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.     

Vacuum-UV photoemission-photoion coincidence spectroscopy of neutral metal atom clusters

A newly developed photoion-photoelectron Vacuum-UV coincidence spectrometer has been coupled to a supersonic metal atom cluster beam source and has been used to investigate the electronic structure of isolated mercury clusters in the size range from 1 to 110 at several selected discrete excitation energies between 11.3 and 7.1 eV. Excitation of the van der Waals cluster Hg₁₀ at the center of the strongD _3/2-autoionization line at 10.7 eV yields a photoelectron kinetic energy distribution between 0 and 2.5 eV indicating the population of Hg₁₀ ionic states, which are also accessible by threshold ionization.     

Radiative neutralization of small clusters impinging on solid surfaces

Light emission during the interaction of slow singly-charged clusters with solid surfaces is studied theoretically. More precisely, we consider positive ions of Ag _n clusters (n = 1...5) impinging on silver surfaces. In such systems, the charge transfer process involved during the cluster-surface interaction is mainly resonant capture. However, photon emission due to radiative capture is also a possible charge transfer channel. Our simple theoretical model including both processes allows us to calculate the light spectra and the total photon yield for the different cluster sizes, n. Our results show that light emission strongly depends on the electronic level dynamics of the clusters in front of the surface, providing a tool for electronic structure analysis of atoms, clusters and solid surfaces.     

The electronic structures of vanadate salts: Cation substitution as a tool for band gap manipulation

The electronic structures of six ternary metal oxides containing isolated vanadate ions, Ba₃(VO₄)₂, Pb₃(VO₄)₂, YVO₄, BiVO₄, CeVO₄ and Ag₃VO₄ were studied using diffuse reflectance spectroscopy and electronic structure calculations. While the electronic structure near the Fermi level originates largely from the molecular orbitals of the vanadate ion, both experiment and theory show that the cation can strongly influence these electronic states. The observation that Ba₃(VO₄)₂ and YVO₄ have similar band gaps, both 3.8 eV, shows that cations with a noble gas configuration have little impact on the electronic structure. Band structure calculations support this hypothesis. In Pb₃(VO₄)₂ and BiVO₄ the band gap is reduced by 0.9-1.0 eV through interactions of (a) the filled cation 6s orbitals with nonbonding O 2p states at the top of the valence band, and (b) overlap of empty 6p orbitals with antibonding V 3d-O 2p states at the bottom of the conduction band. In Ag₃VO₄ mixing between filled Ag 4d and O 2p states destabilizes states at the top of the valence band leading to a large decrease in the band gap (E_g=2.2 eV). In CeVO₄ excitations from partially filled 4f orbitals into the conduction band lower the effective band gap to 1.8 eV. In the Ce_1−xBi_xVO₄ (0≤x≤0.5) and Ce_1−xY_xVO₄ (x=0.1, 0.2) solid solutions the band gap narrows slightly when Bi³⁺ or Y³⁺ are introduced. The nonlinear response of the band gap to changes in composition is a result of the localized nature of the Ce 4f orbitals.     

Bond length alternation and energy gap in (CH)x. Application of the intermediate exciton formalism

Previous ab initio LCAO Hartree-Fock (HF) self-consistent field (SCF) band structure calculations predicted the alternating all trans (CH)_x (polyacetylene or polyene) to be energetically more stable than the equidistant metallic chain, however, the gap of those calculations was too large (≈7 eV). The present discussion based on semi-empirical PPP hamiltonian explains how electronic correlation neglected in the HF theory reduces this gap to ≈4 eV, while the excitation energy for the creation of excitons is estimated to be below 2 eV. The formalism for this estimation is an intermediate-binding exciton theory with explicit use of numerical Wannier functions. It is concluded that the gap is not due to a spin-density-wave state of the equidistant chain, as expected by Ovchinnikov .     

Effect of hydrogen chemisorption on the photoionization threshold of isolated transition metal clusters

Large increases in the photoionization threshold energies of small V_x, NB_x, and Fe_x clusters (x = 3–25) induced by chemisorption of H₂ have been observed using photoionization time-of-flight mass spectrometry of a molecular beam. These shifts exhibit a definite dependence both on the number of atoms constituting the bare metal cluster and on the number of chemisorbed hydrogens, and are particularly large (≳ 0.8 eV) for multiple-H₂ chemisorption on small clusters. A simple frontier orbital model for the chemisorption process predicts the direction of adsorbate-induced shifts in cluster ionization threshold for both H₂ and NH₃ as adsorbates.     

