Bare Metal Clusters Research Articles

Photochemical processes on transition metal atoms and small clusters

Reported here are the first results of a new approach to studying the chemistry of metal clusters. Bare metal atoms and clusters complexes with one or two reactant molecules (either ethanol or propanol) are generated in an ion trap where they are excited with radiation from a tuneable infrared laser. Although a single photon of the latter is not sufficient to break a chemical bond, because the clusters spend ≈ 1 ms within the radiation field, they absorb 15–20 photons (1.5–2 meV) which, in some cases, is sufficient to drive a chemical reaction. The photofragment yields exhibit a pronounced dependence on laser wavelength and at least one infrared absorption feature has a peak centre close to that seen for alcohol molecules adsorbed on to the surfaces of metals and metal oxides.

Tetrahedral Clusters of GaMo4S8-Type Compounds: A Metal Bonding Analysis

Extended Hückel tight binding calculations have been performed on ligated as well as on ligand-free Mo4 and Mo6 extended frames, in order to analyze the metal-metal bonding within the clusters and particularly the appreciable changes of the metal-metal bond lengths through the M4 tetrahedral units contained in GaM4X8 (M = Mo, Nb, V, Ta; X = S, Se, Te), Mo4S4Y4 (Y = Cl, Br, I). A comparison with the M6 octahedral units of the M Mo6X8 (M = Pb, Ag, La; X = S, Se) series is made. By means of DOS, COOP curves, and overlap populations, results clearly display the strong reorganization of the electronic structure of the bare metal clusters network while the ligand interactions occur, inducing a strong reduction of the strength of the metal-metal bonds. We outline the relationship between the metal-metal bond lengths and various parameters such as the valence electron count (VEC) per cluster and the nature of the ligands. Our results indicate that the two series M4 and M6 differ: M-M bond lengths are unaffected by the VEC in the regular M4 cluster, whereas some M-M bond lengths undergo a significant change when the VEC increases in the distorded M6 clusters. Likewise, it is worthy to note that metal d orbitals have a more significant effect in M4 cluster series. In contrast, the metal-ligand covalency induces similar elongations of metal-metal bonds in the two series.

Metal cluster-rare gas van der Waals complexes: physisorption on a microscopic scale

Van der Waals complexes consisting of argon or krypton atoms bound to clusters of iron, cobalt, nickel, and niobium atoms have been generated by supersonic expansion from a liquid nitrogen-cooled laser vaporization source. Laser photoionization time-of-flight mass spectrometry reveals significant nonmonotonic variations in the efficiency of formation of these species as a function of metal cluster size. A correlation between these variations and the size-dependent reactivities of the bare metal clusters toward molecular hydrogen is observed. This correlation suggests that those metal cluster surface sites which readily support the binding of rare gas atoms are also highly reactive sites in direct cluster-hydrogen encounters. Alternatively, this correlation may imply that these cluster-hydrogen reactions occur by a two-step mechanism involving the production of a physisorbed M{sub n}{hor_ellipsis}H{sub 2} intermediate. Analysis of the post-threshold behavior in the photoionization efficiency spectra of niobium cluster-argon van der Waals species (Nb{sub n}Ar{sub m}) suggests that argon atoms bind with different efficiencies to each of the structural isomers of Nb{sub 7}, Nb{sub 9}, and Nb{sub 11}. 25 refs., 11 figs.

Mathematical inorganic chemistry: from gas-phase metal clusters to superconducting solids

This Account surveys application of mathematical methods to studies of gas-phase bare metal clusters to infinite solid-state materials including bulk metals and superconducting solids. Further details on each of these applications are available in the references cited herein.

Electronic shell closings in metal cluster plus adsorbate systems: Cu+7CO and Cu+17CO

The stability of CO-chemisorbed small clusters of copper have been studied both by first principles calculations and by experiment. Evidence is found that the shell model (which predicts that clusters of 8, 18, and 20 electrons are particularly stable) is useful both for the bare metal clusters, and for these clusters with a chemisorbed CO−provided the CO is considered to have contributed two electrons. Experiments supporting this view are reported for gold clusters as well.

