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.     

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).     

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.     

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.     

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.     

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.     

Reaction studies of carbon clusters

Reaction studies of carbon clustersC _n in the rangen=8–37, produced by laser vaporisation in a supersonic nozzle, have been investigated using time-of-flight mass spectrometry. Clear differences in reaction products formed on hydrogenation are detected which show that even and odd clusters behave quite differently and furthermore thatat least two different types of even cluster appear to exist. The reactivity patterns for clusters C _n withn=16, 18 and 22 are in a different class from those withn=20, 24, 26 ..., a behaviour consistent with the existence of closed cage fullerene structures for even clusters with 20 or more carbon atoms (other thann=22).     

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).     

Electron bombardment induced fragmentation of size selected neutral (D2O) n clusters

The fragmentation behaviour of size selected neutral (D₂O) _n clusters withn4 after ionization with 70 eV electrons is subject of this work. Size selection by scattering the cluster beam from a He target beam in combination with a quadrupole mass filter and time resolved measurements at specific laboratory angles enables us to determine the neutral precursor masses of the detected ions. The measured fragment pattern is dominated by deuterated ions of the form (D₂O) _n–x D⁺ withx1. The dimer fragmentation which leads with a probability of 62.5% to the D₃O⁺ ion and with 37.5% to D₂O⁺ can be explained by fast intracluster ion-molecule reactions of charged monomer fragments reacting with the partner molecule. For larger clusters the fragmentation process can be rationalised by the creation of an initially highly excited D₃O⁺ (D₂O) _x complex which is stabilized by evaporating additional monomer units with the main fragment channel (D₂O)D⁺ forn=3 and (D₂O)₂D⁺ forn=4. With increasing cluster size an increasing tendency of evaporation of more than one water monomer unit has been observed.     

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.     

A new formulation of the dynamical response of many-electron systems and the photoabsorption cross section of small metal clusters

Static polarizabilities and photoabsorption cross sections of clusters Na ₇ ^– , Na₈, Na ₁₉ ^– Na₂₀ are calculated, based on the spherical jellium model including the self-interaction correction (SIC) of Perdew and Zunger. To this end, a new formulation of the theory of the linear response is presented, which is suitable for general, self-interaction corrected, many-electron systems. The results obtained display an overall agreement with available experimental data, offering a systematic improvement with respect to the standard TDLDA. Furthermore, the cross sections of the negatively charged clusters are found to be dominated by a broad peak in the visible region, whose line width can be related to the lifetime of the surface plasmon against electron detachment.On leave of absence from the University of Coimbra, Portugal     

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.     

Ionization potential measurements of hydrogenated lithium clusters

Ionization Potentials of Li_nH_m clusters have been measured by photoionization. As in Li_n, odd-even alternations and shell closing effects are observed. In a first approximation, we find that Li_nH clusters behave like Li_n–1 and Li_nH₂ like Li_n–2. The results may be interpreted by assuming that the bonding of one hydrogen localizes one electron and that the other electrons remain delocalized.     

Atomic bonding of neutral and cationic Mn clusters

By using the spin-polarized DV-X-LCAO method, electronic states of neutral and cationic Mn _N clusters (N=25) are calculated to study atomic bonding of Mn clusters. For the neutral Mn₂ cluster, the equilibrium interatomic distance is much larger than that of the bulk crystal. The 3d-derived states are nearly half-filled, and the 4s-derived states are almost fully occupied,i.e. the electronic configuration is close to that of the isolated atom. These indicate that the Mn₂ cluster is bound by the van der Waals force. The same situation is true for the larger neutral clusters while the equilibrium interatomic distance becomes smaller and thes-d mixing becomes larger. For the cationic clusters, the behaviors tend to become metallic. The equilibrium interatomic distances are smaller and thes-d mixings are larger than those of the corresponding neutral clusters. However, the Mn ₂ ⁺ and Mn ₄ ⁺ clusters still remain the van der Waals characters. Contrary to these clusters, the Mn ₅ ⁺ cluster, whose interatomic distance is smaller than that of the bulk crystal, shows strong metallic bonding. These results seem to correspond to the magic number observed on the mass spectroscopy of cationic Mn clusters.     

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.     

Solvent effects in thermal electron attachment to clusters containing molecular oxygen

Thermal electron attachment to oxygen is strongly modified when this molecule is bound in heterogenous van der Waals clusters. With the help of a laser-excited Rydberg electron source, we investigate the influence upon electron capture of the solvent nature by comparing O₂(H₂O)_N and O₂(C₆H₆)_N attachment rate constants, and we observe size effects down to nearly zero energy.     

Supported Pdn (n=1?20) cluster reactivity : CO chemisorption

The Pd_n 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 -Alumina. We show that the reactivity and the kinetics of the CO-Pd_n association depends on the cluster size.     

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.     

High-resolution electron microscopy and electron energy-loss spectroscopy of giant palladium clusters

Combined structural and chemical characterization of cationic polynuclear palladium coordination compounds Pd₅₆₁L₆₀(OAc)₁₈₀, where L=1,10-phenantroline or 2,2-bipyridine has been carried out by high-resolution electron microscopy (HREM) and analytical electron microscopy methods including electron energy-loss spectroscopy (EELS), zero-loss electron spectroscopic imaging, and energy-dispersive X-ray spectroscopy (EDX). The cell structure of the cluster matter with almost completely uniform metal core size distributions centered around 2.3 ±0.5 nm was observed. Zero-loss energy filtering allowed to improve the image contrast and resolution. HREM images showed that most of the palladium clusters had a cubo-octahedral shape. Some of them had a distorted icosahedron structure exhibiting multiple twinning. The selected-area electron diffraction patterns confirmed the face centered cubic structure with lattice parameter close to that of metallic palladium. The energy-loss spectra of the populations of clusters contained several bands, which could be assigned to the delayed Pd M_{4, 5}-edge at 362 eV, the Pd M₃-edge at 533 eV and the Pd M₂-edge at 561 eV, the NK-edge at about 400 eV, the O K-edge at 532 eV overlapping with the Pd M₃-edge and the carbon C K-edge at 284 eV. Background subtraction was applied to reveal the exact positions and fine structure of low intensity elemental peaks. EELS evaluations have been confirmed by EDX. The recorded series of the Pd M-edges and the N K-edge in the spectra of the giant palladium clusters obviously were related to Pd-Pd- and Pd-ligand bonding.V. Oleshko is on leave from the N.N. Semenov Institute of Chemical Physics of the Russian Academy of Sciences, 117421 Moscow, Russia