Electronic and geometrical structure of noble metal clusters

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.This article was processed using Springer-Verlag T_EX Z.Physik D macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.     

Effects of motional narrowing on the plasmon resonance in small metal clusters

A model accounting for the time dependence of shape fluctuations in metal microclusters and their effect on the damping width of the plasmon resonance is discussed. An estimate of the relaxation time of the quadrupole shape is given, and calculations demonstrating the effects of time-dependent thermal fluctuations on the plasmon in K ₉ ⁺ are presented.Lee A. DuBridge research fellow in Physics     

Computer simulations of HREM images of metal clusters

Multislice calculations have been performed for Ag, Pd and Au clusters in the size range of 5.0 nm diameter of cuboctahedral, icosahedral and decahedral structures. It could be shown that tilt series are necessary for the classification of the structures. Particularly for arbitrary orientations, i.e. deviations from main directions such as 2-, 3- and 5-fold axes, the performance of computer simulations is mandatory. The influence of absorption is also studied for the case of a 100 kV microscope by introducing a complex potential.     

Evaporation rates for liquid clusters

An expression for the evaporation rate of neutral atoms from a hot liquid cluster is suggested. It combines Weisskopf's statistical model with a level density that is derived from the experimentally known free energy of a macroscopic droplet of the cluster material. For the case of sodium clusters, it is compared with the rate based on the level density of a system of oscillators. For the same internal energy the latter gives substantially larger evaporation rates, whereas for equal temperature the increase is moderate. For large clusters the electronic contribution to the entropy can no longer be neglected.     

Isomerisation and phase transitions in small sodium clusters

Using a distance-dependent tight-binding hamiltonian, we have studied the influence of the temperature on the geometries of small alkali clusters (Na₄, Na₈, and Na₂₀). We have applied a Monte-Carlo thermodynamical method which consists in performing canonical samplings for various temperatures, these samplings being reexpressed in the microcanonical ensemble. This method provides thermodynamical values such as the entropy and the specific heat. Their behaviour shows one phase transition in the case of Na₄ and Na₈, and two phase transitions for Na₂₀. As concerns Na₄ and Na₈, the transition occurs at 200 K, between a solid-like phase and a phase for which the geometry of these clusters oscillates between numerous shapes. In the case of Na₂₀, the two observed phase transitions can be described as a melting of the surface atoms (at 200 K) preliminarily to the fluctuation of an inner icosahedron seed (at 300 K).     

Energy transfer in collisions of alkali-metal clusters and ions

The electronic energy loss of a positively charged projectile passing through a sodium cluster is calculated by the semiclassical Vlasov method. The electron density before impact is obtained within the localdensity approximation in the jellium model. For the dynamical response, the Vlasov equations are solved by test particle discretization. The model gives the correct breathing mode and dipole Mie frequencies. The energy deposit is calculated as a function of projectile velocity, charge and impact parameter and is compared with theoretical and tabulated values for bulk sodium.     

Theoretical investigation of small MgO, CaO and NaCl clusters

We study the size dependence of the electronic and structural properties of small MgO, CaO and NaCl clusters (up to 12 atoms), thanks to a selfconsistent tight binding approach in which we treat electrostatic and covalent effects on the same level. We discuss the conditions under which the ionic charges get reduced and the interatomic distances are contracted or dimerized.     

Dynamics of the barium-molecule system within large argon clusters

The effects of adding molecules on the LIF at 540 nm of a barium atom at the surface of an argon cluster (average size 420) has been investigated. We showed that molecules like ethanol,n-hexane and O₂ from stable complexes with ground state barium. In the case of molecules like N₂, CH₄ and SF₆, the collisional quenching of solvated Ba(¹ P) is observed. The large quenching rates obtained are interpreted by a surface mobility of the collisional partners. Moreover, we showed that this collisional quenching leads to the ejection of free Ba(³ P ₁).     

Infrared multiphoton excitation of refractory metal clusters

Photoexcitation and photoionization experiments on small Tungsten and Niobium clusters were performed with a Q-switched Nd:YAG laser (1064 nm) and a reflectron type time-of-flight mass spectrometer. For low laser fluences the monomer and very small clusters do not show up in the mass spectra. Furthermore, the detected cluster ions show very asymmetric peak shapes caused by delayed ionization (thermionic emission). For high photon fluences photoions with up to charge state +3 could be detected.     