Multiphoton excitation, ionization, and dissociation decay dynamics of small clusters of niobium, tantalum, and tungsten: Time-resolved thermionic emission

The ionization dynamics of transition metal clusters have been investigated using time-of-flight mass and electron spectroscopy following single-photon (220 nm) and two-photon (351, 308, and 248 nm) excitation by pulsed laser light. At 220 nm, the ionization is direct and only prompt photoelectrons are produced. At 308 nm, delayed photoelectrons are produced. In consequence of this delayed ionization process, the time-of-flight mass spectrum peaks show exponential tails (decay time 0.67, 0.40, and 1.54 μs for Nb+7, Ta+7, and W+7, respectively). The decay time is shown to have an explicit dependence upon the cluster nuclearity and the laser wavelength. Experiments, in which the acceleration voltage of the time-of-flight spectrometer is pulsed on after the photoionization laser pulse, reveal that the precursor to the delayed ion signals is a neutral molecule, further evidence for a delayed ionization process. Similar effects are also seen for transition metal carbide clusters. Clusters of the same nuclearity have approximately equal decay times independent of the number of carbon atoms in the cluster. Transition metal oxide clusters do not give a two-photon ionization signal. These observations are explained using a model for the two-photon excitation, dissociation, and ionization dynamics. The central feature of this model is that following single photon excitation of an electronic transition below the ionization potential, there is rapid internal conversion among all vibronic states. The absorption of a second photon then creates a vibrationally excited cluster which contains internal energy greater than the ionization potential, but which can only ionize by a nonadiabatic process. This delayed ionization process occurs in competition with dissociation. As clusters of niobium, tantalum, and tungsten and their carbides are very strongly bound, the dissociation rate is slow and the delayed ionization may be observed. Oxidized clusters are expected to be less strongly bound as the diatomic transition metal oxide provides an excellent leaving group; in consequence, no delayed ionization is observed for partially oxidized clusters. The rates for dissociation and ionization of the bare metal clusters have been calculated within the framework of a generalized statistical theory for cluster processes. These rates are in general agreement with the measured decay times. In addition, the rates have been estimated by a procedure which uses tabulated thermodynamic parameters for the bulk elemental materials and makes an explicit correction for the size dependence. Once again, a reasonable agreement is obtained. These results provide the first experimental observation of a delayed ionization process for a neutral polyatomic molecular system. In analogy with materials properties, they also represent the first experimental observation of time-resolved thermionic emission.

Characterization of cluster ions produced by the sputtering or direct laser vaporization of group 13 metal (Al, Ga, and In) oxides

The stabilities and reactivities of cluster ions generated from the fast-atom bombardment (FAB) or the direct laser vaporization (DLV) of the Group 13 metal oxides (Al2O3, Ga2O3, and In2O3) were examined by mass spectrometry. The nascent cluster ion distributions, fragmentations, and reactions were studied. The observed patterns of stability and reactivity were compared with the structures and heats of formation calculated from theoretical studies of aluminum oxide cluster ions using MNDO, Xα, and Born–Mayer pair potentials. The method of production of the metal oxide cluster ions, whether by FAB, DLV, or through the reaction of sputtered bare metal cluster ions with oxygen, had little influence on the abundance distribution observed. In agreement with the known M–O binding energies, a trend of increasing cluster oxidation state was observed in the abundance distributions of the cluster ions for decreasing metal atom z value. Dissociation of the oxide cluster ions occurred through the loss of particularly stable neutral fragments which together with theoretical considerations suggest a preference for particular parent cluster stoichiometries. Although gallium oxide and indium oxide cluster ions exhibited little reactivity toward oxidation agents, the aluminum oxide ions reacted readily with most oxidants. ‘‘Oxygen saturation’’ effects were observed for the ions with 2 and 3 aluminum atoms. These saturation effects could be explained by the formation of structures in which the Al atoms are three-fold planar coordinated.

A novel target manipulation technique used in generating bare metal cluster ions for catalytic research

A novel method of target manipulation for laser vaporization of metals has been described. The simplicity of fabrication offers a number of advantages over existing techniques. The yield of stable clusters M+x, x=1 to 8 is quite good and mass selection using a Wien (E×B) mass filter has been achieved. Examples of Nb, Co, and Mn mass spectra are presented.

Formation of metal cluster ions by gas-phase ion-molecule reactions: The bond energies of Cr 2+ AND Mn 2+

Ion-molecule reactions occurring in Cr(CO) 6, Fe(CO) 5, and Mn 2(CO) 10 systems are carried out in a Fourier transform ion cyclotron resonance (FTICR) spectrometer. Abundant intensities of bare metal cluster ions can be formed by employing a multiple pulse sequence including selective ejections and photodissociation. In subsequent collisional activation experiments, the binding energies of Cr 2 + and Mn 2 + are determined as 1.6±0.2 and 2.1±0.3 eV, respectively, the latter being in conflict with a previously reported value.

