Formation and Characterization of Large (Ar) n , (N2) n , and Mixed (Ar) n (N2) m van der Waals Clusters Produced by Supersonic Expansion

This paper reports the formation and characterization of large (Ar) _n , (N₂) _n , and mixed binary (Ar) _n (N₂) _m van der Waals clusters produced at room temperature in the process of supersonic expansion. The average cluster size is determined by the buffer gas induced beam-broadening technique. For both Ar and N₂ clusters, power variations of the average cluster size with the gas stagnation pressure P ₀ give size scaling as . The average cluster sizes of argon vary from 2950 to more than 30900 atoms per cluster with the argon gas stagnation pressures ranging from 4 to 14 bars, and of nitrogen vary from 600 to more than 10400 molecules per cluster with the nitrogen gas stagnation pressures ranging from 8 to 38 bars. The mixed binary (Ar) _n (N₂) _m cluster is produced by supersonic expansion of an Ar–N₂ mixture. The large mixed binary (Ar) _n (N₂) _m clusters with the average sizes n + m between 1000 and 16000 are obtained. In coexpansion of Ar–N₂ mixture, we find that the argon concentration becomes higher in the beam than before the expansion. This finding is discussed and may be helpful for further insight into the phenomenon of clustering.     

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.     

CO<Subscript>2</Subscript> Cluster Ion Beam,an Alternative Projectile for Secondary Ion Mass Spectrometry

The emergence of argon-based gas cluster ion beams for SIMS experiments opens new possibilities for molecular depth profiling and 3D chemical imaging. These beams generally leave less surface chemical damage and yield mass spectra with reduced fragmentation compared with smaller cluster projectiles. For nanoscale bioimaging applications, however, limited sensitivity due to low ionization probability and technical challenges of beam focusing remain problematic. The use of gas cluster ion beams based upon systems other than argon offer an opportunity to resolve these difficulties. Here we report on the prospects of employing CO₂ as a simple alternative to argon. Ionization efficiency, chemical damage, sputter rate, and beam focus are investigated on model compounds using a series of CO₂ and Ar cluster projectiles (cluster size 1000–5000) with the same mass. The results show that the two projectiles are very similar in each of these aspects. Computer simulations comparing the impact of Ar₂₀₀₀ and (CO₂)₂₀₀₀ on an organic target also confirm that the CO₂ molecules in the cluster projectile remain intact, acting as a single particle of m/z 44. The imaging resolution employing CO₂ cluster projectiles is improved by more than a factor of two. The advantage of CO₂ versus Ar is also related to the increased stability which, in addition, facilitates the operation of the gas cluster ion beams (GCIB) system at lower backing pressure.

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In-situ HEED structure analysis of AlNx films grown by the simultaneous use of a radical beam source and ICB technique

A reactive ionized cluster beam technique (RICB) which was composed of a conventional ICB source and a radical beam source has been used to deposit stable and metastable polycrystalline AlN_x (0x1) films. Using in-situ high energy electron diffraction (HEED) at grazing incidence geometry, crystallographic properties such as structure, preferred orientation and interplanar dspacing values were determined and the relation to deposition parameters investigated. It could be shown that the simultaneous use of the ICB technique and a radical beam source to separately control the kinetic energy of the Al ions and the dissociation rate of molecular nitrogen, allows AlN films to be deposited with variable composition and crystal structures. In-situ HEED used in the transmission mode is an effective tool to investigate the crystallography of growing compound films such as AlN_x.     

Laser induced fission of C60 with kinetic energy release

A neutral C₆₀ fullerene beam is ionised by 308 nm laser pulses. For each cluster sizeC _n ⁺ , 0n60 of the typical bimodal mass distributions known from the literature [1] velocity distributions have been determined by a time of flight method. A consistent interpretation of the measured mean velocities is obtained when binary fission of the parent molecule is assumed to be responsible for the fragmentation patterns, the total kinetic energy release being 0.45±0.1 eV independent of fragment mass and of laser fluence.     

Reactive scattering of sodium clusters with molecular oxygen

The first reactive differential scattering study for atomic clusters is reported. Oxidation of Na _x (x8) with O₂ is investigated in a crossed beam apparatus. Sodium oxide (Na _n O,n4) and sodium dioxide (Na _n O₂,n6) are produced with a total reactive cross section from 50 to 80 Ų, depending on the cluster size. The excess energies for these reactions are estimated by an SCF type ab initio calculation and range from 0.5 to 5 eV. The large cross section may then be understood quantitatively in terms of a harpooning mechanism as a first step in the reaction path. Angular distributions have been determined for the most abundant products, showing strong forward scattering. Two different schemes are discussed for the reaction: while the dioxides Na _n O₂ may be formed by an evaporative cooling process from a highly excited collision complex, formation of Na _n O appears to originate from a direct process. In both cases the experimental data suggest that most of the exothermicity remains in the reaction products.     

