odd-even staggering. In the ground state the HOMO-LUMO gap is larger than the neighbouring energy gaps. The reduction of the odd-even staggering due to exchange and correlation is studied using the local-spin-density approximation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01445004
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  • 9
    Electronic Resource
    Electronic Resource
    Molecular dynamics study of A18B lennard-jones clusters (1994)
    Springer
    The European physical journal 31 (1994), S. 299-301 
    Details
    ISSN: 1434-6079
    Keywords: 36.40 ; 61.20.J
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Low temperature structures, melting and evaporation temperatures of A18B clusters with Lennard-Jones interactions have been studied using Molecular Dynamics simulations for different values of the parameters representing (a) the size ratio between A and B atoms, and (b) the ratio between the strengths of the A - B and A - A interactions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01445011
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  • 10
    Electronic Resource
    Electronic Resource
    Molecular dynamics study of orientational order and rotational melting in clusters of TeF6 (1994)
    Springer
    The European physical journal 31 (1994), S. 117-123 
    Details
    ISSN: 1434-6079
    Keywords: 36.40 ; 64.60.Qb ; 82.20.Wt
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Molecular dynamics simulations of the behavior of molecules in crystalline clusters of TeF6 were carried out on systems of 100, 150, 250, and 350 molecules. Several diagnostic functions were applied to investigate whether rotational melting occurred before translational melting. These functions included the coefficient of rotational diffusionD θ(T), the “orientational Lindemann index” δθ(T), the “orientational angular distribution function”Q(θ,T), and the “orientational pair-correlation function”g θ(r, T). All indicators implied that rotational melting occurred before translational melting, that it began with the outermost molecules, and that its onset for smaller clusters was at lower temperatures than for larger clusters. Results also showed that the rotational transition coincided with the transition from a lower symmetry phase (monoclinic) to cubic, a phenomenon that had been noted by others to occur with some regularity for systems of globular molecules.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01426586
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  • 11
    Electronic Resource
    Electronic Resource
    Picosecond photoelectron studies of cluster dynamics (1994)
    Springer
    The European physical journal 30 (1994), S. 1-12 
    Details
    ISSN: 1434-6079
    Keywords: 34.30 ; 36.40 ; 82.30.N
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Time-resolved photoelectron spectroscopy is proving to be a versatile tool for investigating dynamical properties of molecules and clusters. In this account we review our recent work involving studies of cluster reactivity by photoelectron spectroscopy. Two topics are discussed. The first, involves measurements of the rate of solvent reorganization and the magnitude of the solvent reorganization energy (i.e., the solute-solvent interaction). The solvent rearrangement is induced by an excited-state proton transfer in phenol-ammonia clusters. These studies are motivated by the opportunity to probe condensed-phase solvation effects on a molecular level in clusters. The second topic describes how the chemical properties of small clusters (e.g., dimers) provides detailed information regarding the collision complexes of bimolecular gas-phase reactions. We show an example of how the combination of photoelectron spectroscopy and ion fragmentation mass spectrometry can be used to determine barrier heights for bimolecular reactions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01437471
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  • 12
    Electronic Resource
    Electronic Resource
    Oscillations of finite quantal fermi systems (1994)
    Springer
    The European physical journal 31 (1994), S. 181-185 
    Details
    ISSN: 1434-6079
    Keywords: 21.00 ; 36.40 ; 33.80.Eh ; 71.45.−d
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Mean field theory and linear response methods as used in the description of nuclei and clusters are reviewed. These methods are applied to the electromagnetic response of C60.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01437833
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  • 13
    Electronic Resource
    Electronic Resource
    Fragmentation of doubly charged argon clusters (1994)
    Springer
    The European physical journal 31 (1994), S. 245-251 
    Details
    ISSN: 1434-6079
    Keywords: 33.20.Rm ; 33.80.G ; 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Fragmentation of doubly charged argon clusters is reported. Neutral argon clusters are excited with monochromatized synchrotron radiation in the energy regime of the argonL 3/L 2 absorption edges (240–260 eV) leading predominantly to cluster dication formation. All charged particles are detected in a photoelectron-photoion-photoion-concidence (PEPIPICO) experiment. Symmetric and asymmetric charge separation reactions (Coulomb explosion) are identified for clusters below the critical size of stable dication formation. The peak shapes of the coincidence signals are investigated as a function of neutral cluster size. Characteristic changes in peak shape are observed which are used to derive fragmentation mechanisms involving sequential evaporation of neutrals before and after charge separation. The spectra indicate in accordance with low kinetic energy releases occurring in charge separation of large dissociative cluster dications (Ar n 2+ , withn〉50) that due to large charge separation distances the momenta of both singly charged fragments are not any more directed into opposite direction, as it is typical for Coulomb explosion. The results are compared to collision induced fragmentation of mass selected argon cluster dications as well as photon stimulated desorption spectra of condensed argon.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01445002
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  • 14
    Electronic Resource
    Electronic Resource
    Optical response of bimetallic Li x Na8−x (0≦x≦8) and of doped Na8Zn clusters (1994)
    Springer
    The European physical journal 31 (1994), S. 269-273 
    Details
    ISSN: 1434-6079
    Keywords: 36.40 ; 31.15 ; 31.20
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Using the time dependent local density approximation we calculate the photoabsorption spectra of bimetallic Li x Na8−x (0≦x≦8) and of doped Na8Zn clusters. Ground state cluster geometry and electronic structure are obtained by minimization of total energy from Kohn-Sham self-consistent calculations within the spherically averaged pseudopotential method. For Li x Na8−x clusters we obtain two nearly isomeric geometries. In both cases the corresponding photoabsorption spectra show a single collective excitation (surface plasmon) at increasing frequency as the number of lithium atoms increases. For a given number of lithium atoms, the plasmon frequency of the cluster in the ground state geometry is ∼0.1 eV blueshifted with respect to that in the isomeric state. The equilibrium geometry of doped Na8Zn cluster is formed by a nearly spherical shell of eight Na atoms enclosing the Zn impurity at the cluster center. The corresponding optical spectrum shows two peaks at higher frequencies than the surface plasmon of pure Na8 cluster, in agreement with recent photodepletion spectroscopy experiments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01445005
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  • 15
    Electronic Resource
    Electronic Resource
    On the electronic shell structure of metallic spheres and circular discs (1994)
    Springer
    The European physical journal 31 (1994), S. 287-289 
    Details
    ISSN: 1434-6079
    Keywords: 36.40 ; 33.10
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The electronic shell structure of spherical and circular potential wells of finite depth is examined as an extension of the work of Balian and Bloch. We obtain analytical expressions for the electronic density of states, which are in good agreement with numerical results. The position of the maxima and minima in the density of states depends strongly on the depth of the potential via the quantummechanical scattering phases. For circular discs the oscillating structure in the density of states shows a different supershell pattern than for spheres. This is a consequence of the difference in the effective quantummechanical multiplicity of closed paths, which determine the oscillating structure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01445009
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  • 16
    Electronic Resource
    Electronic Resource
    Electron impact and single photon ionization cross sections of neutral silver clusters (1994)
    Springer
    The European physical journal 32 (1994), S. 137-144 
    Details
    ISSN: 1434-6079
    Keywords: 34.80.G ; 36.40 ; 32.80.F ; 33.80.E
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Neutral silver atoms and small clusters Ag n (n=1...4) were generated by sputtering, i.e. by bombarding a polycrystalline silver surface with Ar+ ions of 5 keV. The sputtered particles were ionized by a crossed electron beam and subsequently detected by a quadrupole mass spectrometer. In alternative to the electron impact ionization, the same neutral species were also ionized by single photon absorption from a pulsed VUV laser (photon energy 7.9 eV), and the photoionization cross sections were evaluated from the laser intensity dependence of the measured signals. By in situ combining both ionization mechanisms, absolute values of the ratio σ e (Ag n )/σ e (Ag) between the electron impact ionization cross sections of silver clusters and atoms could be determined for a fixed electron energy of 46 eV. These values can then be used to calibrate previously measured relative ionization functions. By calibrating the results using literature data measured for silver atoms, we present absolute cross sections for electron impact ionization of neutral Ag2, Ag3 and Ag4 as a function of the electron energy between threshold and 125 eV.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01425933
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  • 17
    Electronic Resource
    Electronic Resource
    Determination of C60/C70 ratios in fullerene mixtures and film characterization by scanning tunneling microscopy (1993)
    Springer
    Applied physics 56 (1993), S. 197-205 
    Details
    ISSN: 1432-0630
    Keywords: 36.40 ; 61.1.P ; 68.20
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Fullerene powder mixtures with different C60/C70 ratios have been analyzed by a variety of techniques, and results have been compared. The fullerence mixtures have been characterized as solutions in n-hexane by high-pressure liquid chromatography (HPLC) and UV-VIS spectroscopy. Thin films of fullerenes on Au(111) have been prepared from the mixtures by sublimation. The sublimation process has been studied by simultaneous thermogravimetric and differential thermal analyses. Thin fullerene films on Au(111) have been investigated by scanning tunneling microscopy (STM). The STM images show primarily two types of ballshaped molecules arranged in a lattice with hexagonal symmetry (fcc(111) face, nearest neighbour distance: 1 nm). The two species differ in diameter. STM images of films made of mixtures of different C60/C70 ratios show that C70 molecules display a larger apparent diameter (0.8 nm) and corrugation than C60 molecules (0.7 nm). The C60/C70 ratios obtained by counting the corresponding molecular species in the STM images of the thin films are compared to the C60/C70 ratios determined by HPLC on hexane solutions of the mixtures. The observed differences might be explained by different rates of sublimation for the two species. The STM images reveal film defects (vacancies and boundaries) and dynamic processes (displacement of C70 molecules and vacancies). In films prepared to have a C60 coverage of less than one monolayer, stable structural units of the C60(111) surface consisting of three or seven C60 molecules are revealed by STM. Occasionally, substructure within individual fullerene molecules is observed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00539474