Gas phase studies of Zn+2, Ag+3, and Ag+5

Laser desorption from ZnO and AgO produces small bare metal cluster ions. Laser desorption from a ZnO/AgO mixture produces an enhancement of the silver cluster ion signal with complete suppression of the zinc signal. The chemistry of Zn+2 indicates IP(Zn2)=9.0±0.2 eV and D0(Zn+–Zn)=0.56±0.2 eV. The reactivity of Zn+2 with alkenes and alcohols is characterized by displacement of a zinc atom and formation of Zn+–B (B=alcohol, alkene). The silver cluster ions are produced with excess kinetic energy; however, collisional cooling is achieved by trapping the cluster ions in a static pressure of argon. Charge transfer reactions indicate IP(Agn)<7.0 eV (n=3,5). Ag+3 and Ag+5 are unreactive with small alkanes, alkenes, and alcohols, but AgnL+2 (n=3,5; L=sec-butylamine) reacts with sec-butylamine via deamination and dehydrogenation indicating D0 (AgnL+2–butadiene) >1.73 eV.

Size dependence of surface cluster models: CO adsorbed on Cu(100).

We have studied the strong variation of cluster-model binding energies of the CO/Cu(100) adsorbate system for clusters with between 1 and 34 atoms to represent the Cu surface. Based on the constrained space orbital variation approach the metal-CO interaction is decomposed into three terms: (1) charge superposition of the free CO and metal subunits, (2) charge polarization within the subunits, and (3) charge transfer between the subunits. The results show that one can identify bonding contributions which vary slowly with cluster size and shape. Further, relations between the cluster representation of the Cu(100) conduction band and the metal--CO bond strength can be established. These relations make it possible to determine the importance of cluster artifacts for the metal--CO bonding from electronic properties of the bare metal cluster. Our results provide further support for the validity of using clusters with small numbers of substrate atoms to describe chemisorption on metal surfaces.

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 H 2 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-H 2 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 H 2 and NH 3 as adsorbates.

Dependence of the reaction probability of benzene on the size of gaseous niobium clusters

Small niobium atomic clusters (Nb/sub x/, x = 1-15), synthesized by laser vaporization in a supersonic molecular beam system, are mixed with benzene in a fast flow reactor and detected by excimer laser photoionization time-of-flight mass spectrometry. The ratio of the observed mass peak ion intensity of Nb/sub x/C/sub 6/ to that of Nb/sub x/C/sub 6/H/sub 6/ + Nb/sub x/C/sub 6/H/sub 6/ is used as a measure of the observed conversion of Nb/sub x/C/sub 6/H/sub 6/ into Nb/sub x/C/sub 6/. This probability is found to have a threshold for conversion near x = 4 with maxima at x = 5 or 6 and 11. The ratio also shows a minimum for those clusters containing 8 and 10 atoms, similar to the previously observed reactivity of these two clusters and their cations toward H/sub 2/ and N/sub 2/. This observed variation in the Nb/sub x/C/sub 6/H/sub 6/ to Nb/sub x/C/sub 6/ conversion reaction is discussed in terms of the stabilities of benzene and the bare metal clusters relative to Nb/sub x/C/sub 6/ and 3H/sub 2/.

The role of cluster ion structure in reactivity and collision-induced dissociation: Application to cobalt/oxygen cluster ions in the gas phase

Ion/molecule reaction products of cobalt cluster ions have been characterized using mass spectrometric techniques. Atomic and bare-metal cluster ions were desorbed from foils by particle bombardment within a high-pressure (0.1–0.2 Torr) ion source. Sputtered metal cluster ions react with O2 to produce abundant stoichiometric or nearly stoichiometric cobalt(II) oxide cluster ions. The positive cluster product ions consist of three types: oxygen-deficient [Co(CoO)x]+ clusters, oxygen-equivalent [(CoO)x]+ clusters, and (in less abundance) metal-deficient [(CoO)xO]+ clusters. Tandem mass spectrometry and collision spectroscopy provide structural information about the more abundant cobalt cluster product ions. A major collision-induced fragmentation pathway for the oxygen-equivalent [(CoO)x]+ clusters is the loss of a CoO moiety to form [(CoO)x−1]+ fragments. A major collision-induced fragmentation pathway for the oxygen-deficient [Co(CoO)x]+ clusters is the loss of a cobalt atom to yield [(CoO)x]+ fragments. Geometric structures of the cobalt/oxygen cluster ions were calculated using a Coulomb plus Born–Mayer pair-potential model. The oxygen-equivalent cluster structures were found to be ‘‘globular’’ cages, rings, or ladders. The oxygen-deficient cluster structures were found to be strained and ‘‘angular’’ with protruding cobalt atoms. The structures are discussed in terms of the observed collision-induced fragmentations. The fragmentations are rationalized using an ‘‘instantaneous’’ dissociation model of the collision-induced dissociation of the cluster ions. Preliminary trajectory calculations using classical dynamics support the use of this instantaneous dissociation model. The role of cluster ion structure in reactivity and collision-induced dissociation is discussed in terms of the experimental data and theoretical structures.