3D ToF-SIMS Analysis of Peptide Incorporation into MALDI Matrix Crystals with Sub-micrometer Resolution

The analytical sensitivity in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is largely affected by the specific analyte-matrix interaction, in particular by the possible incorporation of the analytes into crystalline MALDI matrices. Here we used time-of-flight secondary ion mass spectrometry (ToF-SIMS) to visualize the incorporation of three peptides with different hydrophobicities, bradykinin, Substance P, and vasopressin, into two classic MALDI matrices, 2,5-dihydroxybenzoic acid (DHB) and α-cyano-4-hydroxycinnamic acid (HCCA). For depth profiling, an Ar cluster ion beam was used to gradually sputter through the matrix crystals without causing significant degradation of matrix or biomolecules. A pulsed Bi₃ ion cluster beam was used to image the lateral analyte distribution in the center of the sputter crater. Using this dual beam technique, the 3D distribution of the analytes and spatial segregation effects within the matrix crystals were imaged with sub-μm resolution. The technique could in the future enable matrix-enhanced (ME)-ToF-SIMS imaging of peptides in tissue slices at ultra-high resolution.

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Synthesis and Structure of a Molybdenum–Cobalt Bimetallic Carbide Cluster [N(PPh3)2][Mo3Co3(μ6-C)(μ-CO)3(CO)15] Bearing Only Carbonyl Ligands

The reaction of a trinuclear cobalt cluster [ClCCo₃(CO)₉] with [Mo(CO)₃(CH₃CN)₃] gave a molybdenum–cobalt bimetallic cluster complex [Mo₃Co₃( ₆-C)(-CO)₃(CO)₁₅]^–. The cluster anion has a carbide-centered Mo₃Co₃ octahedral metal core, where the three molybdenum and three cobalt atoms are placed in facial positions. The six metal atoms are coordinated by only carbonyl ligands. The cluster is suitable for a model of heterogeneous desulfurization catalysts.     

Symmetry-adapted cluster f-orbitals and a vector method for generally oriented f-orbital overlaps

The irreducible representations consisting of linear combinations of cluster atomic f-orbitals are obtained for f, f,f and f orbitals in M₃(D _3h ), M₄(D _4h ), M₄(T _d ), M₆(O _h ) and M₈(O _h ) clusters. The charge overlap of any pair of two atoms in a cluster is decomposed in terms of a set of coefficients for the , , , and overlaps, respectively. A vector method is devised for this decomposition which may be extended to any arbitrary orientation and to higher orbitals. The decomposition coefficients represent fundamental geometrical properties of the cluster and are applicable to clusters of arbitrary dimensions.Contribution No. 0004 ITI Basic Research Laboratory. Presented in part at the 187th ACS National Meeting in St. Louis, MO, USA     

Comparison of the Bonding of Benzene and C60 to a Metal Cluster: Ru3(CO)9(μ3-η2,η 2,η2-C6H6) and Ru3(CO)9(μ3-η2,η2,η2-C60)

The electron distributions and bonding in Ru₃(CO)₉( ³- ², ², ²-C₆H₆) and Ru₃(CO)₉( ³- ², ², ²-C₆₀) are examined via electronic structure calculations in order to compare the nature of ligation of benzene and buckminsterfullerene to the common Ru₃(CO)₉ inorganic cluster. A fragment orbital approach, which is aided by the relatively high symmetry that these molecules possess, reveals important features of the electronic structures of these two systems. Reported crystal structures show that both benzene and C₆₀ are geometrically distorted when bound to the metal cluster fragment, and our ab initio calculations indicate that the energies of these distortions are similar. The experimental Ru–C_fullerene bond lengths are shorter than the corresponding Ru–C_benzene distances and the Ru–Ru bond lengths are longer in the fullerene-bound cluster than for the benzene-ligated cluster. Also, the carbonyl stretching frequencies are slightly higher for Ru₃(CO)₉( ³- ², ², ²-C₆₀) than for Ru₃(CO)₉( ³- ², ², ²-C₆H₆). As a whole, these observations suggest that electron density is being pulled away from the metal centers and CO ligands to form stronger Ru–C_fullerene than Ru–C_benzene bonds. Fenske-Hall molecular orbital calculations show that an important interaction is donation of electron density in the metal–metal bonds to empty orbitals of C₆₀ and C₆H₆. Bonds to the metal cluster that result from this interaction are the second highest occupied orbitals of both systems. A larger amount of density is donated to C₆₀ than to C₆H₆, thus accounting for the longer metal–metal bonds in the fullerene-bound cluster. The principal metal–arene bonding modes are the same in both systems, but the more band-like electronic structure of the fullerene (i.e., the greater number density of donor and acceptor orbitals in a given energy region) as compared to C₆H₆ permits a greater degree of electron flow and stronger bonding between the Ru₃(CO)₉ and C₆₀ fragments. Of significance to the reduction chemistry of M₃(CO)₉( ³- ², ², ²-C₆₀) molecules, the HOMO is largely localized on the metal–carbonyl fragment and the LUMO is largely localized on the C₆₀ portion of the molecule. The localized C₆₀ character of the LUMO is consistent with the similarity of the first two reductions of this class of molecules to the first two reductions of free C₆₀. The set of orbitals above the LUMO shows partial delocalization (in an antibonding sense) to the metal fragment, thus accounting for the relative ease of the third reduction of this class of molecules compared to the third reduction of free C₆₀.     