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  • 18
    Electronic Resource
    Electronic Resource
    Excitation and decay processes in helium clusters studied by fluorescence spectroscopy (1993)
    Springer
    The European physical journal 26 (1993), S. 175-177 
    Details
    ISSN: 1434-6079
    Keywords: 36.40 ; 67.90+Z ; 71.90+Q
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Excitation and decay processes of helium clusters are investigated with fluorescence methods. The results differ remarkably from that obtained for the heavier rare gas clusters. They are discussed in view of the unusual structural and electronic properties of helium.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01429135
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  • 19
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    Electronic Resource
    Symmetry substructures for MQ-NMR of [A]20 spin clusters of dodecahedranes, [13CH]20 or [13CD]20, metallic M @M20 and [H2O]+ @[H2O]20 exo-cage cluster molecules and their higher-n SO(3) ×l n spin algebras, within the context of mapping,l n -ITP algebras and number partitions (1993)
    Springer
    The European physical journal 25 (1993), S. 275-284 
    Details
    ISSN: 1434-6079
    Keywords: 76.60 ; 03.65.F ; 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract For both highern andI i ≧1 spin clusters, combinatorics provides powerful arguments with which to investigate the substructural forms of cluster spin algebras; this is especially so for SO(3) ×l n symmetries for 12≦n≦60 wherex (.) [λ] character tabulations become extensive. Bijective enumerative mappings over the combinatorialp-tuples (number partitions) afford insight into the general functionf(p,n) as well as into {|IM(I 1−I n [λ]〉} M -structure of spin algebras, even where the full details of the explicitx () [λ] (l n ) characters are not readily available. Both simply-reducible and higher aspects ofl n -inner tensor product (ITP) algebras are derived from dimensionality considerations, as part of combinatorial hooklength formalisms for $$\chi _{(1^n )}^{[\lambda ]} $$ . TheI i ≦3/2 forms of [A] n clusters forn≦20, (forp≦3, 4) of multiple-quantum NMR (MQ-NMR) are considered here as part of current interest in giant cage-clusters. In addition, the SU2 substructural hierarchy over Liouville space is derived for [A]20(l n ) (I i =1/2) spin cluster of the cage-cluster molecule dodecahedrane; aspects ofI i =1 spin cluster over {|IM (...)〉} space are derived as high temperature model of the exo-cage of [H2O]+ @[H2O]20 cluster ion; 20-fold higher-I i lusters provide models for M @M20 metal-clusters and further applications ofl 20-number partitions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01426891
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  • 20
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    Structural transitions and melting of copper clusters (1993)
    Springer
    The European physical journal 26 (1993), S. 255-257 
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    ISSN: 1434-6079
    Keywords: 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Molecular dynamics is used to study the melting and structural transitions of small copper clusters. The melting temperature is found to be proportional to the average coordination number. Small icosahedral clusters melt at slightly higher temperatures than the cubic structures. Small cuboctahedral clusters are not stable but transform via a nondiffusive transition to icosahedral structure.
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    URL: http://dx.doi.org/10.1007/BF01429161
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  • 21
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    Nuclear shell model applied to metallic clusters (1993)
    Springer
    The European physical journal 26 (1993), S. 261-263 
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    ISSN: 1434-6079
    Keywords: 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We apply the nuclear shell model to jellium clusters of up to twenty-one Na atoms. Binding energies, ionization potentials, and photoabsorption cross sections are calculated and compared with mean-field results.
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    URL: http://dx.doi.org/10.1007/BF01429163
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  • 22
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    The evolution of electronic structure in AlnCom (1993)
    Springer
    The European physical journal 26 (1993), S. 322-324 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 33.80.E
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The ionization potentials of AlnCom clusters (n〉m) have been bracketed using laser photoionization mass spectrometry. We find the electronic shell structure manifested in the ionization potentials of Aln for n≥7 is observed also for AlnCo and AlnCo2, and is consistent with cobalt contributing one electron to the conduction band of the cluster. However, with increasing number of cobalt atoms, this simple picture breaks down; all vestiges of aluminum cluster electronic shell structure are absent for m≥4.
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    URL: http://dx.doi.org/10.1007/BF01429182
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  • 23
    Electronic Resource
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    Molecular dynamics studies of homogeneous nucleation in solid-state transitions (1993)
    Springer
    The European physical journal 26 (1993), S. 364-366 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 64.60.Qb ; 64.70.Kb ; 82.20.Wt
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Simulations of clusters containing 100 to 250 molecules of TeF6 successfully reproduce the crystalline packing arrangements observed in electron diffraction investigations of large molecular clusters (∼ 104 molecules) of the same material. More remarkably, when the clusters are cooled step by step in MD computations at a rate of ca. 1011 K/s they spontaneously undergo the same bcc to monoclinic phase transition that has been observed experimentally in supersonic flow, despite the million-fold difference in the timescales involved. The existence of such a correspondence over so many orders of magnitude, in itself, imposes severe constraints on what type of molecular mechanism can underlie the transformation. Even more revealing evidence about the molecular behavior associated with the phase change is provided by the simulations. They show the formation of the actual transition complexes along the transition pathway, namely, the critical nuclei of the new phase. These nuclei, which are made up of approximately 20 molecules, can be recognized in the midst of the surrounding matter. Techniques based on molecular orientations, involving Pawley-Fuchs projections and orientational angular distribution functions, make it possible to estimate the size of critical nuclei. One noteworthy result established in the simulations is that the solid-state transition temperature from bcc to monoclinic depends upon particle size in the same manner as does the freezing point.
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    URL: http://dx.doi.org/10.1007/BF01429196
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  • 24
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    Excitation and decay processes in helium clusters studied by fluorescence spectroscopy (1993)
    Springer
    The European physical journal 26 (1993), S. 382-384 
    Details
    ISSN: 1434-6079
    Keywords: 36.40 ; 67.90+Z ; 71.90+Q
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Excitation and decay processes of helium clusters are investigated with fluorescence methods. The results differ remarkably from that obtained for the heavier rare gas clusters. They are discussed in view of the unusual structural and electronic properties of helium.
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    Characterization of the fluxes of neutral and positively charged clusters (Agn and Ag n + ; n≤4) produced by argon ion sputtering of silver (1993)
    Springer
    The European physical journal 26 (1993), S. 101-103 
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    ISSN: 1434-6079
    Keywords: 79.20N ; 36.40
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    Topics: Physics
    Notes: Abstract The emission of neutral and positively charged silver clusters during sputtering of a polycrystalline silver target by 5 keV Ar+ ion bombardment has been studied and the sputter ejected silver flux has been characterized. As a result, the silver flux is found to be strongly dominated byneutral clusters rather than cluster ions. The contribution of neutral clusters in the overall silver flux decreases rapidly and monotonically with increasing cluster size n and decreases, in addition, with decreasing bombarding energy. The well known alternation of the secondary ion intensities of Ag n + as a function of cluster size (higher intensities for odd n) is found to be correlated with the effective “ionization potentials” of the corresponding sputtered neutral clusters.
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    Melting behavior of beryllium clusters (1993)
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    The European physical journal 26 (1993), S. 134-136 
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    Keywords: 36.40
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    Notes: Abstract The melting properties of Ben, n=7−14, clusters have been studied using molecular dynamics simulations and an interatomic potential which contains two- and three-body terms, derived from local spin density calculations. The cluster meltinglike behavior has been studied calculating characteristic physical quantities like short-time averages of the kinetic energy per particle as a function of time, caloric curves and root-mean-square bond length fluctuations as a function of the cluster total energy. The meltinglike transition in Ben clusters shows more complex features than those found in van der Waals and ionic clusters. The estimated melting temperatures for n=11−14 are higher than the bulk value. This result indicates that the parameters of the cluster interaction potential could be inadequate to describe the thermodynamic properties of Ben clusters.
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    Interaction of CO with silver clusters: Dependence on cluster size (1993)
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    The European physical journal 26 (1993), S. 162-164 
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    ISSN: 1434-6079
    Keywords: 36.40
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    Topics: Physics
    Notes: Abstract The interaction of matrix-isolated silver clusters with CO has been investigated for clusters containing as few as 100 atoms/cluster. A vibrational mode at a monolayer coverage, which is found on bulk silver at 2110 cm−1, is present even for these small particles. For these small-sized particles however, an additional, broad, unresolved band at 1950 cm−1 appears, which does not exist for larger clusters. Whether this band has its origin in a modified chemical behaviour for particles smaller than 100 at/cl is not clear.
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    The formation and photolysis of tantalum sulfide cluster ions (1993)
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    The European physical journal 26 (1993), S. 246-248 
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    ISSN: 1434-6079
    Keywords: 82.30.N ; 36.40 ; 82.50.F
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    Notes: Abstract Tantalum sulfide cluster ions were produced by direct laser ablation, and were studied with a tandem time-of-flight mass spectrometer. The main dissociation channel of the UV-photolysis (248 nm) of tantalum sulfide cluster ions is sequent S2 loss. Structures with Ta3S4 and Ta4S6 as frameworks were suggested for the large tantalum sulfide cluster ions.