Laser-Stimulated Noble Metal Deposition From Gaseous and Condensed Acetylacetonates. Photoreactions in the Supersonic Jet and Above Silicon

ABSTRACTLaser-stimulated deposition from the acetylacetonates of Pt and Ir has been found to take place on silicon surfaces. The gas phase photoreactions have been observed in a supersonic free jet TOF mass spectrometer and above silicon. The bare metal ions and metal cluster ions are formed from the complexes in the gas by multiphoton excitation. On the substrates deposits are detected by microprobe analysis with a metal content higher than 95 %. Also from condensed complexes laser-induced metal deposition, and as competitive process photodesorption, takes place.

Metal cluster ion cyclotron resonance. Combining supersonic metal cluster beam technology with FT-ICR

An apparatus is described for the injection and trapping of supersonic bare metal cluster ions in a Fourier transform ion cyclotron resonance spectrometer (FT-ICR). Metal clusters were generated in a supersonic beam by laser vaporization of a metal target rod inside a pulsed nozzle. The cold clusters were ionized by an argon-fluoride excimer laser, mass-selected, and then directed down the bore of a superconducting magnet to be trapped in the rectangular cell of the FT-ICR. Efficient injection was accomplished with a series of Einzel lenses which directed the clusters along the magnetic field lines leading into the magnet. Once in the magnet center, the ions were trapped and analyzed by standard FT-ICR methods. Bare niobium clusters ranging in size from two to six have been successfully injected, trapped and detected at high mass resolution. Measurements indicated near perfect injection efficiency and a net trapping efficiency of ∼ 60%.

XPS study of bonding in ligated Au clusters

Ligated metal cluster compounds containing a core of metal atoms with well defined structure surrounded by a variety of organic and inorganic ligands are closely related to the bare metal clusters that are only now becoming available in uniform cluster size. The evolution of band structure and the development of metallic properties as a function of cluster size are of considerable interest. We report here a comparison of these two types of systems based on a study by X-ray photoelectron spectroscopy. The valence band spectra of ligated Au clusters compounds are similar in many respects to those of bare clusters, indicating significant participation of the d electrons in bonding. The core electron binding energy shifts of the central Au atom in Au 11(PPh 3) 7Cl 3 corresponds to the loss of approximation one 6 s electron. The total charge transferred to the halogens is accounted for by the shifts of the central and three halogenbonded Au atoms. No indication of metallic behaviour is found in the core of the ligated clusters.

The photoionization of oxidized metal clusters

Oxidized metal clusters (NaxO and KxO for 2≤x≤4) were formed in a gas phase reaction between metal clusters and an oxidizing gas using a double expansion technique. Their appearance potentials were measured using a molecular beam-photoionization mass spectrometer system. These first photoionization data for oxidized clusters provide information on trends of ionization potentials as a function of the degree of aggregation. The ionization potentials do not differ greatly from the analogous metallic species, but in the case of the sodium tetramer the value does fall below that of the bare metal cluster. This finding is in accord with what has been observed as an influence of impurities on the work function of the bulk sodium. The results are also of interest concerning questions of octet rule violations and hypervalency.

Supersonic metal cluster beams of refractory metals: Spectral investigations of ultracold Mo2

A novel technique involving pulsed laser vaporization of the bulk metal within a pulsed supersonic nozzle has been shown to successfully produce ultracold bare metal clusters of even the most refractory of metals, tungsten and molybdenum. Clusters of up to 25 atoms may be readily prepared using this technique. Mass-selective resonant two-photon ionization spectra of Mo2 produced in this fashion show that the dimer is efficiently cooled in the expansion Ttrans<6 K, Trot∼5 K, and Tvib∼325 K. We have rotationally resolved the A 1Σ+u←X 1Σ+g (0–0) band for 92Mo2 and determined the bond length in the ground and excited states to be 1.940±0.009 and 1.937±0.008 Å, respectively. This confirms and extends the analysis of Efremov et al. [J. Mol. Spectrosc. 73, 40 (1970)] who prepared 98Mo2 by flash photolysis of isotopically pure Mo(CO)6. We have also observed the (1–1), (2–2), and (3–3) sequence bands which together with the ground state data of Efremov et al. determine vibrational constants ω′e=449.0±0.2 cm−1 and ωex′e=2.3±0.2 cm−1 for the A 1Σ+u state. The lifetime of the A 1Σ+u v=0 state of Mo2 has been measured to be 18±3 ns by time-delayed two-photon ionization. The ionization potential of Mo2 is found to be less than 6.42 eV (compared to 7.10 eV for atomic Mo) indicating a substantially stronger chemical bond in Mo+2 than in Mo2. A discussion of the electronic structure of Mo2 and the implications of these findings for bonding in other transition metal dimers is also presented.

Production of gas-phase bare transition metal clusters by laser photodissociation of organometallic cluster compounds

Production of gas-phase bare transition metal clusters by laser photodissociation of organometallic cluster compounds