SiC bonding in ( C 60 ) n Si m clusters

This paper deals with a new type of SiC bonding where silicon atom seems to bridge C₆₀ molecules. We have studied films obtained by deposition of (C₆₀)_nSi_m clusters prepared in a laser vaporization source. Prior deposition, free ionized clusters were studied in a time-of-flight mass spectrometer. Mixed clusters (C₆₀)_nSi_m were clearly observed. Abundance and photofragmentation mass spectroscopies revealed the relatively high stability of the (C₆₀)_nSi _n ⁺ , (C₆₀)_nSi _{n - 1} ⁺ and (C₆₀)_nSi _{n - 2} ⁺ species. This observation is in favor of the arrangement of these complexes as polymers where the C₆₀ cages may be bridged by a silicon atom. Free neutral clusters are then deposited onto substrate making up a nanogranular thin film (≃ 100 nm). The film is probed by Auger and X-ray photoemission spectroscopies, but above all by surface enhanced Raman scattering. The results suggest an unusual chemical bonding between silicon and carbon and the environment of the silicon atom is expected to be totally different from the sp³ lattice: ten or twelve carbon neighbors might surround silicon atom. The bonding is discussed to the light of the so-called fullerene polymerization as observed for pure fullerite upon laser irradiation. This opens a new route for bridging C₆₀ molecules together with an appreciable energy bonding, since the usual van der Waals bonding in fullerite could be replaced by an ionocovalent bond. Such an assumption must be checked in the future by XAS and EXAFS experiments. Received 15 November 2000     

Structures and stability of Al<Subscript><Emphasis Type="Bold">7</Emphasis></Subscript>C and Al<Subscript><Emphasis Type="Bold">7</Emphasis></Subscript>N clusters

We report results of the atomic and electronic structures of Al₇C cluster using ab initio molecular dynamics with ultrasoft pseudopotentials and generalized gradient approximation. The lowest energy structure is found to be the one in which carbon atom occupies an interstitial position in Al₇ cluster. The electronic structure shows that the recent observation [Chem. Phys. Lett. 316, 31 (2000)] of magic behavior of Al₇C^- cluster is due to a large highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) gap which makes Al₇C^- chemically inert. These results have further led us to the finding of a new neutral magic cluster Al₇N which has the same number of valence electrons as in Al₇C^- and a large HOMO-LUMO gap of 1.99 eV. Further, calculations have been carried out on (Al₇N)₂ to study interaction between magic clusters. Received 28 July 2001     

Ultraviolet photoelectron spectroscopy of Si 4 - to Si 1000 -

Using a new experimental setup we have measured UV (h ν = 6.4 eV) photoelectron spectra of cold silicon cluster anions Si _n ^- in a very broad size range. For sizes up to n = 46 the spectra exhibit rich structures. For larger sizes only smooth spectra have been obtained. No trace of a bandgap has been found even for clusters with more than 1000 atoms. Received 30 November 2000     

Theoretical study of small silicon clusters on a graphite layer

In a series of recent experiments, the HOMO-LUMO energy gaps of small Si clusters deposited on a graphite substrate have been determined by Scanning Tunneling Microscopy (STM). The values obtained were found to be substantially smaller than the energy gaps of corresponding passivated clusters. This work considers dimensional reduction as a possible mechanism for a sizeable energy gap narrowing by the example of the system Si₅. The impact of the graphite substrate on the deposited species is investigated in the framework of a pseudocluster model. Received 30 November 2000     

Size and concentration effects in the optical properties of alloyed ( Au x Ag 1 - x ) n and core-shell ( Ni x Ag 1 - x ) n embedded clusters

Optical properties of mixed clusters (Au_xAg _{1 - x} ) _n and (Ni_xAg _{1 - x} ) _n , produced by laser vaporization and embedded in an alumina matrix, are reported. The size effects are investigated for different concentrations (x = 0.25, 0.5 and 0.75) in the diameter range 2-4 nm. For alloyed clusters (Au_xAg _{1 - x} ) _n of a given size an almost linear evolution of the surface plasmon frequency ω _s with the concentration is observed (between those of pure gold and pure silver clusters). Moreover the blue-shift and the damping of the resonance with decreasing size is all the more important as the gold concentration in the particles increases. Such results are in agreement with theoretical calculations carried out in the frame of the time-dependent local-density-approximation (TDLDA) including an inner skin of ineffective screening and the porosity of the matrix. The optical response of (Ni_xAg _{1 - x} ) _n clusters exhibits a surface plasmon resonance in the same spectral range as the one observed for pure silver clusters, but considerably damped and broadened. For a given mean cluster size 3.0 nm, a blue-shift of the resonance is observed when increasing the nickel concentration (between x = 0.25 and x = 0.75). The results are in good qualitative agreement with classical predictions in the dipolar approximation, assuming a core-shell geometry. Received 21 November 2000     