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    Ring-stacking consideration on higher fullerene growth (1993)
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    The European physical journal 26 (1993), S. 258-260 
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    Keywords: 36.40
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    Notes: Abstract A hypothetical ring-stacking procedure combined with the isolated-pentagon rule(IPR) shows variety of formation pathways of many fullerene structures with a 5- and 6-membered ring carbon cage. Combining the ring-stacking procedure with energetic considerations, the numbers of possible reaction channels are dramatically reduced within the framework of the ring-stacking considerations. This, in turn, gives the results that only some specific isomers of large fullerenes are selectively produced. The resulting fullerene structures specified are surprisingly well consistent with the recent experimental results.
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    Electronic and geometrical structure of noble metal clusters (1993)
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    The European physical journal 26 (1993), S. 12-14 
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    Keywords: 36.40
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    Notes: Abstract We investigate the electronic and structural properties of small (N ≲ 20) and medium sized (N ≲ 500) clusters of Cu, using the first principles Tight-Binding Linear Muffin-Tin Orbitals (TB-LMTO) method in connection with the real-space recursion scheme. We find the electronic structure resembling the one of simple alkali metal clusters: Pronounced shell closing effects can be identified in the ionization potentials as well as in the HOMO-LUMO gaps for the magic sizesN=8, 20, 34 and 40. The low-energy equilibrium geometries show considerable Jahn-Teller distortions, just as in the case of alkali metals.
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    Electron energy loss spectroscopy of free C60 and C70 (1993)
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    The European physical journal 26 (1993), S. 294-296 
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    Keywords: 36.40 ; 34.80
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    Notes: Abstract Electron energy loss spectroscopy has been performed with samples of pure C60 and C70 using primary electron energies between 10 and 1100 eV. The experimental data could be explained in terms of optically forbidden and allowed transitions, and by collective excitations (plasmons), which could be distinguished by their dependence on the energy of the primary electron.
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    Reactivity of carbon cluster ions with Ar, O2 and D2 (1993)
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    The European physical journal 26 (1993), S. 305-307 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 35.20
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    Notes: Abstract Carbon cluster ions generated by laser evaporation have been reacted with atoms (Ar) and molecules (O2, D2). In order to vary the reaction conditions for the cluster ions two gas inlets at different locations have been added to the cluster source. Addition of D2 increased the line width in the TOF-spectrum indicating the appearance of reaction products of the type CmDn. Both O2 and D2 proved very effective in removing the odd numbered carbon cluster ions in the vicinity of C60. Application of O2 produced a significant effect: depending on the gas pressure it reacted with small cluster ions forming products like CnO+ and CnO2 +. Such reaction channels are not found for n〉12 thus indicating a change in cluster geometry. Another sequence of experiments has been undertaken by adding Ar atoms to the carbon cluster ions. Some stable ionic reaction products like ArC+,..., ArC4 +, ArC5 + can be detected. It is known from previous studies that the ArC+ is a stable ion with an dissociation energy of about 1 eV.
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    Dynamics of cluster-surface interaction: Evidence of structural isomers of C 60 − (1993)
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    The European physical journal 26 (1993), S. 320-322 
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    Keywords: 36.40
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    Notes: Abstract Collision induced electron detachment of the fullerene C 60 − and the other C 60 − produced through the laser vaporization of a carbon rod was studied. Both of C 60 − showed quite different behavior for the process. It was suggested that the laser vaporized C 60 − has a different structure from the fullerence C 60 − .
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    Quantum mechanical study of the structure and stability of molecular van der Waals' clusters II: (NH3)nHCN (1993)
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    The European physical journal 26 (1993), S. 329-331 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 31.20.D
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    Notes: Abstract The structures and stabilities of small mixed clusters of ammonia with hydrogen cyanide have been studied usingab initio electronic structure calculations at the Hartree Fock and Møller Plesset levels of theory. Stable structures for (NH3)nHCN, n=1, 2, 3 have been located. Two stable structures and a transition state for the heterodimer are described.
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    Stability, structures and a hypothetical growth mechanism of carbon 5/6 network (1993)
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    The European physical journal 26 (1993), S. 69-73 
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    Keywords: 36.40
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    Notes: Abstract A novel picture of growth of fullerene and fullerene-like structures is proposed. The ring stacking model have been studied in detail in connection with the stability, structures and growth mechanism of carbon 5-and 6-membered ring network. Combining the model with energetic considerations, the selective formation of sizes and isomers of large fullerenes has successfully been described.
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    Physical and chemical probes of metallic clusters (1993)
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    The European physical journal 26 (1993), S. 1-7 
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    Keywords: 36.40
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    Notes: Abstract Two different approaches for studying the electronic structure of clusters are presented. The physical probe uses the photoexcitation of size selected alkali-atom clusters. Giant resonances govern the absorption cross-section. It is due to the collective excitation of delocalized valence electrons. The behavior of the resonance up to large sizes, 900-atom clusters for potassium and 1500-atom clusters for lithium puts into evidence the failure of the jellium model for lithium. The chemical approach makes use of the nucleation process in the presence of oxygen. From the analysis of mass spectra of oxidized products, informations are gained on the valence of metallic atom in the cluster oxide. The method is applied to europium and thulium clusters. A comparison is done with the simple case of alkali clusters.
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    Excitation and radiative decay of neutral and ionic ArN clusters studied by fluorescence spectroscopy (1993)
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    The European physical journal 26 (1993), S. 370-372 
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    Keywords: 36.40 ; 78.55-M
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    Notes: Abstract Excitation and decay processes of neutral and ionized ArN clusters are analysed using fluorescence spectroscopy with synchrotron radiation. The fluorescence yield of ionized ArN clusters is resonantly enhanced after excitation from states related to the atomic 3s levels.
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    Two dimensional detection of size selected and focused neutral carbon clusters using image intensified charge coupled device (ICCD) system (1993)
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    The European physical journal 26 (1993), S. 317-319 
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    Keywords: 36.40
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    Notes: Abstract An apparatus for getting size selected and focused neutral carbon clusters was developed. Using image intensified charge coupled device (ICCD) system, the beam profiles of C 10 − and C10 were investigated. The results indicated that the beam density of C10 surpasses that of C 10 − of space charge limit.
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    Solvent-induced electronic spectral shifts: Benzene·Ar n revisited (1993)
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    The European physical journal 26 (1993), S. 323-325 
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    Keywords: 33.10.C ; 36.40 ; 77.40 ; 78.40.D
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    Notes: Abstract Just what can be learned about cluster dynamics (and, more generally, about solvation dynamics) from spectral studies of small clusters that have been doped with a chromophore is still an open question. In the present work we suggest a novel procedure for calculating the shift in the electronic absorption spectrum of a chromophore deriving from the attachment to or the incorporation in a cluster. The particular system of interest here is benzene·Ar n , for which experimental results are readily available although their interpretation has been a point of controversy. In addition, since the present formalism is equally applicable to a chromophore isolated in a bulk phase (either liquid or solid), we are able to venture an explanation for the apparent observation that the spectral shift of cluster-isolated benzene does not approach the asymptotic values characteristic of the bulk-isolated species.
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    Quantum mechanical study of the structure and stability of molecular van der waals' clusters III: Dimers of hydrogen sulfide (1993)
    Springer
    The European physical journal 26 (1993), S. 332-334 
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    ISSN: 1434-6079
    Keywords: 31.20.D ; 36.40 ; 82.40.W
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    Notes: Abstract Electronic structure calculations employingab initio methods have been carried out for the H2S monomer and two of its associated dimers: its homodimer, (H2S)2 and its heterodimer with water. The results reported here include geometric parameters, the zero-point and minimum energies, rotational and vibrational constants, and dipole moments. The contribution of the monomers to the dimer spectrum has been suggested. Two structures for the homodimer corresponding to potential surface stationary points have been found. One of these has been identified as a transition state.
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    The role of Ga-droplet formation in nanometer-scale GaAs cluster synthesis from organometallic precursors (1993)
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    The European physical journal 26 (1993), S. 219-221 
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    Keywords: 36.40
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    Notes: Abstract Recently we demonstrated the formation of crystalline nanometer scale GaAs clusters from organometallic precursors using an aerosol process derived from organometallic vapor phase epitaxy (OMVPE) [P. C. Sercelet al., Appl Phys. Lett.61, 696 (1992)]. Here, we explore the influence of precracking the Ga precursor (trimethyl-gallium), forming nanometer scale Ga droplets, prior to introduction of AsH3 to the reaction. We find the GaAs clusters so formed have an entirely different morphology than do those formed when the reactants are pre-mixed prior to reaction. This data supports our earlier contention that homogeneous nucleation is the dominant reaction mechanism for the formation of the clusters.
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    The optical extinction of ligand-stabilized Au13- and Au55-clusters: The vanishing of the Mie-resonance (1993)
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    The European physical journal 26 (1993), S. 1-3 
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    Keywords: 36.40 ; 33.20K
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    Notes: Abstract We present optical extinction spectra of ligand-stabilized Au-clusters AuN with sizes of N=13 and N=55. They are compared to larger, embedded clusters up to N=5·103 (diameter 5nm). The Mie resonance due to excitation of the dipole plasmon polariton vanishes with decreasing cluster size. For N〉55, this effect has recently been interpreted as caused by smearing out of the interband transition edge of Au.