Preparation and study of silver clusters stabilized by matrices of different chemical nature

Small silver clusters characterized by an absorption band (a.b.) at 320–340 nm were stabilized in matrices of different chemical nature. Among them Ag-alumosilicate was a most suitable system for cluster study by different physical and chemical methods. Clusters introduced into the zeolite of mordenite kind were found to be the most stable. Size of stabilized silver clusters measured by different methods is discussed.     

Effects of the cluster surface on the electronic shell structure: faceting,roughness and softness

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.This paper was originally submitted in connection with the 2nd. Int. Conference on Atomic and Nuclear Clusters held in Santorini from 28. June–2. July 1993 and is published here as a regular article after an independent refereeing procedure according to the standards of Z. Phys. D     

Ionic structure and global deformation of axially symmetric simple metal clusters

The ground-state structure and shape parameters of neutral and singly charged simple metal clusters with up to 40 valence electrons have been calculated within the cylindrically averaged pesudopotential scheme (CAPS). Na and Ba have been chosen as typical monovalent and divalent metals. The obtained structures agree very well with the results of more elaborate methods. A simple growth pattern for the most stable ionic geometries is deduced. The ionic and electronic multipole moments from CAPS are very similar to those of the structure averaged jellium model (SAJM) and of the plasma model. The surface energies derived from the CAPS results agree with experimental values.     

Calculation of the electronic properties of metal dimers

The low-energy excitation spectra of metal dimers are determined by solving exactly a realistic many-body Hamiltonian with inter- and intra-atomic Coulomb interactions. Our results for Cu₂ and Ag₂ are in very good agreement with the excitation energies derived from recent photodetachment experiments. The characteristics of the many-body excited states in these clusters are briefly discussed.     

Photoionization of small silicon clusters: ionization potentials for Si2 to Si40

Photoionization thresholds of Si _n (n=2–40) were examined by vacuum UV radiation (5.7–8.5 eV) generated by stimulated Raman scattering of narrow-bandwidth 193-nm radiation in high pressure hydrogen and deuterium gases. A strong threshold energy dependence on cluster size is observed, featuring major maxima at 10 and 20. The magic behavior atn=10 is consistent with the results of the photofragmentation and CID experiments reported previously.     

Exchange effects on electronic states in jellium clusters

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.deceased     

Molecular dynamics study of A18B lennard-jones clusters

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.This paper was originally submitted in connection with the 2nd. Int. Conference on Atoic and Nuclear Clusters held in Santorini from 28. June–2. July 1993 and is published here as a regular article after an independent refereeing procedure according to the standards of Z. Phys. D     

Effective magnetization of rotating free ferromagnetic metal clusters

The deflection of free magnetic metal clusters in a Stern-Gerlach magnetic field is studied. In particular we investigate magnetic resonance effects resulting from lattice anisotropy and cluster rotation. In analogy to small suspended particles in an oscillating magnetic field the anisotropy field fixed to the rotating atomic lattice of the cluster acts on the cluster magnetization like an rf field in NMR experiments. In our calculation we have used the Bloch equations and assumed different anisotropy field symmetries (uniaxial, cubic). A minimum in the magnetization as a function of the Stern-Gerlach field and also of the cluster size, as observed recently, is obtained under certain conditions. However, such a resonance behavior occurs only if the distribution of the rotation frequency _rot is relatively narrow, while a broad distribution of _rot yields an almost superparamagnetic behavior. In addition, the strength of the anisotropy field and the relaxation time are important variables which determine the magnetic behavior of the clusters.     

Sodium doped binary clusters I: Ionization potentials of SinNam clusters

Sodium doped silicon clusters (Si_nNa_m, 3n11, 1m4) were produced by two independent laser vaporization methods and their ionization potentials were measured by scanning the wavelength of the UV dye laser. The IPs of most Si_nNa_m clusters decrease monotonously with the number of Na atoms, but IPs of Si₇Na_m and Si₁₀Na_m clusters show an apparent even-odd alternation; odd numbers of Na atoms efficiently decrease the IP but even numbers of Na atoms never significantly decrease the IPs. In addition, the reactivity of Si_nNa_m clusters for NO molecules was investigated with a fast flow reactor, and an anti-correlation between IP and the reactivity was clearly observed; clusters having high IP show low reactivity andvice versa.     

Angular momentum stability of small atomic clusters

The limits of stability of small atomic clusters with angular momentum are investigated as a function of the size and charge of the aggregates. Critical angular momenta are obtained in a di-cluster picture from the balance of a cohesive surface-surface interaction and the disruptive centrifugal and electric interactions. The calculation scheme incorporates the mass and charge asymmetry degrees of freedom and it is thus particularly suited to explore cluster-cluster reaction processes. Deformation of the fragments is taken into account.