From anisotropic dots to smooth RFe 2 (110) single crystal layers (R = rare earth)

Single crystal RFe₂(110) films were grown by molecular beam epitaxy to a total thickness of 1000 ? at different substrate temperatures ranging from 450 ^° C to 660 ^° C. The first stages of growth and the surface morphology of the deposited layers have been studied using Reflection High Energy Electron Diffraction (RHEED) and Atomic Force Microscopy (AFM). The growth is first strained but further deposit induces the formation of three-dimensional fully relaxed islands. Subsequently, the morphology of the RFe₂(110) nanosystems evolves from anisotropic dots to a smooth surface, as a function of the preparation parameters, i.e. nominal thickness and substrate temperature. It also depends on the rare earth involved in the compound. Received 29 June 2000     

Solvation and chemical reaction of sodium in water clusters

Na_m(H₂O)_n Clusters ( n = 1...200, m = 1...50) are formed in a recently build pick-up arrangement. Preformed water clusters traverse a sodium oven, where sodium atoms are picked up. At low sodium vapour pressure ( < 1×10^-4 mbar) pure Na(H₂O)_n clusters are observed in the mass spectra. At high sodium vapour pressure ( > 1×10^-3 mbar) the water cluster pick up more than 50 Na atoms and reaction products Na(NaOH)_n ( n = 2, 4...50) dominate the mass spectra. The even number of NaOH units in the products indicate that also in a finite cluster the reaction occurs in pairs as in the macroscopic reaction. Received 4 December 2000     

Coalescence process of monodispersed Co cluster assemblies

Cluster-cluster coalescence process of monodispersed Co clusters with mean diameter d = 8.5 and 13 nm deposited a plasma-gas-condensation-type cluster beam deposition system was investigated by in situ electrical conductivity measurements and ex situ scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and analyzed by percolation concept. The electrical conductivity measurement and TEM observation indicated that, below temperature T≈ 100^°C, the Co clusters in the assemblies maintain their original structure as deposited at room temperature, while that the inter-cluster coalescence takes place at T > 100^°C, although the size distribution and the interface morphology of the clusters showed no marked change at substrate temperatures T _s≤200^°C. Received 29 November 2000     

Observation of multiply charged silver-cluster anions

Singly charged silver-cluster anions are produced in a laser vaporization source and transferred into a Penning trap. After size selection the clusters are subjected to an electron bath in the trap, which results in the attachment of further electrons. The relative abundance of dianions or trianions as a function of the clusters' size is analyzed by time-of-flight mass spectrometry. Silver-cluster dianions are observed for sizes n≥ 24 and trianions for n > 100. In addition, a detailed study of the cluster sizes 24 ?n? 60 shows a pronounced resistance to electron attachment for singly charged anions Ag_n ^- with a closed electronic shell, in particular Ag₂₉ ^-, Ag₃₃ ^-, and Ag₃₉ ^-. Both the threshold size for the observation of dianionic silver clusters and the shell effects in the production yield correlate favorably with previous theoretical investigations of the respective electron affinities. Received 24 November 2000     

Collision-induced fragmentation and neutralization of methanol cluster cations

This contribution addresses the inelastic interaction of positively charged molecular cluster ions with a solid surface at kinetic energies up to 30 eV/molecule. We report experimental results on the scattering of mass-selected, protonated methanol cluster cations (CH₃OH)_nH⁺, n = 4-32, off a diamond-coated silicon surface. In particular we provide fragment size distributions of methanol cluster ions following their impact on the target, as well as surface-induced neutralization probabilities of methanol cluster ions as a function of the size and the kinetic energy of the parent clusters. Received 30 November 2000     

Relative stability of Sin and SinSc- clusters in the range n = 14–18

We present a first-principles pseudopotential optimization of the lower energy equilibrium structure of Si_nSc^- anions for n=14-18. We find that Si₁₆Sc^- is more stable than its neighbors clusters, in agreement with recent experimental mass spectra. We also optimize the geometry of pure Si_n neutral clusters in the range n=14-18, and compare our results with those from previous first-principles calculations.     