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    Quantum mechanical study of the structure and stability of molecular van der Waals' clusters I: hydrates of SO2 (1993)
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    The European physical journal 26 (1993), S. 326-328 
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    Keywords: 36.40 ; 31.20.Gm ; 31.20.D ; 82.40.We
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    Topics: Physics
    Notes: Abstract The structure and stability of the mixed clusters SO2(H2O)n, n ranging from 1 to 4, have been examined using semi-empirical quantum mechanical Hartree Fock methods with the improved PM3 parameter set as implemented in MOPAC [1]. Limited molecular dynamics studies together with investigation of the potential surface reveal potential barriers of 1 kcal/mol or less between many of the stable structures for a cluster of a given size. For n=1,2 the results are compared with those ofab initio calculations using a double zeta basis at the MP4 level.
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    Energy exchanges following very low energy electron attachment to neat CS2 and CS2-containing clusters (1993)
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    The European physical journal 27 (1993), S. 365-369 
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    Keywords: 36.40 ; 34.70.+e
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    Notes: Abstract We have studied electron transfer from state-selected Rydberg atoms to (CS2) N and CS2(Ar) N clusters and compared the results to Rydberg electron transfer to isolated CS2 molecules. At large Rydberg principal quantum numbers (n〉20), the influence of the positive ionic core becomes negligible and we are able to directly investigate the competition between electronic autodetachment of the anions and intracluster energy exchanges between the anions and their environment. We show that argon atoms are unable to achieve efficient internal energy exchanges in heterogeneous clusters as compared to the high efficiency of CS2 molecules in homogeneous clusters.
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    Exchange effects on electronic states in jellium clusters (1993)
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    The European physical journal 28 (1993), S. 73-80 
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    Keywords: 36.40 ; 71.70.G
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    Notes: Abstract Hartree-Fock calculations are done for finite number of valence electrons (8, 20, 40, and 58) in a positively charged uniform background (jellium) with the density of bulk sodium. Differences and similarities with the results of Kohn-Sham local-density calculations are discussed. State-dependence and suppression of the wave functions are the two important effects coming from the nonlocality of the Hartree-Fock potential. These two effects, however, cancel in the density profile of electrons. The Kohn-Sham calculations consequently produce similar density profiles as in the Hartree-Fock calculations. The dipole polarizability is also calculated for 8-mers. The calculated value is still smaller than the measured one and is not an improvement of the insufficient Kohn-Sham result.
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    Static, statistical, and dynamical properties of small sodium clusters (1993)
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    The European physical journal 26 (1993), S. 232-235 
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    Keywords: 31.20.D ; 31.30 ; 36.40
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    Topics: Physics
    Notes: Abstract This paper reports results obtained in the study of small alkali metal clusters (2≤n≤34). Properties of interest include static descriptions of the Potential Energy Surfaces (PES) using a Distance-Dependent Tight Binding hamiltonian (DDTB) and new results obtained with the same hamiltonian through a Monte-Carlo Growth Method (MCGM) which is shown to give an efficient way to sample the PES, but may also be helpful to estimate the statistical occurrences of the various isomers (catching areas). The probability for generating a given configuration is proportional to a Boltzmann distribution factor and hence a temperature dependence is introduced in the study of the PES. In section 2, the temperature dependence of the optical response for small clusters (on the example of Na4) is simulated throughab initio calculations of the electronic structure coupled with Monte-Carlo dynamics on the absorbing system in the ground state.
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    Multiphoton ionization and photodissociation of (NO)mArn clusters (1993)
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    The European physical journal 26 (1993), S. 220-222 
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    Keywords: 36.40 ; 33.80 ; 35.20
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    Notes: Abstract Picosecond multiphoton ionization of (NO)mArn clusters produced in a supersonic expansion of NO/Ar gas mixtures has been studied using time-of-flight mass spectrometry. Two-photon ionization with 266 nm photons show that dilute gas mixtures (1% NO/Ar) yield clusters limited to m≤7, but with as many as 37 argon atoms. Magic numbers are observed for NO+Ar12, NO+Ar18, (NO) 2 + Ar17, NO+Ar22, and (NO) 2 + Ar21 and are understood in terms of solvation of the NO+ and (NO) 2 + by argon in icosahedral arrangements. Four-photon ionization with 532 nm light produces dissociation of the clusters to yield only NO+Arn with n up to 54. This distribution exhibits an additional magic number at n=54, consistent with the completion of a second solvation sphere about the NO+. The known wavelength dependence for photodissociation of (NO) 2 + and (NO) 3 + and comparison of MPI spectra obtained with 266, 355, and 532 nm light indicate that the dissociation is occurring in the cluster ions.
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    Interplay between collective and single electron excitations in large metal clusters (1993)
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    The European physical journal 26 (1993), S. 267-269 
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    Keywords: 36.40 ; 73.20 ; 78.65 ; 82.65
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    Topics: Physics
    Notes: Abstract Collective and single electron excitations of large metal clusters supported on transparent substrates are investigated. The applied experimental techniques include extinction spectroscopy and laser induced dissociation accompanied by the ejection of individual atoms. The optical spectra depend on the electromagnetic far field and reflectcollective electron excitations of the conduction electrons, i.e. surface plasmons. Dissociation, however, is correlated to repulsivesingle electron energy levels. The characteristics of these localized excitations indicate a strong influence of collective excitations. In particular, it is found that nonlocal optical effects are important. In this picture surface plasmons catalytically enhance the number of single electron excitations and therefore of the metal atoms ejected as a result of the absorption of visible light. Results will be presented, which illustrate this interplay between collective and single electron excitations.
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    Surface melting in Ni55 (1993)
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    The European physical journal 26 (1993), S. 304-306 
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    Keywords: 36.40
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    Topics: Physics
    Notes: Abstract The phenomenon of surface melting in a Ni55 cluster is predicted from results of molecular dynamics simulations.
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    Peculiarities of structures and meltinglike transition in gold clusters (1993)
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    The European physical journal 26 (1993), S. 316-318 
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    Keywords: 36.40
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    Notes: Abstract Structural and dynamical properties of gold clusters are derived from a Gupta-type potential using numerical simulations and are compared to features of nickel clusters derived from the same potential. The major implication of the comparison is that the properties of clusters mimicked by this many-body potential depend, in general, on the values of its parameters.
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    Structural models for intermediate-sized Si clusters (1993)
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    The European physical journal 26 (1993), S. 346-348 
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    Keywords: 36.40 ; 31.20.Sy
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    Notes: Abstract We have investigated two generic structural models for Si clusters in the size range 20–33 atoms. The first model consists of stacks of puckered six-fold rings, with capping atoms on the two ends. The second model comprises compact structures with as many bulk-like four-fold coordinated atoms as possible. We have found the cohesive energy of the ring structures to be nearly constant as a function of cluster size, while that of the compact structures decreases dramatically with increasing size as more four-fold coordinated atoms are incorporated into the structures. We compare our results with recent experiments which suggest a crossover in stability between two cluster types in the size range studied here.
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    Photoelectron spectroscopy of (CO2)n(H2O) m − clusters (1993)
    Springer
    The European physical journal 26 (1993), S. 367-369 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 33.80
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    Topics: Physics
    Notes: Abstract Photoelectron spectra of (CO2)nH2O− (2≤n≤8) and (CO2)n(H2O) 2 − (1≤n≤2) were measured at the photon energy of 3.49 eV. The spectra show unresolved broad features, which are approximated by Gaussians. The vertical detachment energies (VDEs) were determined as a function of the cluster size. For (CO2)nH2O−, the VDE-n plots exhibit a sharp discontinuity between n=3 and 4; the VDE value is ≈3.5 eV at n=3, while it drops down abruptly to 2.59 eV at n=4. This discontinuity in VDE is ascribed to "core switching" at n=4; a C2O 4 − dimer anion forms the core of (CO2)nH2O− for n≤3, while a monomer CO 2 − is the core for n≥4. The (CO2)2(H2O) 2 − ion has a VDE of 2.33 eV, indicating the presence of a CO 2 − monomer core in the binary clusters containing two H2O molecules.
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    In-situ STM observation of coalescence of metal particles in liquid (1993)
    Springer
    The European physical journal 26 (1993), S. 42-44 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 61.16.D ; 82.65.F
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have madein-situ observation of electrodeposition on the graphite surface in electrolyte(0.5 mM-Bi2O3, 10 mM-HClO4) by means of Scanning Tunneling Microscopy(STM). We have observed the growth and the stripping of Bi metal particles of about 10 nm in diameter, with increase and decrease in the electrode potential, respectively. The STM images are taken in real time in a period of about 30 sec. They show that small particles grow in the initial stage, come in contact with each other, and coalesce in the contact area connecting two neighboring particles.
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    Supported Pdn (n=1−20) cluster reactivity : CO chemisorption (1993)
    Springer
    The European physical journal 26 (1993), S. 64-66 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 82.30
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The Pdn clusters are prepared by vapour deposition in an UHV chamber providing static secondary ion mass spectrometry (SSIMS) and thermo-programmed desorption analysis (TPD) facilities. By adjusting deposition parameters we are able to obtain isolated Pd species, dimers etc... In the present work we used as substrates thin films of γ-Alumina. We show that the reactivity and the kinetics of the CO-Pdn association depends on the cluster size.