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.     

First-principles calculations of Cd-doped ZnO thin films deposited by pulse laser deposition

Zn_1−xCd_xO thin films are deposited on quartz substrate by pulse laser deposition. Their band structure and optical properties are experimentally and theoretically investigated. By varying Cd concentration, the band gap of Zn_1−xCd_xO films can be adjusted in a wide range from 3.219 eV for ZnO to 2.197 eV for Zn_0.5Cd_0.5O, which produces different emissions from ultraviolet to Kelly light in their photoluminescence spectra. Simultaneity, the electronic structure and band gap of Zn_1−xCd_xO are investigated by the density functional theory (DFT) with a combined generalized gradient approximation (GGA) plus Hubbard U approach, which precisely predicts the band-gaps of ZnO and Zn_1−xCd_xO alloys. Both the experimental results and theoretical simulation reveal that with increasing Cd concentration in Zn_1−xCd_xO alloys, their absorption coefficients in visible light range are evidently enhanced. The adjustable photoluminescence emission and enhanced visible light absorption endow Zn_1−xCd_xO alloys potential applications in optoelectronic and photocatalytic fields.     

Insights on the origin of the structural phase transition in BaV10O15 from electronic structure calculations and the effect of Ti-doping on its structure and electrical transport properties

Band structure calculations at the level of LMTO-ASA provide insight into the electronic structure of BaV₁₀O₁₅ and the origin of the structural phase transition. A crystal orbital Hamiltonian population/integrated crystal orbital Hamiltonian population analysis provides evidence that the crystallographic phase transition is driven by V-V bond formation. As well, the energy bands near the Fermi level are very narrow, <1 eV, consistent with the fact that the observed insulating behavior can be due to electron localization via either Mott-Hubbard correlation and/or Anderson disorder. The partial solid solution, BaV_10−xTi_xO₁₅, was examined to study the effect of Ti-doping at the V sites on the structure and electronic transport properties. In spite of the non-existence of “BaTi₁₀O₁₅”, the limiting x=8, as indicated by a monotonic increase in the cell volume and systematic changes in properties. This limit may be due to the difficulty of stabilizing Ti²⁺ in this structure. For x=0.5 both the first order structural phase transition and the magnetic transition at 40 K are quenched. The samples obey the Curie-Weiss law to x=3 with nearly spin only effective moments along with θ values which range from −1090 K (x=0.5) to −1629 K (x=3). For x>3 a very large, ∼2×10⁻³ emu/mol, temperature independent (TIP) contribution dominates. Conductivity measurements on sintered, polycrystalline samples show semiconducting behavior for all compositions. Activation energies for Mott hopping derived from high temperature data range from ∼0.1 eV for x=0-1 and fall to a plateau of 0.06 eV for x=3-7. Low temperature data for x=3, 5 and 7 show evidence for Mott variable range hoping (VRH) with a T^1/4 law and in one case between 5 and 17 K, a Efros-Shklovskii correlated hopping, T^1/2 law, was seen, in sharp contrast to BaV₁₀O₁₅ where only the E-S law was observed up to 75 K. Seebeck coefficients are small (<35 μV/K), positive, roughly TIP and increase with increasing x up to x=5. This may point to a Heikes hopping of holes but a simple single carrier model is impossible. The compositions for x>3 are remarkable in that local moment behavior is lost, yet a metallic state is not reached. The failure of this system to be driven metallic even at such high doping levels is not fully understood but it seems clear that disorder induced carrier localization plays a major role.     

Fragmentation dynamics of ammonia cluster ions after single photon ionisation

A reflecting time of flight mass spectrometer (RETOF) is used to study unimolecular and collision induced fragmentation of ammonia cluster ions. Synchrotron radiation from the BESSY electron storage ring is used in a range of photon energies from 9.08 up to 17.7 eV for single photon ionisation of neutral clusters in a supersonic beam. The threshold photoelectron photoion coincidence technique (TPEPICO) is used to define the energy initially deposited into the cluster ions. Metastable unimolecular decay (µs range) is studied using the RETOF's capacity for energy analysis. Under collision free conditions the by far most prominent metastable process is the evaporation of one neutral NH₃ monomer from protonated clusters (NH₃) _{n ? 2}NH ₄ ⁺ . Abundance of homogeneous vs. protonated cluster ions and of metastable fragments are reported as a function of photon energy and cluster size up ton=10.     