Magnetization curves of DyN clusters

Magnetic properties of isolated Dy_N clusters are studied in a molecular beam performing Stern-Gerlach experiments. The magnetizations μ _z of Dy_N are measured in dependence of the magnetic field strength B = 0-1.6 T and at nozzle temperatures T _n = 18 K and T _n = 300 K. At room temperature the magnetization augments linear with the field following a simple paramagnetic model. At T _n = 18 K the magnetization curves saturate at field strengths B ≥0.8 T. To explain the magnetization process at low temperatures two models are discussed: A model for adiabatic magnetization based on cluster rotation effects and a modified van-Vleck model. Received 30 November 2000     

Interaction between aromatic amine cations and nonpolar solvents: Infrared spectra of isomeric aniline -Ar n ( ) complexes

Infrared (IR) photodissociation spectra of the aniline⁺-Ar_n cations, An ⁺ - Ar _n (n = 1, 2), are analyzed in the vicinity of the N-H stretch fundamentals. The complexes are produced in an electron impact (EI) ion source which produces predominantly the most stable cluster isomers. Two isomers of An⁺-Ar are identified by their characteristic N-H stretch frequencies: the planar proton-bound global minimum, in which the Ar ligand forms a nearly linear H-bond to the amino group, and the less stable π-bound local minimum, in which the Ar atom is attached to the π-electron system of the aromatic ring. This result is the first unambiguous detection of the most stable H-bound An⁺-Ar dimer. All previous spectroscopic studies of An⁺-Ar employed resonance enhanced multiphoton ionization (REMPI) of neutral An-Ar and identified only the less stable π-bound cation due to restrictions arising from the Franck-Condon principle. The EI-IR spectrum of An⁺-Ar₂ shows that the most stable structure of this trimer features two equivalent H-bonds (C_2v symmetry). The interpretation of the experimental data is supported by quantum chemical calculations. The ab initio potential of An⁺-Ar calculated at the UMP2/6-311G(2df, 2pd) level features H-bound global minima ( D _e = 513 cm^-1) and π-bound local minima ( D _e = 454 cm^-1), with a barrier of V _b ≈ 140 cm^-1 for isomerization from the π-bound toward the H-bound minimum. Received 4 February 2002 Published online 13 September 2002     

Tight-binding study of structural and electronic properties of silver clusters

Tight-binding model is developed to study the structural and electronic properties of silver clusters. The ground state structures of Ag clusters up to 21 atoms are optimized by molecular dynamics-based genetic algorithm. The results on small Ag_n clusters (n = 3-9) are comparable to ab initio calculations. The size dependence of electronic properties such as density of states, s-d band separation, HOMO-LUMO gap, and ionization potentials are discussed. Magic number behavior at Ag₂, Ag₈, Ag₁₄, Ag₁₈, Ag₂₀ is obtained, in agreement with the prediction of electronic ellipsoid shell model. We suggest that both the electronic and geometrical effect play significant role in the coinage metal clusters. Received 7 August 2000     

Ultrafast dynamics in the excited hydrogen atom transfer states of ammonia clusters

Neutral ammonia clusters (NH₃)_m are photo-excited to the electronic state by a deep UV femtosecond laser pump pulse. Within a few hundred femtoseconds a significant fraction of the clusters rearrange to form an H-transfer state (NH₃)_m-2NH₄(3s)NH₂ with the subunit NH₄ in its 3s electronic ground state. This state is then electronically excited by a time-delayed infrared control pulse of variable wavelength. Finally, a third (probe) pulse in the UV ionizes the clusters for detection. The lifetime of the excited (NH₃)_m-2NH₄(3p)NH₂ states is found to vary between 2.7 and 0.13 ps depending on cluster size and excitation energy. It increases drastically upon deuteration. The corresponding cluster size-dependent photoelectron spectra allow us to disentangle the underlying energetics of the excitation and ionization process and reveal additional processes, such as nonresonant ionization or dissociative ionization. The experimental findings suggest that the excited H-transfer ammonia complexes with m > 2 are deactivated by an internal conversion process back to the electronically lowest H-transfer state followed by fast dissociation. Received 22 September 2001 and Received in final form 31 January 2002     