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    An optical method for temperature measurement of silver clusters (1993)
    Springer
    The European physical journal 26 (1993), S. 115-117 
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    ISSN: 1434-6079
    Keywords: 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Generating silver clusters by means of the gas aggregation technique, we have observed that in addition to larger clusters, a substantial amount of silver dimers is always present. This allows us to determine the vibrational population in the electronic ground state and, hence, the vibrational temperature of Ag2 optically. Temperatures between 70–300 K were measured in this experiment, dependent on cell parameters, e.g. evaporation rate, argon flow rate, length of the drift tube, etc. The high volume collisional rate of 1021 cm−3s−1 at the given pressures allows us to assume that the cluster temperature does not differ considerably from that of dimer.
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    Comparison of small metal clusters: Ni, Pd, Pt, Cu, Ag, Au (1993)
    Springer
    The European physical journal 26 (1993), S. 156-158 
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    ISSN: 1434-6079
    Keywords: 36.40
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    Topics: Physics
    Notes: Abstract Structures of small clusters of Ni, Pd, Pt, Cu, Ag, and Au with n=2−34 and n=55 atoms are calculated as functions of number of atoms and temperature by the simulated annealing Monte Carlo method using an embedded atom potential.
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    Gas-phase chemical kinetics of small metal clusters: developmen and characterization of a fast-flow reactor for neutral clusters (1993)
    Springer
    The European physical journal 26 (1993), S. 168-170 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 82.20.Pm ; 82.30.Cf
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    Topics: Physics
    Notes: Abstract The development and characterization of a new fast-flow reactor for the study of the gas-phase chemical kinetics of small metal clusters is reported. The flow reactor has been designed to study the reactivity of well-thermalized clusters over a range of well-determined pressures in order to obtain quantitative absolute kinetic rate constants, especially in the low pressure limit. The apparatus has been characterized and proven through the study of known reactions of atoms and is capable of measuring bimolecular rate constants ranging five orders of magnitude down from gas kinetic at total pressures from 0.2 to 6 Torr. Application to the study of cluster kinetics has been demonstrated by measurements made on reactions of the copper dimer, Cu2.
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    Clustering in expanding nozzle flows (1993)
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    The European physical journal 27 (1993), S. 89-92 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 61.50.C
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A simplified model for the calculation of the maximum possible cluster size in expanding nozzle flows, assuming a coagulation growth, is presented. For this model we assume seeded beams with lean mixtures, where the ratio of carrier gas pressure to vapour pressure is much larger than one. In this case the heat of condensation is effectively absorbed by the carrier gas. To estimate the maximum possible cluster size it is assumed that every collision of condensable material contributes to the cluster growth. The cluster sizeN, which is the number of monomers or atoms per cluster, is then given by the number of cluster-cluster collisionsZ N by the relation $$N = 2^{Z_N } $$ .
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    Photoelectron spectroscopy of sodium chloride anions with two excess electrons (1993)
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    The European physical journal 26 (1993), S. 184-186 
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    ISSN: 1434-6079
    Keywords: 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Photoelectron spectroscopy measurements of (NaCl)nNa− (n=1–13, except n=10) are reported. The observed electron energy spectra fall into three distinct types, reflecting different correlations between the two excess electrons and ions. Depending on the host clusters' structures, the experimental evidence indicates that the two excess electrons in these sodium chloride clusters could be spin paired, forming a bipolaron or a Na− anion. The two excess electrons could also be spin parallel, forming a double F-center state.
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    The dynamics of phase changes in molecular clusters (1993)
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    The European physical journal 26 (1993), S. 101-104 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 61.14.F ; 64.60.Q ; 82.20.M
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    Topics: Physics
    Notes: Abstract The significant advantages offered by systems of molecular clusters in the study of homogeneous nucleation are discussed. Determinations of nucleation rates in clusters can be followed experimentally in supersonic jets or computationally in molecular dynamics simulations. Extraordinarily high rates may be encountered, both in freezing and in solid-state transitions. From such information can be inferred the interfacial free energies, σsl and σSS, mechanisms of solid-state transitions. and an explanation of why certain crystalline phases not found in bulk systems can be seen in large molecular clusters.
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    Shell structure and level spacing distribution in metallic clusters (1993)
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    The European physical journal 26 (1993), S. 28-32 
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    ISSN: 1434-6079
    Keywords: 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The lattice gas Monte Carlo and tight binding method is used to study the electronic shell structure in large metallic clusters. The average density of states of a large ensemble of deformed clusters shows the same shell structure as the most spherical geometry. The level spacing distribution at the Fermi level is a Wigner distribution.
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    Collision-induced reactions of size-selected molecular cluster anions (1993)
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    The European physical journal 26 (1993), S. 223-225 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 34.90
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Collision-induced reactions of size-selected cluster anions, (CO2) n − and (N2O)nO− with He and Kr atoms were studied at collision energies from 0.1 to 2.0 eV (center-of mass) by means of a tandem mass-spectrometer equipped with a pair of octapole ion guides. The dominant process was evaporation of the constituent molecules from the parent cluster ion. The absolute cross section for the evaporation was measured as functions of the size of the parent cluster ion and the collision energy. The reaction was explained by collisional excitation of the parent cluster ion followed by its unimolecular dissociation. The observed cross sections which correspond to those for the collisional excitation agree with those calculated in terms of charge-induced dipole and induced dipole-induced dipole interactions between the parent cluster ion and the target atom. The distributions of the product ions resulting from the unimolecular dissociation were reproduced by a simple calculation based on RRK theory. In the collision of (CO2) n − , the cross sections for (CO2) 10 − and (CO2) 14 − were significantly small and their abundances in the product ion distributions were particularly large. These findings indicate that (CO2) 10 − and (CO2) 14 − are stable species. On the other hand, stable species in (N2O)nO− was found to be (N2O)5O−.
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    Surface effects in vibrational and melting properties of Pb clusters (1993)
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    The European physical journal 26 (1993), S. 45-47 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 64.70D
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    Topics: Physics
    Notes: Abstract We have studied the structural, dynamical and thermodynamical behaviour of Pb clusters in theN=147−2057 range, using a phenomenological many-body potential and molecular dynamics. Cuboctahedral structures, previously found to be particularly stable for this system, are used as starting configurations at lowT. Calculated size-dependent melting temperatures appear to be reasonably accounted for by Pawlow's law,T m (N)=T m (∞)−AN −1/3. The behaviour of the mean square displacements and direct structural observations suggest that melting initiates at the surface, in agreement with recent experimental observations. High-frequency surface peaks in the vibrational spectrum disappear asT approachesT m , in coincidence with the onset of surface diffusion.
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    Ionization potential of clusters in liquids (1993)
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    The European physical journal 26 (1993), S. 82-86 
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    ISSN: 1434-6079
    Keywords: 82.50 ; 36.40 ; 35.20.V
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Time-resolved observations of the fast electron transfer from an electron donor to metal ions adsorbed on metal clusters in solution have shown that a critical size of the cluster is required to make it capable of accepting electrons. The threshold is attributed to a size dependent redox potential of the cluster, increasing with the nuclearity (in contrast with the ionization potential in the gas phase which decreases when n increases): it corresponds to the nuclearity for which the cluster redox potential becomes more positive than the potential of the electron donor acting as a monitor. New data of redox potentials (or IP) of Agn clusters (hydroquinone as monitor) and Cun cluster (sulfonatopropylviologen anion as monitor) are derived. The influence of n and of the solvation or the ligand is discussed.
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    Magic cluster ion distributions: (NO) 3 + (N2O)n and (NO) 3 + (CO2)n (1993)
    Springer
    The European physical journal 26 (1993), S. 183-185 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 33.80 ; 35.20
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Supersonic jet expansions of mixtures of nitric oxide with either nitrous oxide or carbon dioxide have been investigated over a wide range of relative concentrations. Mixed molecular cluster ions of the form (NO) m + (N2O)n and (NO) m + (CO2)n are detected following non-resonant two-photon ionization. Over a wide range of intermediate concentrations, the cluster ion distributions (NO) 3 + (N2O)n and (NO) 3 + (CO2)n with n≤30 are significantly more intense than clusters containing other numbers of nitric oxide molecules. The extra abundance of these species is attributed to their especially stable structures and several possible forms are discussed. An intriguing possibility involves a stable cyclic nitric oxide trimer (or ion) when combined with nitrous oxide or carbon dioxide clusters.
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    Threshold electron spectra of semiconductor cluster ions (1993)
    Springer
    The European physical journal 26 (1993), S. 216-218 
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    ISSN: 1434-6079
    Keywords: 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A simple threshold electron analyzer was constructed and laser photodetachment threshold electron spectra of Si− and Si 4 − were measured.
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    Reactions of carbon cluster ions stored in an RF trap (1993)
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    The European physical journal 26 (1993), S. 252-254 
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    ISSN: 1434-6079
    Keywords: 36.40
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    Topics: Physics
    Notes: Abstract Reactions of carbon clusterions with O2 were studied by using an RF ion trap in which cluster ions of specific size produced by laser ablation could be stored selectively. Reaction rate constants for positive and negative carbon cluster ions were estimated. In the case of the positive cluster ions, these were consistent with the previous experimental results using FTMS. Negative carbon cluster ions C n − (n=4−8) were much less reactive than positive cluster ions. The CnO− products were seen only in n=4 and 6.