Threshold photoionization behaviour of (Li2O)Lin clusters produced by a laser vaporization source

We report on the production of small and medium size lithium and lithium oxide clusters by a laser vaporization cluster source. The isotopomeric distribution of natural lithium allowed to identify Li_kO clusters as the most abundant components in the mass spectrum. Photoionization efficiency curves of Li_kO clusters with photon energies from 3.4 to 4.7 eV were measured for 8 ≤ k ≤ 27. Using linear extrapolation of the increase in photoionization efficiency with photon energy, ionization potentials were extracted. With the chemical bond of the O^2- anion to two Li atoms, leaving n = k-2 valence electrons in the (Li₂O)Li_n clusters, clear shell closure effects are present at n = 8 and n = 20.     

Electron impact and single photon ionization cross sections of neutral silver clusters

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.     

Cation Distribution of the Transparent Conductor and Spinel Oxide Solution Cd1+xIn2−xSnxO4

The cation distribution in the transparent conducting oxide Cd_1+xIn_2−xSn_xO₄ was investigated to determine if there is a correlation between structure and electronic properties. Combined Rietveld refinements of neutron and X-ray diffraction data and ¹¹⁹Sn Mössbauer spectroscopic analysis were used to show that the cation distribution changed with x(0≤x≤0.7) from a primarily normal spinel (x=0) to an increasingly random spinel. CdIn₂O₄ quenched from 1175°C has an inversion parameter of 0.31 (i.e., (Cd_0.69In_0.31)^tet(In_1.69Cd_0.31)^octO₄). The inversion parameter decreases to 0.27 as the quench temperature is lowered from 1175°C to 1000°C. The decrease in inversion parameter with temperature correlates with an increase in optical gap from 3.0 eV to 3.3 eV for specimens prepared at 1175°C and 800°C, respectively. We show that this is a consequence of an increase in the fundamental band gap.     

Electronic and Structural Properties of Neutral, Anionic, and Cationic Rh x Cu4−x (x = 0–4) Small Clusters: A DFT Study

In this study, electronic structure, stability, and tendency to exchange electron of neutral, anionic, and cationic Rh _x Cu_4?x (x = 0–4) small clusters were investigated by density functional theory calculations. For neutral small clusters, it was found that the most stable structures of Rh₄, Rh₃Cu and Rh₂Cu₂ have distorted tetrahedral shape while the most stable structures of RhCu₃ and Cu₄ have quasi-planer shape. Adding charges to the clusters, caused shapes of the most stable structures undergo variations. Stabilities of the neutral, anionic, and cationic clusters decrease linearly with increasing the copper content. In addition, calculated chemical harnesses indicated that the small cluster with 75 % copper content has the least chemical hardness. Interestingly, prevailing number of electronegative (Rh) and electropositive (Cu) atoms in the anionic and cationic clusters coincides with high dipole moment in these species that occur at 25 and 75 % copper respectively.     

Fragmentation of doubly charged argon clusters

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.     

Dicarboxylate assisted synthesis of the monoclinic heterometallic tetrathiocyanato bridged copper(II) and mercury(II) coordination polymer {Cu[Hg(SCN)4]}n: Synthesis, structural, vibration, luminescence, EPR studies and DFT calculations

The synthesis of the monoclinic polymorph of {Cu[Hg(SCN)₄]}_n is reported. The compound, as determined by X-ray diffraction of a twinned crystal, consists of mercury and copper atoms linked by μ_1,3-SCN bridges. The crystal packing shows a highly porous infinite 3D structure. Diagnostic resonances for the SCN^- ligand and metal-ligand bonds in the IR, far-IR and Raman spectra are assigned and discussed. The electronic band structure along with density of states (DOS) calculated by the DFT method indicates that the compound is an indirect band gap semiconductor. The DFT calculations show that the observed luminescence of the compound arises mainly from an excited LLCT state with small MLCT contributions (from the copper to unoccupied π^? orbital of the thiocyanate groups). The X-band EPR spectrum of the powdered sample at room temperature reveals an axial signal with anisotropic g factors consistent with the unpaired electron of Cu(II) ion in the d_x²−y² orbital.     

