Threshold electron spectra of semiconductor cluster ions

A simple threshold electron analyzer was constructed and laser photodetachment threshold electron spectra of Si^– and Si ₄ ^– were measured.     

Reactions of carbon cluster ions stored in an RF trap

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 C_nO^– products were seen only in n=4 and 6.     

Theory for the atomic shell structure of the cluster magnetic moment and magnetoresistance of a cluster ensemble

We present a simple theory for the cluster size dependence of the average cluster magnetic moment of transition metal clusters. Assuming a local environmental dependence of the atomic magnetic moments, the cluster magnetization exhibits a magnetic shell structure, reflecting the atomic structure of the cluster. Thus, the observed oscillations of the average cluster magnet moment may serve as a fingerprint of the cluster geometry. We also discuss the giant magnetoresistance (GMR) exhibited by an ensemble of magnetic clusters embedded in a metallic matrix. It is shown that the magnetic anisotropy affects strongly the magnetization of the cluster ensemble under certain conditions. Since the GMR depends on the cluster ensemble magnetization, it can be used to determine the cluster magnetic anisotropy energy.     

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.     

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.     

Comparison of small metal clusters: Ni, Pd, Pt, Cu, Ag, Au

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.     

Collisional process involving atomic cluster ions

Collision of Ar cluster ions, Ar _n ⁺ (n=3–16), with He and Ne atoms was investigated by use of mass spectroscopic techniques. The cross sections for the production of Ar _n ⁺ (nn) were measured as functions of the size of the parent cluster ion and the collision energy (0.1–10 eV in the center-of-mass frame). These results were analyzed in the scheme of hard-sphere spectator collision with RRK theory. It was concluded that the reaction proceeds via collisional excitation of the parent cluster ion and following sequential loss of the constituent Ar atoms.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     

Nickel cluster structure determined from the adsorption of molecular nitrogen: Ni49-Ni71

The geometrical structures of nickel clusters in the size range from 49 to 71 atoms are studied by the chemical probe method. Saturation coverages of molecular nitrogen are determined for each cluster and from this data specific structures are proposed (except for Ni₆₆ and Ni₆₇). The results indicate that icosahedral packing is the dominant structural configuration throughout this size range, in agreement with earlier results based on water and ammonia adsorption. In addition, it seems that for clusters larger than Ni₅₄ the excessive strain in the surface of the 55-atom regular icosahedron often leads to rear-rangements of the surface atoms to relieve that strain. Ni₅₅, in particular, is found to have two isomers, the regular icosahedron and a structure in which a single apex atom is displaced to the center of an opposite face. Ni₇₁ occurs as a 55-atom regular icosahedron with a 16-atom cap. The results suggest that the atoms in the cap adopt an ABA configuration relative to the underlying icosahedron rather than an icosahedral arrangement. For some clusters the saturation with nitrogen causes a small degree of surface reconstruction that leads to the adsorption of additional nitrogen molecules.Work supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, under Contract No. W-31-109-Eng-38     

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     

Kinetics of cluster aggregation in competition with a chemical growth reaction

The kinetics of abundances and the distribution in size of clusters at a given time are obtained by numerical simulation in the case where a chemical growth competes with the coalescence of clusters originating from isolated monomers. To take into account the size dependence of cluster reactivity, it is assumed that the chemical reaction occurs only beyond a critical valuen _c . The kinetics of the total cluster concentration are unchanged by addition of the reactant but the size distribution is suddenly perturbed for sizes beyondn _c . The decay of the reactant, the kinetics of a cluster of a given nuclearity and the distribution of sizes at a given time depend on the relative values of the rates of coalescence and of growth due to the reactant. It is shown how the initial conditions control the kinetics and the amplitude or the time of the maximum cluster abundance. The case of an attack by a chemical reactant on clusters, restricted to low values ofn, is also considered. The concentration kinetics (for all size clusters) display much lower amplitudes than for pure coalescence case. No parity effects are observed.     

Picosecond photoelectron studies of cluster dynamics

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.     

Structural and dynamical properties of water dimer and tetramer: molecular dynamics study

A detailed evaluation of the structural and dynamical properties of isolated water dimers and tetramers using the Lemberg, Stillinger and Rahman potential energy surface and microcanonical molecular dynamics simulations are reported. The lowest-energy configurations of these clusters were obtained by simulated thermal quenching. Relative root-mean-square bond length fluctuations were used to characterize phase transitions. Solid-like, pre-melting, liquid-like, and dissociated states were identified. Global-local minimum transitions and the minimum energy path to dissociation of the dimer were also characterized.     

Electronic structure and reactivity of Mg+(H2O)n cluster ions

Electronically excited states of magnesium-water cluster ions, Mg⁺(H₂O) _n ,n=1–5, are studied by photodissociation after mass selection. The observed photodissociation spectra are assigned to the²P–²S type transitions localized on the Mg⁺ ion with the aid of ab initio CI calculations. In addition to evaporation of water molecules, photoinduced intracluster reaction to produce MgOH⁺(H₂O) _n is found to occur efficiently, with pronounced size dependence. The intriguing features observed in the mass spectrum of nascent cluster ions are discussed in relation to the stepwise solvation of this reaction.     

The optical extinction of ligand-stabilized Au13- and Au55-clusters: The vanishing of the Mie-resonance

We present optical extinction spectra of ligand-stabilized Au-clusters Au_N 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.     

SCA calculations of the inner shell ionization with Dirac-Fock electronic wave functions

The theory of inner shell ionization for arbitrary atomic shells is reviewed. Emphasis is onL- andM-shells in order to show how the proper screening formalism entering the electronic form factor affects the ionization probabilities. The radial wavefunctions in the form factor are computed as relativistic Hartree-Fock orbitals for both bound and continuum states. The continuum orbitals were evaluated in theV(N–1) potential with correct exchange. These results are then compared with the previous ones using screened hydrogen-like wavefunctions and also with the experimental data in some cases.     

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.     

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.     

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).     

Magic cluster ion distributions: (NO)3+ (N2O)n and (NO)3+ (CO2)n

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 n30 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.     

Kinetics of cluster growth by aggregation

The classical problem of the coalescence of isolated species to produce growing clusters/colloids/polymers by successive statistical encounters having the same rate constant, is revisited using numerical simulation for a maximum nuclearity value of a few 10³ units. The evolution with time of the abundance of clusters of a given nuclearity and of the total population, and the distribution of sizes at a given time are obtained and compared with models from the literature. A remarkable feature of these curves is that they exhibit parity effects for the nuclearity, even clusters being systematically more abundant than odd ones. For easier comparison with experiments, some simulated curves are presented in the form of an approximated analytical expression: kinetics of the total population, and of the monomer, dimer and higher oligomers populations, amplitudes at the maximum and delay for the maximum as functions of the nuclearity, size distribution at a given time. The validity of the approximations is discussed.