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    Investigations on processes induced by single- and multiple-scattering of electrons in clusters involving excimer-induced fragmentation and a new metastable fragmentation channel observed for ArnO+* cluster ions (1993)
    Springer
    The European physical journal 26 (1993), S. 335-337 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 33.40.Hp ; 32.30.Bv
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    Topics: Physics
    Notes: Abstract A new metastable decay reaction of ArnO+ cluster ions produced by the electron impact ionization of argonoxygen clusters has been discovered, leading to the evaporation of more than two monomers in the metastable time regime. It is estimated that an energy of approx. 1 eV has to be released from the intramolecular to intermolecular modes prior to this metastable decay. In contrast to previously observed excimer-induced fragmentation of pure argon cluster ions, it has been concluded that the localized metastable state of an ionic rather than a neutral species is involved in the fragmentation. A reaction mechanism involving an electronic transition from the a2Π ArO+* (O(3P)-Ar+(2P)) excited state to the X4Σ− ArO+ (O+(4S)-Ar(1S)) ground state of the ArO+ complex ion within the cluster and subsequent release of considerable amount of vibrational energy to cluster modes is suggested to account for the observed fragmentation. Based on these measurements, a lifetime of approx. 2 µsis proposed for the excited state a2Π ArO+*.
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    Two maxima dependencies of isomerism contribution to heat capacity of N2O-HCN & CO-C2H2 systems in low temperature region (1993)
    Springer
    The European physical journal 26 (1993), S. 338-340 
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    ISSN: 1434-6079
    Keywords: 35.20 ; 36.40
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    Topics: Physics
    Notes: Abstract Temperature effects on heat capacity in four and three component systems N2O-HCN and CO-C2H2, respectively, were computed. Structural and vibrational parameters originated from the second order Møller-Plesset perturbation treatment with the 6-31++G** basis set while relative energies were of a still advanced nature. Isomerism contribution to heat capacity exhibits two temperature maxima at 43 and 205 or 15 and 83 K in N2O-HCN or CO-C2H2. The highest reported isomeric enhancement of the total heat capacity is about 27 %.
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    Theory of nonlinear optical properties of small metallic spheres (1993)
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    The European physical journal 28 (1993), S. 169-175 
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    ISSN: 1434-6079
    Keywords: 42.65.K ; 36.40 ; 73.35
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    Topics: Physics
    Notes: Abstract We calculate the nonlinear optical properties of small metallic spheres using electromagnetic theory and assuming that the local response of the conduction electrons is the same as for a plane surface. Electromagnetic Mie-resonances cause a strong increase of the second and higher harmonics in the reflected light. Detailed results are given for the second and third harmonic generation, its dependence on the frequency and polarization of the incident light, and on the cluster size. An enhancement of the second harmonic generation by a factor of about 5000 is obtained for small spherical metallic clusters. This is in good agreement with experiments on artificially roughened metal surfaces.
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    Modified Nilsson model for large sodium clusters (1993)
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    The European physical journal 28 (1993), S. 235-245 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 35.20.Wg ; 71.45.Nt
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    Topics: Physics
    Notes: Abstract We propose a modified Nilsson model for spheroidal sodium clusters and investigate the modification of shell structure by deformation for sizes up toN=850. For spherical clusters, our potential is fitted to the single-particle spectra obtained from microscopically selfconsistent Kohn-Sham calculations using the jellium model and the local density approximation. Employing Strutinsky's shell-correction method, the surface energy of the jellium model is renormalized to its experimental value. We find good agreement between our theoretically predicted deformed magic numbers and the experimentally observed ones extracted from recent sodium mass abundance spectra.
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    Dynamics of C 60 + surface impact: Rolling, deformation, disintegration, and deposition on HOPG graphite (1992)
    Springer
    Applied physics 55 (1992), S. 461-467 
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    ISSN: 1432-0630
    Keywords: 36.40 ; 61.80.L ; 79.20.N
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Cluster surface collisions of C+ 60 (Buckminsterfullerene) ions with pyrolytic graphite are studied by time of flight mass spectroscopy in the energy range of 100 eV to 1500 eV at oblique angles of incidence extending our previous studies with steep angles of incidence. Analysis of the velocity dependence of the scattered cluster ions reveals that the normal and tangential component of the ion velocity have different significance for the collision dynamics. The tangential component is partially transformed into rotational energy of the C+ 60 during the interaction with the surface as may be explained by an intuitive and astonishingly simple rolling ball model. In contrast, the normal component of the velocity appears to determine the amount of energy transferred into vibrational and deformational energy of projectile and target. At high “normal energies” the C+ 60 disintegrate significantly and can also be partially deposited onto the surface.
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    Laser induced fission of C60 with kinetic energy release (1992)
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    The European physical journal 24 (1992), S. 307-309 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 33.80 ; 33.80.G
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    Topics: Physics
    Notes: Abstract A neutral C60 fullerene beam is ionised by 308 nm laser pulses. For each cluster sizeC n + , 0〈n≦60 of the typical bimodal mass distributions known from the literature [1] velocity distributions have been determined by a time of flight method. A consistent interpretation of the measured mean velocities is obtained when binary fission of the parent molecule is assumed to be responsible for the fragmentation patterns, the total kinetic energy release being 0.45±0.1 eV independent of fragment mass and of laser fluence.
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    Structural fluctuation of Au55 and Au147 clusters: Substrate effect (1992)
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    The European physical journal 24 (1992), S. 377-384 
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    ISSN: 1434-6079
    Keywords: 36.40
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    Topics: Physics
    Notes: Abstract Structural fluctuation of Au55 and Au147 clusters with and without the substrate interaction are examined using model potentials and the transition state theory. It is shown that, for Au55 both with and without the substrate interaction, the cuboctahedral structure (COCT) has a lifetime too short to be observed with an electron microscope while the icosahedral structure (IC) has a lifetime long enough. On the other hand, for Au147 without the substrate interaction, both COCT and IC have lifetimes long enough but the lifetime of IC is too long for both of them to be observed in an observation period, say in several minutes. However, the lifetime is so much reduced by the substrate interaction that both the structures can be observed in an observation period. These results are compared with the experimental facts.
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    Core-hole photoionization study of polysilane compounds (1992)
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    The European physical journal 23 (1992), S. 159-164 
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    ISSN: 1434-6079
    Keywords: 33.10 ; 33.20.N ; 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present the results obtained with a new experimental set-up designed for the study of free semi-conductor clusters. This set-up is aimed to study the mass distribution of particles and the evolution of electronic properties as a function of the size, using the technique of core hole photoionization. The cluster production is based on the technique of radio-frequency discharge decomposition of a gas. We study the gaseous particles (Si n H x , 0〈x〈2n+2) generated by pure silane (SiH4) discharges at low pressures (〈10 millitorr) under continuous RF (Radio-Frequency) excitation conditions. We have studied the neutral species present in the post discharge zone and the positive ions present in the discharge. We identify the neutral species as polysilane compounds. We have also compared the ionization spectra obtained near Si-2p edge for the particles containing few silicon atoms with the spectra of SiH4 and Si2H6 molecules. For these molecules, the experimental observations are in agreement with theoretical calculations.
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    Formation of water and ammonia cluster anions by electron transfer from laser excited Rydberg atoms (1992)
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    The European physical journal 24 (1992), S. 297-306 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 82.30.f ; 82.60.L
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    Topics: Physics
    Notes: Abstract Using a laser excited Rydberg atomic beam as a very low energy (10-270 meV) electron source, we have investigated the experimental conditions for observation of water (from dimer to 40-mer) and ammonia cluster anions. From simple model calculations, estimates of the small water cluster adiabatic electron affinities have been determined as well as the orders of magnitude of autodetachment lifetimes and attached electron critical energies.
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    A diatomics-in-molecules model for singly ionized neon clusters (1992)
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    The European physical journal 23 (1992), S. 171-179 
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    ISSN: 1434-6079
    Keywords: 31.20.G ; 36.40 ; 34.20
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A minimum-basis diatomics-in-molecules (DIM) model previously developed for singly-ionized argon clusters is applied to neon clusters, Ne n + , forn=3, 4,...,22. A search for the global minimum energy of each cluster yields structures with the positive charge localised on a dimer-ion. This appears to be due largely to the linear unsymmetrical configuration which the model finds for Ne 3 + . For this reason, the structures of the clusters at their minimum energy are different from those for Ar n + computed with the same model. On the other hand, the behaviour of the charge distribution as a function of the geometrical configuration is similar to that for Ar n + , as are the overall shapes of the potential energy surfaces. The results are discussed in terms of the charge distributions and the ratios of equilibrium properties of the dimers and dimer-ions which constitute the input to the model.
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    URL: http://dx.doi.org/10.1007/BF01436741
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    Coherent raman spectroscopy of nitrogen molecules and clusters in supersonic jets (1991)
    Springer
    Applied physics 52 (1991), S. 84-89 
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    ISSN: 1432-0649
    Keywords: 36.40 ; 42.65 ; 47.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Coherent Stokes and anti-Stokes Raman scattering CSRS and CARS have been employed to study the spectroscopy of nitrogen molecules and clusters in the expansion of a supersonic jet. In the vibrational spectrum, at strong stagnation conditions, an intense redshifted peak is observed which can be assigned to the intramolecular vibrations in large N2 clusters having adopted the β-phase structure. Another weak feature is assigned to nitrogen clusters in the α-phase. In the rotational region of the spectrum only monomer features have been observed. The failing to observe librational motions is consistent with the finding that the nitrogen clusters are predominantly in the orientationally unordered β-phase. The low rotational temperature suggests supercooling of the β-phase.
