Calculation of the size dependence of the ionization energy and autoionization energy of Hg-clusters

To study the transition from van der Waals to metallic bonding we calculate the size dependence of the ionization energy and 5d→6p autoionization energy of Hg _n -clusters using a parametrized LCAO model. Our results are in good qualitative agreement with experiment. Comparison with experimental results suggests that electron correlations play an important role for the transition from localized (van der Waals-like) to delocalized (covalent or metallic) electronic states occuring in Hg _n atn?13–19.     

Calculation of the electronic properties of neutral and ionized divalent-metal clusters

The electronic properties of neutral and ionized divalent-metal clusters have been studied using a microscopic theory, which takes into account the interplay between van der Waals (vdW) and covalent bonding in the neutral clusters, and the competition between hole delocalization and polarization energy in the ionized clusters. By calculating the ground-state energies of neutral and ionized Hg _n clusters, we determine the size dependence of the bond character and the ionization potentialI _p (n). For neutral Hg _n clusters we obtain a transition from van der Waals to covalent behaviour at the critical sizen _c ～10–20 atoms. Results forI _p (Hg _n ) withn≤20 are in good agreement with experiments, and suggest that small Hg _n ⁺ clusters can be viewed as consisting of a positive trimer core Hg ₃ ⁺ surrounded byn?3 polarized neutral atoms.     

Electronic properties of divalent-metal clusters

The transition from van der Waals to metallic bonding expected to occur in divalent-metal clusters (e.g., Be _n , Mg _n , Hg _n ) as a function of cluster size is discussed. Theoretical results for several electronic properties reflecting this transition in Hg _n -clusters are briefly reviewed and compared with available experiments. The limitations of the present theory particularly concerning the role of correlations and van der Waals interactions are discussed and possible improvements are suggested.     

Theory for the short-time response of small Hg n clusters after excitation

The short-time behavior of small Hg _n clusters immediately after single or double ionization is studied. We calculate self-consistently the ground state electronic energyE of ionized Hg _n clusters. Upon ionization changes of the potential energy surface (PES) occur, which govern the atomic motion in the cluster. These changes depend on cluster size and charge and are determined by the interplay between the localization of the holes within an ionic core and the polarization energy of the neutral rest of the cluster. In the case of single ionization of the cluster the PES results mainly from hole delocalization. In contrast, in the case of double ionization the PES is governed almost only by strong environment polarization. We use our theory to explain the physical origin of the oscillations in the ionization cross-section of singly and doubly excited Hg _n clusters observed in recent pump-probe experiments.     

Structural stability and energetics of C,Si, and Ge microclusters: empirical many-body potential energy function calculation

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 C_3?4 microclusters are linear withD _∞h symmetry but C_5?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.     

Distribution of interatomic distances in large metallic clusters

Spherically averaged pseudopotential (SAPS) calculations have been done for Mg _n clusters, withn up to 250 within the framework of density functional theory. The electronic structure is computed resorting to the Thomas-Fermi-Dirac-Weizsäcker (TFDW) approximation for the kinetic energy. The equilibrium geometries have been obtained by minimizing the total cluster energy with respect to the atomic positions using the steepest-descent method. The ground state geometries obtained in this way are formed by spherical atomic shells, the number of them increasing with cluster size, up to a number of four for the biggest sizes considered here. An analysis of the distribution of the interatomic distances shows that the more internal is the shell, the more contracted are the interatomic distances. This effect diminishes progressively with increasing cluster size. For the purpose of comparison, similar calculations have been done with Cs _n clusters in the same size range, allowing us to reproduce previous results obtained using a more elaborated density functional technique (Kohn-Sham method). The inhomogeneous contraction of interatomic distances then appears as a general fact for simple metallic clusters and not only for alkaline ones.     

Observation of doubly charged mercury cluster ions Hg n 2+, n=1-10 using secondary ion mass spectrometry

Mass spectra of doubly charged mercury clusters (m/z=30-1065) were investigated by secondary ion mass spectrometry. Positively charged ions were generated from an amalgam of mercury and silver by bombardment with a xenon ion beam and mass analysis by a grand-scale sector type mass spectrometer. Hg _n ²⁺, n=1-10 and Hg _n +, n =1- 5 were observed. Some doubly charged mercury clusters, (Hg _n ²⁺) survived at least for 0.1 ms.     