Small angle neutron scattering experiments on solid electrolyte (AgI) x (AgPO3)1−x

Superionic conductor glasses have generated considerable technological interest in the applications such as batteries, fuel cell, sensors, etc. In AgPO₃ glass doped by AgI, (AgI) _x (AgPO₃)_1?x , small size of AgI clusters were formed and dispersed in the AgPO₃ glass. The size of clusters in the sample depends on the AgI content and influences the electrical properties of the sample. To understand the microscopic structure, in particular the shape and size of the clusters, a series of small angle neutron scattering experiment on (AgI) _x (AgPO₃) _x with x?=?0.0, 0.5, and 0.7 were performed at the Neutron Scattering Laboratory–National Nuclear Energy Agency, Indonesia. By assuming the clusters are spherical, a radius of gyration R _g of the clusters was determined from a Guinier plot. As there are different clusters sizes in the samples, a polydisperse model is also used to analyze the data. The results show that the average radius of clusters dispersed in (AgI) _x (AgPO₃)_1?x samples with x?=?0.0, 0.5, and 0.7 are around 236.2, 252.6, and 257.5 Å, respectively.     

Electronic structure and related properties for quasi-binary (GaP)1−x (ZnSe) x crystals

The results of pseudopotential calculations of the band structure and related electronic and optical properties of quasi-binary (GaP)_1?x (ZnSe) _x crystals in the zinc blende structure are presented. Trends in bonding and ionicity are discussed in terms of electronic charge densities. Moreover, the composition dependence of the refractive index and dielectric constants are reported. The computed values are in reasonable agreement with experimental data. The results suggest that for a proper choice of the composition x, (GaP)_1?x (ZnSe) _x could provide more diverse opportunities to achieve the desired electronic and optical properties of the crystals which would improve the performances of devices fabricated on them.     

Electrochemical studies of homocysteine and homocystine at mercury electrodes

The behaviour of homocysteine and cysteine at mercury electrodes is compared. The one-electron oxidation associated with thiols is shown to be the same for both compounds in acidic phosphate buffer, giving rise to an adsorbed thiol—mercury complex, (RS)₂Hg, at the electrode surface. Formation of this complex is utilized in the cathodic stripping voltammetric determination of homocysteine; the detection limit is 10^?9 M after a deposition time of 90 s at a hanging mercury drop electrode. The similar E_{$\text{1}\text{2}$} values for homocysteine and cysteine mean that prior separation is needed for their individual determination. Amperometric detection with a mercury-coated goal electrode after separation by cation-exchange liquid chromatography provides a method for the simultaneous determination of both compounds. Reduction of homocystine at the mercury electrode is also compared to that of cystine. The more negative reduction potential, and the maximum observed for homocystine on d.c. polarograms, which is not seen for cystine, is attributable to different reaction kinetics at the mercury electrode; the products of both the 2-electron reductions are the corresponding thiol-containing amino acids.     

Crystallochemistry of mercury chalcogenides. IV. Crystalline structure of two polymorphous modifications of the mercury sulfochalcogenide Hg3S2Br1.5Cl0.5

Two modifications of a new mercury sulfohalide of Hg₃S₂Br_2−x Cl_x (x = 0.5) composition have been grown from the gas phase and explored by X-ray structural analysis. The compounds were obtained at an attempt to synthesize an analogue of the rare mineral arzakite Hg₃S₂(Br, Cl)₂ (Br > Cl). The refinement of the crystalline structures of monoclinic (I) and cubic (II) phases (I: a = 17.824(4) Å, b = 9.238(2) Å, c = 10.269(2) Å, β = 115.69(1)°, V = 1523.8(5) ų, space group C2/m, Z = 8, R = 0.0513; II: a = 18.248(2) Å, V = 6076.4(12) ų, space group Pm3ˉn, Z = 32, R = 0.038) has shown that they are polymorphous modifications of the compound of Hg₃S₂Br_1.5Cl_0.5 formula. The monoclinic modification I is isostractural to the synthetic compound α-Hg₃S₂Br₂. Modification II is isostructural to synthetic β-Hg₃S₂Cl₂. In both structures, each atom S has in its surrounding three atoms of Hg forming umbrella-type groups SHg₃ with spaces Hg—S 2.366–2.430 Å and angles HgSHg 95.66–97.60°. SHg₃-fragments are bound by Hg-apices with the formation of isolated cubic groups [Hg₁₂S₈]. Like that in other structures of mercury chalcohalides, the main role in structure-forming of the investigated compounds is played by atoms of halogens creating a cubic sublattice in which radicals Hg—S are arranged. Original Russian Text Copyright ? 2006 by N. V. Pervukhina, S. A. Magarill, D. Yu. Naumov, S. V. Borisov, V. I. Vasil’yev, and B. G. Nenashev __________ Translated from Zhurnal Strukturnoi Khimii, Vol. 47, No. 2, pp. 318–323, March–April, 2006.