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    URL: http://dx.doi.org/10.1007/BF00357660
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    Laser-fluorescence diagnostics for condensation in laser-ablated copper plasmas (1991)
    Springer
    Applied physics 53 (1991), S. 353-361 
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    ISSN: 1432-0649
    Keywords: 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We are investigating the thermodynamic conditions under which condensation occurs in laser ablated copper plasma plumes. The plasma is created by XeCl excimer laser ablation (308 nm, 300 mJ/pulse) at power densities from 500–1000 MW/cm2 into backing pressures of helium in the range 0–50 torr. We use laser-induced fluorescence (LIF) to probe velocity and relative density of both atomic copper and the copper dimer molecule, Cu2, which is formed during condensation onset. At low pressure (10 mtorr), the atomic Cu velocity peaks at approximately 2×106 cm/s. Copper dimer time-of-flight data suggest that condensation onset occurs after the Cu atoms have slowed very significantly. Excitation scans of the Cu2A-X (0,0) and (1,1) bands yield a rotational and vibrational temperature in the neighborhood of 300 K for all conditions studied. Such low temperatures support the theory that Cu2 is formed under thermally and translationally cold conditions. Direct laser beam absorption is used to determine the number density of atomic copper. Typical densities attained with 5 torr of helium backing gas are 6–8×1013 cm−3. Rayleigh scattering from particulate is easily observable under conditions favorable to particulate production.
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    URL: http://dx.doi.org/10.1007/BF00331827
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    Cluster emission under ion bombardment of metallic targets (1991)
    Springer
    Applied physics 52 (1991), S. 213-217 
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    ISSN: 1432-0630
    Keywords: 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Mass spectroscopic studies of the neutral particles sputtered by Ar+ ions at 8 keV from polycrystaline samples have been performed, using non-resonant laser ionization and subsequent time-of-flight mass spectroscopy. Besides sputtered atoms, also dimer and trimer contributions in the order of 10−1 to 10−2 and 10−3 to 10−4, respectively, are found in the sputtered flux. The data obtained here together with previously published data by other groups for different bombarding energies provide strong support for the validity of the recombination model.
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    URL: http://dx.doi.org/10.1007/BF00324422
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    Laser-stimulated desorption of potassium atoms (1991)
    Springer
    Applied physics 52 (1991), S. 445-447 
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    ISSN: 1432-0630
    Keywords: 82.65 ; 36.40 ; 73.20
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Desorption of K atoms by laser-excitation of surface plasmons in small K particles is reported. The desorption rate has been measured for different laser wavelengths and particle sizes. Time-of-flight measurements reveal a kinetic energy of the desorbed atoms of Ekin=0.13(3) eV. From the experimental data it is concluded that the desorption mechanism is non-thermal in nature. Comparison of the results reported here with our earlier work on Na desorption is made.
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    URL: http://dx.doi.org/10.1007/BF00323657
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    Studies of Sb and Bi cluster produced by laser desorption (1991)
    Springer
    The European physical journal 19 (1991), S. 203-206 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 79.20
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Direct production of cations and anions of metal clusters of Sb and Bi by laser evaporation in a vacuum has been studied. Bulk sample substrates are irradiated by 1064, 532 and 355 nm beams at variable intensity, and the ions produced are accelerated and identified by time-of-flight mass spectrometry. At 1064 nm, the cation distributions show that Sb 3 + and Bi 3 + are the most abundant species, while the monomer and dimer cations are almost non-existent. The anion spectra indicate very low yields of Sb− and Bi− with dominant dimer anion species. These patterns persist with laser power variation within the stable operation domain. With lower incident laser wavelength, the mass distributions are modified, favouring the production of the light cluster ions. In no circumstances were Sb and Bi ions withn〉5 observed. Many of the observed phenomena can be explained if one assumes that for these elements, clusters withn〈6 are formed on the substrate surface. Cluster ions are produced via a prompt desorption process, and are subjected to photon induced reactions due to the same incident laser beam. However, more detailed investigation of the desorption properties will be necessary to confirm such a desorption mechanism.
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    STM observation of Au fine-particles on graphite (1991)
    Springer
    The European physical journal 19 (1991), S. 333-335 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 61.16.D ; 82.65.F
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Gold particles deposited on graphite in vacuum have been studied by STM observation in air. Liquid-like coalescence between small gold-particles has been observed near room temperature. Preparation of small particles in vacuum is discussed. Small particles are formed in nucleation process if the degree of coalescence of particles is reduced. Over 400 Au particles of 5 nm in diameters with a narrow size-distribution with FWHM 2 nm and a high density of 3×1012/cm2 is prepared by evaporating gold in a vacuum of about 2×10−5 Torr and at the substrate (HOPG) temperature of 20°C.
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    Adsorption — desorption kinetics on epitaxially oriented palladium clusters (1991)
    Springer
    The European physical journal 19 (1991), S. 347-351 
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    ISSN: 1434-6079
    Keywords: 82.65.Jv ; 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Palladium clusters with a low size dispersion and a single crystallographic shape have been obtained by epitaxial growth on (100) MgO. The isothermal adsorption — desorption kinetics of CO is obtained by a molecular beam method allowing the direct determination of the global adsorption probability and of the mean life time of CO molecules on the clusters. Three main results are deduced from these measurements: — an increase of the adsorption rate of CO on the clusters by the capture of CO molecules physisorbed on the substrate, — a faster desorption rate from (100) than from (111) facets at high coverage, — an increase of the adsorption energy for clusters smaller than 3 nm.
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    Structural stability and energetics of C, Si, and Ge microclusters: empirical many-body potential energy function calculation (1991)
    Springer
    The European physical journal 19 (1991), S. 423-425 
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    ISSN: 1434-6079
    Keywords: 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The structural stability and energetics of carbon, silicon, and germanium microclusters containing 3−7 atoms have been investigated by using a recently developed empirical many-body potential energy function (PEF), which comprises two- and three-body atomic interactions. The PEF satisfies both bulk cohesive energy per atom and bulk stability exactly. It has been found that the most stable C3−4 microclusters are linear withD ∞h symmetry but C5−7 microclusters are planar withD nh symmetry. Silicon and germanium microclusters show similar structural stability. TheX n (X=Si, Ge;n=3−7) microclusters are found to be most stable in the following forms:X 3 is triangular withD 3h symmetry,X 4 is tetragonal withT d symmetry,X 5 is square pyramidal withD 4h symmetry,X 6 is bipyramidal square withO h symmetry, and finallyX 7 is square pyramidal having two atoms underneath withD 2h symmetry.
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    C n carbon clusters (n=4−6, 60, 70): interesting cases of computation/observation comparisons (1991)
    Springer
    The European physical journal 19 (1991), S. 431-433 
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    ISSN: 1434-6079
    Keywords: 35.20 ; 36.40 ; 82.30Q
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Recent theoretical studies proved some of the small carbon clusters C n (n≈1−10) are represented by several isomeric structures. Evaluation of the isomer relative stabilities is necessary for comparison of theoretical data with observation, this being exemplified by the C4 and C6 cases. For the former cluster a new energy estimation has been suggested based on the isomerism reasoning. Relative-stability evaluation for clusters of different dimensions is discussed on the C60/C70 system, pointing out a strong pressure-dependency of the picture. The scheme is matched to the recent observations of Bethune and Meijer.
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    On the shell structure and geometry of monovalent metal clusters (1991)
    Springer
    The European physical journal 21 (1991), S. 271-279 
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    ISSN: 1434-6079
    Keywords: 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The Hückel model is used to study the electronic structure of monovalent metal clusters. In an fcc cluster the Hückel model gives an estimate to the electronic structure of a free electron cluster. It is shown that the surface faceting of the fcc cluster can destroy the electronic shell structure already when the cluster has about 100 electrons. In the Hückel model the icosahedral structure has smaller total energy than the fcc structures, from which the Wulff construction has the smallest energy already when the cluster has 600 atoms.
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    URL: http://dx.doi.org/10.1007/BF01426385
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    The structure of deposited metal clusters generated by laser evaporation (1991)
    Springer
    The European physical journal 21 (1991), S. 285-288 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 35.20
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Metal clusters have been produced using a laser evaporation source. A Nd-YAG laser beam focused onto a solid silver rod was used to evaporate the material, which was then cooled to form clusters with the help of a pulsed high pressure He beam. TOF mass spectra of these clusters reveal a strong occurrence of small and medium sized clusters (n〈100). Clusters were also deposited onto grid supported thin layers of carbon-films which were investigated by transmission electron microscopy. Very high resolution pictures of these grids were used to analyze the size distribution and the structure of the deposited clusters. The diffraction pattern caused by crystalline structure of the clusters reveals 3-and 5-fold symmetries as well asfcc bulk structure. This can be explained in terms of icosahedron and cuboctahedron type clusters deposited on the surface of the carbon layer. There is strong evidence that part of these cluster geometries had already been formed before the depostion process. The non-linear dependence of the cluster size and the cluster density on the generating conditions is discussed. Therefore the samples were observed in HREM in the stable DEEKO 100 microscope of the Fritz-Haber-Institut operating at 100 KV with the spherical aberrationc S =0.5 mm. The quality of the pictures was improved by using the conditions of minimum phase contrast hollow cone illumination. This procedure led to a minimum of phase contrast artefacts. Among the well-crystallized particles were a great amount of five- and three-fold symmetries, icosahedra and cuboctahedra respectively. The largest clusters with five- and three-fold symmetries have been found with diameters of 7 nm; the smallest particles displaying the same undistorted symmetries were of about 2 mm. Even smaller ones with strong distortions could be observed although their classification is difficult. The quality of the images was improved by applying Fourier filtering techniques.