Collision induced dissociation of stored gold cluster ions

The stability of gold cluster ions Au _n ⁺ (2n23) has been investigated via collision induced dissociation in a Penning trap. Threshold energies and dissociation channels have been determined. The cluster stability exhibits a pronounced odd — even alternation: Clusters with an odd number of atoms,n, are more stable than the even-numbered ones. Enhanced stabilities are found for Au ₃ ⁺ , Au ₉ ⁺ , and Au ₁₉ ⁺ in accordance with the Clemenger-Nilsson and the deformed jellium model of delocalized valence electrons. Excited odd cluster ions withn15 predominantly decay by evaporation of dimers; all others decay by monomer evaporation. From the dissociation channels estimates of the binding energies are deduced.This publication comprises part of the thesis of St. Becker     

Products from the trimethylamine N-oxide activation reaction of Ru4(CO)12(μ 4-C6H8) with cyclohexa-1,3-diene

Summary Reaction of Ru₄(CO)₁₂( ₄-C₆H₈) (1) with three equivalents of Me₃NO and cyclohexa-1,3-diene in CH₂Cl₂ (195–295 K) gives one major product, the benzene butterfly cluster Ru₄(CO)₉( ₄-C₆H₈)( ⁶-C₆H₆) (2), in which the benzene is coordinated to a wing-tip Ru atom. Three other products are formed in lower yield, these comprised of an isomer of the aforementioned cluster in which the benzene is attached to a hinge Ru atom (3), the bis(cyclohexadiene) cluster Ru₄(CO)₈( ₄-C₆H₈)( ⁴-C₆H₈)₂ (4), and the trinuclear species Ru₃(CO)₈( ₃-C₆H₈)( ⁴-C₆H₈) (5). The characterization of the new compounds (4) and (5) is described, including the solid-state structure of (4) determined by a single crystal X-ray diffraction analysis.     

Photoionization of Xe2, Xe3 and Xe4 in the 1024–1113 Å region

The mass spectra of Xe _n ⁺ clusters (n=2–13) were recorded using a supersonic beam and an ion time-of-flight mass analyser. The yield of Xe ₂ ⁺ , Xe ₃ ⁺ and Xe ₄ ⁺ cluster ions was measured with a resolution of 0.1 Å (1 meV) in the 1024–1113 Å (11.1–12.1 eV) region. Autoionizing Rydberg series of Xe₂ converging to theC ²_3/2u state of Xe ₂ ⁺ were observed in the spectrum of Xe ₂ ⁺ . The photoionization yield of Xe ₃ ⁺ and Xe ₄ ⁺ ions each displayed similar broad features that contained no fine structure corresponding to vibrational states. The broad features were assigned to autoionizing Rydberg series by analogy with the dimer ion spectrum.     

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     

Studies in heavy ion activation analysis

¹¹B induced radioactivation was used to study the trace determination of light elements with 1Z17. 49 nuclear reactions were investigated with ion beam energies ranging from E_lab=10 MeV to E_lab=27 MeV. Five elements were found to be determinable nondestructively, selectively and sensitively: Li, Be, B, Mg and Si. Nuclear interferences have also been defined and quantified. The technique has been applied to simultaneous Li-B trace determinations in glass samples and to Mg determination in Al₂O₃ ceramic material.     

Alkyne‐Promoted H2 Loss in a Metallaborane: Nido‐to‐Closo Cluster Transformation and sp C?H Bond Oxidative Addition

A sensitive cluster : The labile rhodathiaborane [(PPh₃)₂(H)‐nido‐RhSB₉H₉(NC₅H₅)] combines the redox and coordinative flexibility of the {(PPh₃)₂(H)Rh} fragment with the capability of the 11‐vertex rhodathiaborane cluster to undergo oxidative nido‐to‐closo transformations induced by coordination of alkynes to the metal centre, which leads to hydrogenation of the triple bond, dehydrogenation of the cluster and oxidative addition of sp C? H bonds.