Photoexcitation of rare-gas neon and argon clusters doped with <Emphasis Type="Bold">H</Emphasis><Subscript><Emphasis Type="Bold">2</Emphasis></Subscript><Emphasis Type="Bold">O</Emphasis>

We have studied the fluorescence of electronically excited OH^*, H^* and H₂O^+* dissociation fragments after VUV excitation ( h ν≥11.6 eV) of rare-gas clusters (Rg = Ne, Ar) doped with H₂O molecules. In contrast to a free molecule, where Balmer H-series dominate the UV-visible spectra, only the OH ^* ( A ² Σ ⁺ ↦ X ² Π) emission band is observed in neon clusters. No emission of excited water ions has been observed. We find that while higher excitation energies (Ne vs. Ar) induce higher vibrational excitation of the OH^* ( A ) fragment, the rotational temperature is lower. This effect is attributed to the difference in the geometric position of the H₂O molecule on the surface or inside the Rg-cluster. The rotational relaxation in neon clusters is rapid while the vibrational relaxation is slow because of the coupling with the low energy matrix phonons. Received 7 March 2002 / Received in final form 27 May 2002 Published online 19 July 2002     

Assembling of hydrogenated aluminum clusters

The electronic and atomic structure of Al₁₃H has been studied using Density Functional Theory. Al₁₃H has closed electronic shells. This makes the cluster very stable and suggests that it could be a candidate to form cluster assembled solids. The interaction between two Al₁₃H clusters was analyzed and we found that the two units preserve their identities in the dimer. A cubic-like solid phase assembled from Al₁₃H units was then modeled. In that solid the clusters retain much of their identity. Molecular dynamics runs show that the structure of the assembled solid is stable at least up to 150 K. A favorable relative orientation of the clusters with respect to their neighbors is critical for the stability of that solid. Received 21 November 2000     

Isomerization and dissociation of small (CH <Emphasis Type="Bold">3</Emphasis>CN) <Emphasis Type="Bold">n</Emphasis> molecular clusters: a computational study

The isomerization and evaporation processes in the neutral homogeneous (CH₃CN)_n molecular clusters (n = 2-7) have been investigated using classical molecular dynamics simulations. The evaporation rate constants and the kinetic energy release in the dissociation have been analysed as a function of the cluster size and as a function of the internal energy in the parent cluster. The competition between monomer and dimer ejections has been also carefully studied. All the dynamical properties in these dissociative processes have been discussed in relation to the static properties of the clusters involved in the dissociation and also in relation to the solid-liquid like transition which appears in these homogeneous molecular clusters. Received 19 November 2002 / Received in final form 5 February 2003 Published online 29 April 2003 RID="a" ID="a"e-mail: pascal.parneix@ppm.u-psud.fr RID="b" ID="b"Laboratoire associé à l'université Paris-Sud.     

Metastability of gold-carbonyl cluster complexes, Au N(CO) M -

Smaller gold-cluster anions, typified by Au ₇ ^- , adsorb multiple CO molecules in a high-pressure, room-temperature flow-reactor, tending toward previously unknown saturation compositions, Au₇(CO) ₄ ^- . The weakness of the gold-carbonyl adsorption bond is evidenced indirectly by the high CO partial pressure required and more directly by the high probability of fragmentation in the field-free flight region of the reflectron-type time-of-flight mass spectrometer. The analysis of this metastability reveals that the actual distribution f_N,M of products Au₇(CO) _M ^- in the reactor may be highly non-statistical, e.g. with only even-M species present. Received 17 April 2001     

Ab initio determination of an extended Heisenberg Hamiltonian in CuO 2 layers

Accurate ab initio calculations on embedded Cu₄O₁₂ square clusters, fragments of the La₂CuO₄ lattice, confirm a value of the nearest neighbor antiferromagnetic coupling (J = 124 meV) previously obtained from ab initio calculations on bicentric clusters and in good agreement with experiment. These calculations predict non negligible antiferromagnetic second-neighbor interaction (J' = 6.5 meV) and four-spin cyclic exchange (K = 14 meV), which may affect the thermodynamic and spectroscopic properties of these materials. The dependence of the magnetic coupling on local lattice distortions has also been investigated. Among them the best candidate to induce a spin-phonon effect seems to be the movement of the Cu atoms, changing the Cu-Cu distance, for which the variation of the nearest neighbor magnetic coupling with the Cu-O distance is Δ J /Δ d _{Cu - O} ∼ 1700 cm^-1?^-1. Received 20 November 2000