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    Neutral mass-selected lead cluster beams (1991)
    Springer
    The European physical journal 19 (1991), S. 199-201 
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    ISSN: 1434-6079
    Keywords: 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Neutral lead cluster beams with ultra-narrow size distribution were produced by neutralization of mass-selected lead cluster ions, Pb n + withn≤12, in sodium vapor under near-resonant conditions. Absolute charge exchange cross sections were measured as a function of cluster size and are on the order of 40 Å2 forn≥4. Possible fragmentation of the clusters associated with charge transfer was examined by translational spectroscopy. No indication of fragmentation was found.
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    Thermionic emission from giant fullerenes (1991)
    Springer
    The European physical journal 19 (1991), S. 409-412 
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    ISSN: 1434-6079
    Keywords: 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Large even-numbered carbon clusters in the size range from 100 to 600 atoms (giant fullerenes) were generated by laser vaporization and directly injected as positive ions via a supersonic beam into the magnetic trap of an ion cyclotron resonance apparatus. Intense laser excitation of the magnetically levitated fullerenes at 4.0 eV was found to result in production of multiply charged fragments in the trap. Details of the time scale and size dependence of this process suggest it is due to thermionic emission from the superheated gas phase clusters.
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    Electronic transitions in C60 and its ions (1991)
    Springer
    The European physical journal 19 (1991), S. 435-438 
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    ISSN: 1434-6079
    Keywords: 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The electronic spectrum of C60 is calculated using quantum mechanical methods. The first allowed transition in C60 is calculated at an energy of 3.5 eV and with an oscillator strength of 0.09. Several transitions are found at higher energies with comparatively strong oscillator strengths, the strongest one being at 5.78 eV (λ=214 nm). The calculated energy level diagram of C60 is also used to predict wavelengths for transitions in C 60 + and C 60 − . A comparison is made with some recently observed diffuse interstellar bands at 1180 nm and 1320 nm, which have been speculated to originate from C 60 + .
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    Internal excitation effects in the photodissociation of size selected ethylene and methanol dimers (1991)
    Springer
    The European physical journal 18 (1991), S. 181-188 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 82.50.F
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The infrared photodissociation spectra of C2H4 and CH3OH dimers are measured for different internal excitation energies ΔE. The dimers are size-selected in a scattering process with He and the internal energy is varied by using (1) different collision energies, (2) different scattering angles and (3) by measuring the complete energy loss spectra with and without laser radiation by time-of-flight analysis of the scattered clusters. For (CH3OH)2 the width Γ of the spectrum increases strongly with ΔE, while the cross section at the maximum is constant, a normal behaviour for an inhomogeneously broadened system. For (C2H4)2 Γ is nearly constant after a sort of threshold and the cross section increases with increasing ΔE. These results are explained by hot band excitation and the coupling to the darkv 10-mode.
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    Sub-doppler-spectroscopy of Na3 (1991)
    Springer
    The European physical journal 18 (1991), S. 257-265 
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    ISSN: 1434-6079
    Keywords: 33.20 ; 35.20 ; 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Rotationally cold Na3-molecules are formed during the adiabatic expansion of an argon beam, seeded with 5% sodium vapor. With a single mode tunable dyelaser crossing the molecular beam perpendicularly within the ion source of a mass spectrometer the Na3-molecules are excited into levels of the2 A 2-state which are then ionized by acw argon-laser. Rotational lines of the electronic2 A 2 ←X 2 B 2 transition and their hyperfine structure could be resolved using an OODR-technique. A preliminary analysis of the measured spectra is discussed and a comparison of the experimental results with ab initio calculations is presented.
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    Calculation of photoionization cross sections of Na2–8 and K2–8 clusters (1991)
    Springer
    The European physical journal 18 (1991), S. 267-280 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 33.80E
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Analysis of free metal clusters studied with photoionization mass spectrometry or photoelectron spectroscopy requires theoretical predictions of the photoionization cross sections to gain a deeper physical understanding. Calculated energy-dependent photoionization cross sections of Na2–8 and K2–8 clusters are presented in this study. The ground state electronic structure of the clusters are calculated using the Local Spin Density method (LSD) which is also the starting point for the cross section calculation with the continuum multiple scattering method. A basic analysis of the photoionization process is given within the independent electron picture. Strong resonances are predicted in the UV cross sections (5–10 eV) of K3–8 but not for Na3–8, interpreted as shape resonances, i.e. quasibound states in which electrons are trapped by a potential barrier. Unfortunately experimental data are only known close to the ionization threshold and a comparison between our values and experimental data in a broad energy range is not possible.
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    Computer simulations of small semiconductor and metal clusters (1991)
    Springer
    The European physical journal 19 (1991), S. 31-36 
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    ISSN: 1434-6079
    Keywords: 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A brief survey is presented of recent simulations of small clusters, made with both ab-initio and classical approaches, with particular emphasis on the application of the Car-Parrinello method. The discussion mainly focusses on the structural properties of a variety of materials and on the effects of temperature.
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    The influence of shells, electron thermodynamics, and evaporation on the abundance spectra of large sodium metal clusters (1991)
    Springer
    The European physical journal 19 (1991), S. 47-50 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 35.20.Wg ; 03.65.-w
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Measurements of the mass abundance spectra of sodium clusters containing up to 600 atoms are presented. The clusters are produced in a seeded supersonic expansion of Ar or Kr gas, and the spectra are obtained by a time-of-flight technique. The sawtooth features in the spectra are interpreted as evidence of a regular spherical shell structure with magic numbers,N 0, scaling approximately with the cube root of the number of sodium atoms. Altogether twelve shell closings are observed,N 0=2, 8, 20, 40, 58, 92, 138, 196, 260, 344, 440 and 558. There is also a pronounced odd-even staggering all the way up toN=70. The experimentally observed intensity changes for the clusters around the magic numbers are discussed in terms of the electronic free energy,F(N), calculated at finite temperature, and the second differences of the free energy Δ2 F(N)=F(N−1)−2F(N)+F(N+1). The processes behind the non-uniform abundance distributions, and the thermodynamics of finite electron systems with non-uniform level spacings are discussed on this basis.
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    Semiclassical variational study of size effects in neutral and charged jellium droplets (1991)
    Springer
    The European physical journal 19 (1991), S. 55-58 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 31.20
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract An analytic expression for the total energy of metallic clusters composed ofN identical atoms of valencev and with net chargeQ is obtained by means of a variational solution of the Thomas-Fermi-Weizsäcker energy density functional within the spherical jellium model. The minimum energy is given as an expansion in decreasing powers of the cluster radiusR=r s Z 1/3, withZ=vN andr s the radius per electron of the bulk metal. The coefficients are obtained as functions ofr s . Terms of volume (R 3), surface (R 2), curvature (R), constant (R 0), (1/R) and (1/R 2) are clearly separated in the formula, as well as the different contributions (kinetic, coulombic and exchange-correlation) to each of them. The asymptotic values (R→∞) for the work functions,W(r s ), and surface energies σ(r s ), are compared to analogous semiclassical and Kohn-Sham calculations of jellium-like surfaces and to the experimental values. The size dependent behaviour of chemical potentials, μ(R), electron affinities,AF(R), and ionization potentials,IP(R), are easily obtained for any kind of metallic clusters. In particular we discuss the Coulomb and quantum corrections to the coefficients β, δ in the asymptotic formulae:IP≃W+β/R andAF≃W+δ/R.
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    Absolute cross sections for electron impact ionization of Ag2 (1991)
    Springer
    The European physical journal 19 (1991), S. 77-79 
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    ISSN: 1434-6079
    Keywords: 34.80G ; 36.40 ; 35.20V
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The previously measured relative cross section function for electron impact ionization (EII) of neutral Ag2 has now been calibrated quantitatively by combining the electron impact ionization with in situ non resonant two photon ionization (NR2PI). By comparing the NR2PI saturation intensities measured for Ag 2 + and Ag+ with the corresponding EII intensities, the ratio between the electron impact ionization cross sections (EIICS) of neutral Ag2 and Ag was determined to be σAg2/σAg=1.53 for an electron energy of 46 eV. This result agrees well with the geometricn 2/3-rule $$(\sigma X_n \sim n^{2/3} )$$ commonly proposed for the dependence of the EIICS of clustersX n on the cluster sizen.
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    Thermal broadening of plasmon absorption in potassium cluster ions (1991)
    Springer
    The European physical journal 19 (1991), S. 85-87 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 78.40-q
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The optical absorption spectrum around the surface plasmon peak is presented for K 21 + . Hot, charged potassium clusters are produced by an adapted liquid metal ion source. The plasmon is found to be slightly red shifted from and distinctly broader than the one reported in an experiment on colder clusters by Bréchignac et al. [3]. Mechanisms responsible for the temperature dependence of the width and position of the surface plasmon in small metal clusters are discussed.
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    Response properties of sodium clusters within a jellium-like model with finite surface thickness (1991)
    Springer
    The European physical journal 19 (1991), S. 93-96 
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    ISSN: 1434-6079
    Keywords: 36.40 ; 71.45.G
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The usual spherical jellium model of alkaline clusters is generalized to allow for a finite surface thickness in the positive background distribution. Static (relative stability, ionization potentials) and response properties have been calculated and the results show a better agreement with experiments.
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    URL: http://dx.doi.org/10.1007/BF01448263
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