Modification of the electronic properties of zigzag (n = 5–10) and armchair (n = 3, 5) carbon nanotubes by K atom adsorption

The adsorption of the potassium atom onto the surface of (n,0) zigzag nanotube (n = 5–10) and (n,n) armchair nanotubes (n = 3, 5) has been studied by density functional theory. The local density approximation calculation of adsorption energy (E _ads) emphasized on the dependency of E _ads to the diameter and chirality of the nanotube. E _ads decreases when the diameter increases. So the (5,0)-K system has the highest adsorption energy among all structures. Furthermore, a significant change was observed in the electronic properties of potassium-adsorbed single-walled carbon nanotube (SWCNT) and the metallic behavior of the nanotube improved. Therefore, our results showed that such modified SWCNTs can be applied in nanodevices such as transistors.     

Interaction behavior study of HCl on (ZnS)n (n = 1–12) clusters and HCl effect on Hg0 adsorption

The interaction behavior of HCl and (ZnS)_n (n = 1–12) clusters and HCl effect on Hg⁰ adsorbed by (ZnS)_n have been studied theoretically. The combined genetic algorithm and density functional theory (GA-DFT) method has been used to obtain the structures of (ZnS)_nHCl and (ZnS)_nHgHCl (n = 1–12) clusters. The structural properties of (ZnS)_nHCl and (ZnS)_nHgHCl have been analyzed. The adsorption energies and interaction energies have been calculated. Bond length and bond order analysis has revealed that S H and Zn Cl bonds form after HCl adsorbed on (ZnS)_n clusters, while Hg⁰ can only weakly bind with (ZnS)_nHCl clusters. According to thermodynamic adsorption analysis, the formation of (ZnS)_nHCl clusters from (ZnS)_n and HCl are spontaneous because of their negative Gibbs free energy changes. The formation of (ZnS)_nHgHCl from (ZnS)_nHCl and Hg are nonspontaneous for n = 1–4 and 9, and the Gibbs free energy changes have small negative values for other sizes. Electron localization function and noncovalent interaction (NCI) analysis of (ZnS)₁₀HgHCl manifest that Hg and its nearest Zn form zinc amalgam. Projected density of state study has been performed to obtain the interaction nature of HCl and (ZnS)_n clusters and Hg⁰ adsorption on (ZnS)_nHCl clusters. Based on our study, HCl is chemical adsorbed by (ZnS)_n clusters except (ZnS)₄ cluster. After (ZnS)_n adsorbs HCl, Hg⁰ can physically adsorb on (ZnS)_nHCl clusters. The strength of Hg⁰ on (ZnS)_nHCl is comparable to that of Hg⁰ on (ZnS)_n, indicating that HCl can hardly affect the adsorption of Hg⁰ on ZnS clusters.     

Multiple ionization and Coulomb explosion of mercury clusters in femtosecond laser fields

We report on studies of multiple ionization and fragmentation of free Hg_n (n ≤ 80) clusters in the femtosecond time domain at wavelengths ranging from 255 nm to 800 nm. After excitation by single laser pulses of an intensity of 5 * 10¹¹ W/cm² we observe prompt formation of multiply charged Hg_n clusters. The Hg_n cluster size distribution observed up to n ≈ 80 shows in additon to singly charged also doubly and triply charged clusters with a surprisingly high amount of doubly charged clusters. The measured cluster size distribution is nearly independent of laser wavelengths. For higher laser intensities (2 * 10¹² W/cm²) we observe multiply charged mercury atoms up to Hg⁵⁺. At 10¹³ W/cm² molecules and clusters eventually disappear due to Coulomb explosion and complete Fragmentation. Only atomic ions, singly and multiply charged, with high kinetic energies are then observed.     

On the localization of electrons and holes in Hg n and rare-gas clusters

We have used a microscopic theory to study the size dependence of the degree of localization of the valence electrons and holes in neutral an ionized rare-gas-and Hg _n clusters. We discuss under which circumstances localization of the electrons and holes is favoured. We have calculated the ionization potential of Xe _n , Kr _n and small Hg _n clusters. Good agreement with experiments is obtained. We have also determined the dependence of the ionization potential on cluster structure.     

Investigation of the defect manganese silicide MnnSi2n−m

Low-temperature magnetization studies upon melt-grown single crystals of the defect manganese silicide Mn_nSi_2n?m have shown this material to contain small quantities of plate-like MnSi precipitates. Metallographic and electron microprobe analyses have confirmed this result. The strongly magnetic MnSi precipitates dominate the diamagnetic Mn_nSi_2n?m matrix, and are responsible for the magnetic behavior reported in the literature. MnSi is metallic, and the plate-like metallic precipitates degrade the thermoelectric efficiency of the degenerate semiconductor Mn_nSi_2n?m.     