     

Synthesis and structure of complexes formed in the reaction between dicobalt octacarbonyl and tetramethyl-biphosphine disulfide

Co₂(CO)₈ and Me₂P(S)P(S)Me₂ react to form the two cluster complexes: Co₄(CO)₉S(PMe₂)₂) (1) and Co₃(CO)₇S(SPMe₂) (2). The strucure of1 and of the disubstituted triphenyl phosphine derivative of2. Co₃(CO)₅(PPh₃)₂S (SPMe₃) (2a) were determined. Compound1 contains a quasi-planar rhomboidal Co₄ cluster formed by two Co₃ isosceles triangles sharing a Co-Co edge. One triangle is capped by a sulfur atom, the other triangle has two edge-bridging PMe₂ moieties. Electron counting gives 64 electrons corresponding to a planar system; the distribution of long Co-Co distances, in particular in the triangle bearing PMe₂ bridges, suggests that the excess electrons are located on Co-Co antibonding ortibals. Compound2a contains a Co₃S cluster with one side bridged by a SPMe₂ unit forming a four-membered Co₂SP ring. The substitution of two CO groups with two PPh₃ causes a large deformation of the cluster Co-Co bondscis to these two phosphorus atoms. Crystal data for1, space group P1,a = 9.728(2) Å,b = 10.288(2) Å,c = 11.860(3) Å, = 86.41(2)°, = 76.20(2)°, = 80.37(5)°,Z = 2, 5300 reflections,R = 0.0398; for2a, space group P1,a = 9.78(3) Å,b = 13.05(4) Å,c = 18.28(6) Å, = 93.23(3)°, = 99.17(2)°, = 97.26(6)°,Z = 2, 2976 reflections,R = 0.0579.     

Electronic structure of the Fe3S4 cluster and its quasi-aromaticity

The localized molecular orbitals and their energy levels for the clusters [Fe₃S₄(SH)₃]^2–, [(HS)₃Fe₃S₄·Ni(PH₃)]^2–, [Mo₃S₄(OH₂)₉]⁴⁺, and [Mo₃S₄·Ni]⁴⁺ have been calculated by mean of the Edmiston-Ruedenberg energy localization technique under the CNDO/2 approximation in order to reveal the resemblance between [Fe₃S₄]⁺ and [Mo₃S₄]⁴⁺ in the geometrical configurations and the addition reactivities with heterometal atoms. It is shown that in these two cluster species with core {M ₃(₃-S)(-S)₃} of similar structure (M = Mo, Fe) there exist three synergically connected three-centered two-electron (M-S-M) -bonds around the puckered six-membered {M₃S₃} rings on account of delocalization of a lone electron pair on each bridging S atom; these (M-S-M) -bonds are thus capable of forming cubane-like heterometal clusters with intruder metal atoms through the ( M) bonding. It is therefore seen that unlike the [Mo₃S₄]⁴⁺ with appreciable bonding between the Mo atoms, the extra d-electrons on the metal atoms in the [Fe₃S₄]⁺ cluster are localized on the Fe atoms, exhibiting an electronic structure significantly different from that of the [Mo₃S₄]⁴⁺ cluster.     

An Inorganic-Organic Hybrid Composite Featuring Metal-Chalcogenide Clusters

The cluster complex, Re₆( ₃-Se)₈(PEt₃)₅(4-vinylpyridine)(SbF₆)₂ (1), featuring the face-capped octahedral Re₆( ₃-Se)₈²⁺ cluster core and site-differentiating triethylphosphine and 4-vinylpyridine ligands has been synthesized and structurally characterized. The complex crystallized in monoclinic space group P2₁ with a=11.748(1)Å, b=15.1212(2)Å, c=19.941(2)Å, =90°, =94.411(3)°, =90°, V=3531.8(7)ų, Z=2, R1=0.0531, wR2=0.0774. Copolymerization of styrene with 1 via the polymerizable 4-vinylpyridine ligand afforded a novel inorganic-organic hybrid composite of high molecular weight and a low polydispersity index. Structural integrity of the cluster building block is maintained upon hybrid formation.     

Rotamerism and roulettamerism of vertex-sharing biicosahedral supraclusters: Synthesis and structure of [(Ph3P)10Au13Ag12Cl8](SbF6)

The relative rotation of two centered icosahedra of a vertex-sharing biicosahedral supracluster about the pentagonal symmetry axis gives rise to cluster rotamerism while the relative rotation of the satellite ring of six bridging halide ligands around the equator with respect to the metal framework gives rise to cluster roulettamerism; the latter is exemplified by the structures of [(Ph₃P)₁₀Au₁₃Ag₁₂ X ₈]⁺ X=Cl (this work) or Br.