Laser induced fluorescence (LIF) of Hg2 and Hg3 via dissociation of HgBr2 at 157 nm

Laser induced fluorescence of the mercury clusters Hg₂ and Hg₃ in the spectral range between 300 nm to 510 nm has been obtained from the dissociation of HgBr₂ at 7.88 eV (157.5 nm) with an F₂ molecular laser, together with fluorescence from mercury atomic transitions from highly excited states. The excitation process involves two photon absorption which dissociates the molecule at 15.76 eV total photon energy with the subsequent formation of the metallic clusters.     

Structural and dynamical properties of magnesium microclusters

We present systematic Density Functional Theory-Local Density Approximation computations for neutral Magnesium clusters Mg _n withn≤13. For the smaller sizes the ground state structure is optimized starting from selected symmetries and allowing for relaxation, Jahn-Teller distorsion and spin polarization. For the larger sizes we perform a simulated annealing based on the ab-initio Molecular Dynamics. By the same method, we study the thermal and dynamical properties of Mg₁₀ and Mg₁₆. The general picture emerging from these computations shows that already atn ≈10 these clusters have acquired many characteristic features of metallic Magnesium.     

A REMPI study of solvation of small Hg n clusters by polar molecules

Free Hg _n (DME) _m clusters (where DME=dimethyl-ether,n=1, 2, 3,m=1÷5) formed in a supersonic expansion were studied by the REMPI (Resonance-Enhanced Multi-Photon Ionization) technique. A large decrease of ionization energies due to solvation of Hg _n clusters is observed. Preliminary results are discussed in terms of different equilibrium configurations of the electronic ground, excited and ionic states of clusters.     

Evaporation of tetramers in Sb4n clusters and conditions for the formation of Sb2n+1 clusters

Antimony clusters are produced by the inert gas condensation technique. They are found to be built from Sb₄ units. The fragmentation by evaporation of Sb₄ units is studied as a function of the excess energy in the cluster. By this way the binding energy of the Sb₄ units in the cluster is found to be about 1.5 eV, well below the binding energy of a Sb atom in the bulk and in Sb₄(?3eV). The evolution of ionization potentials of Sb_4n clusters confirms that their structure is probably non metallic. Finally the possible metastable character of this Sb_4n structure is discussed.     

Study of the fragmentation of small sulphur clusters

The fragmentation of sulphur clusters caused by electron impact ionization was studied. For this purpose, a beam ofS _n -clusters withn≦8 was generated in a gas aggregation source and ionized by electrons of variable energy. Special care was taken to maintain constant nucleation conditions so that the neutral cluster composition remained unchanged. It was found that the cluster ion mass spectra drastically depend on the electron energy. Even near threshold fragmentation processes contribute significantly to the dependence of the ion intensities on the electron energy.     

Charge exchange in alkali cluster collisions

Collisional charge exchange between mass selected alkali cluster ions and Cs has been studied and cross sections have been determined for the processes Na _n ⁺ + Cs and K _n ⁺ + Cs, withn=1–21 andn=1–14, respectively. A strong dependence of the cross sections on the energy defect as well as on cluster size and collision energy is found. The results are analysed by a coupled two state density matrix model, taking account of the relaxation of electronic amplitudes due to interaction with the nuclear motion in the cluster.     

Partial photoionization cross sections of large mercury clusters and liquid mercury in the vacuum ultraviolet photon energy region

Photoelectron spectra for neutral mercury clusters (up to a size of 109 atoms) and liquid mercury have been recorded for several different photon energies between 7.1 eV and 10.6 eV. For both large mercury clusters (Hg _x ,x≥60) and liquid mercury a strong increase of the partial photoionization cross sections near threshold with decreasing photon energy is observed. This shows clearly that the local electronic structure of large mercury clusters is very similar to the electronic structure of the metallic bulk material.     

Studies on nessler's reagent and nessler's precipitate part II

The composition of the precipitate formed when the Nessler's reagent is treated with ammonia, is assumed as NH_n-1Hg₂I_n, aq. and has been studied potentiometrically from iodine-hypoiodite-system. It has been observed that the value of n depends on the concentrations of [HgI₄]^-2 and OH^- ions as well as on the amount of ammonia added and it may vary from 1 to 3. The colour of the precipitate deepens from brown to chocolate as the value of n increases.