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The adsorption of hydrogen (deuterium) and oxygen on neutral platinum clusters has been investigated in a cluster beam experiment. The beam passes through two low-pressure reaction cells and the clusters, with and without adsorbed molecules, are detected by laser ionization and mass spectrometry. Both H2 and O2 adsorb efficiently on the platinum clusters with only moderate variations with cluster size in the investigated range, i.e., between 7 and 30 atoms. The coadsorption of H2 and O2 results in the formation and desorption of H2O, detected as a decreasing number of adsorbed oxygen atoms with an increasing number of collisions with H2 molecules. The water-formation reaction proceeds efficiently on all investigated clusters with more than seven atoms.

The hydrogen abstraction reaction between cyclopentadienyl radicals [Xcpd, X=H, CH3(m), F, CN] and substituted methanes (CH4, C2H6, CH3CH2OH, CH3Cl, CH2F2, CHF3, and CH3OH) is studied for the isolated one-to-one van der Waals clusters created in a supersonic expansion. Three different types of fluorescence excitation spectra are characterized for these cluster systems: (1) sharp spectra are observed for some clusters, suggesting no cluster chemistry for either the ground or excited electronic states of Xcpd—CNcpd/CH3Cl, CH2F2, CHF3, CH3OH; (2) broad spectra are observed suggesting initiation of cluster chemistry on the excited state cluster potential energy surface—CNcpd–CH4, Fcpd–CHF2Cl, CHF3; and (3) only a greatly reduced bare radical signal is observed, but no cluster emission can be detected—cpd, mcpd/all substituted methanes, Fcpd–CH2F2, CH3Cl, CH3CH2OH, CH3OH, C2H6, and CNcpd/C2H6, CH3CH2OH. These results, taken together, suggest that the Xcpd radicals undergo an excited electronic state electrophi...

Photofragmentation mass spectra of metal–fullerene clusters C60Mx and C70Mx (M={Fe, Co, Ni, Rh}; x=0,…,30) reveal the existence of a reaction channel which yields clusters having the composition C59−2nM and C69−2nM (n=0,…,10). Enhanced abundances of clusters with 44, 50, and 60 atoms, as well as the presence of clusters containing almost exclusively an even number of atoms, indicate that one carbon atom of the fullerene cage is replaced by a transition metal atom. Additional tandem time-of-flight (TOF) experiments on mass selected C59M and C69M indicate that the initial fragmentation step of this new kind of substitutionally doped fullerenes is the loss of a neutral MC molecule. Measurements on preselected C70Rh3 and C70Ir2 were performed in order to monitor in detail their laser induced transformation into C69Rh and C69Ir.

Photoelectron spectra of mass-separated Nbn− clusters reveal an even/odd alternation for n=6–17, indicating a closed electronic shell of the neutral even-numbered clusters. The HOMO–LUMO gap of Nb8, Nb10, and Nb16 is found to be larger than that of the other even-numbered clusters, which correlates with the low H2 reactivities of these species. The spectrum of Nb15− is different from all other clusters in this size range, which might be an indication for a geometric bcc shell closing. The influence of the electronic structure of the clusters on the reactivity is discussed.

The reactions of anionic niobium and rhodium clusters Mn−, M=Nb, Rh, n=3–28, with C6H6 are investigated under single collision conditions in a Fourier-transform ion-cyclotron-resonance mass spectrometer and compared with the results of previous studies on corresponding cationic species. This reveals strong effects of the cluster charge state on hydrocarbon activation as a function of cluster size. Both differences and parallels are observed for reactions of anions and cations. Niobium clusters with a given number of atoms react quite differently than those with a single atom more or less. The fact that almost identical such effects are in the present work found for anion clusters, as for cations with the same number of atoms but two less electrons, suggests that the observed reactivity patterns are more a function of the cluster shape and geometry, than of the details of their electronic structure. The variety of interesting trends and effects observed is interpreted in terms of simple physical models.

Catalysis of the oxidation reaction CO+12O2→CO2 by metal clusters is examined by gas phase ion chemistry and mass spectrometry techniques. Small platinum cluster anions, Ptn− (n=3–6), are effective catalysts for the reaction under thermal conditions. The proposed reaction mechanism involves atomic oxygen chemisorbed on the clusters.

Reactions of small thermalized positively charged nickel clusters with carbon monoxide were studied in a molecular beam experiment. The nickel clusters were produced in a high intensity cluster ion source and thermalized in a large helium-filled quadrupole ion guide. The clusters were size selected by a quadrupole mass spectrometer. The mass- and charge-selected nickel clusters then passed through a linear quadrupole drift tube filled with a mixture of helium buffer gas and carbon monoxide. The reaction products were then analyzed by a quadrupole mass-spectrometer. Using this technique, saturation limits for Nin+ clusters with n=4–31 were measured and the competitive reaction channels were identified. Under certain experimental conditions carbide formation was observed in the case of the nickel tetramer, pentamer, and hexamer. The structure of the nickel carbonyl clusters is discussed within the framework of the polyhedral skeletal electron pair theory. The cluster growth may be explained by a pentagonal ...

Reactions of tungsten clusters with molecular nitrogen have been investigated by using a fast‐flow reactor equipped with a laser vaporization cluster source and time‐of‐flight mass spectrometer detector. Absolute rate coefficients are reported for reaction of Wn clusters in the range n=4–26, at temperatures 277, 300, and 370 K in He buffer gas at 1 and 2 Torr pressure. For smaller clusters with n

Reactions of molybdenum clusters with molecular nitrogen have been investigated by using a fast‐flow reactor equipped with a laser vaporization cluster source and laser ionization time‐of‐flight mass spectrometer for detection of clusters. Absolute rate coefficients are reported for reaction of clusters in the range 3–35 metal atoms, at temperatures 279, 300, and 372 K in He buffer gas at 1 and 2 Torr pressure. Rate coefficients generally decrease with increasing temperature, as expected for a mechanism involving formation of a weakly bound precursor complex. By comparison with surface chemistry studies of nitrogen adsorption on molybdenum and tungsten, the reactivity trend with cluster size is interpreted as reflecting the geometric configurations of metal atoms on the surfaces of the clusters. It is suggested that structural transitions occur in molybdenum clusters in the size range 13–20 metal atoms. Evidence is described that suggests that cluster reaction products have nitrogen bound in both atomic a...

We have employed a time‐of‐flight analysis to determine the velocity and spatial distribution of photofragments resulting from excitation of (HI)n cluster species at approximately 240 nm. The formation of clusters in the supersonic expansion broadens the I+ flight‐time distribution and destroys the spatial anisotropy characteristic of HI monomer photodissociation, indicating that these fragments experience strong solvent cage effects when exiting the cluster environment. In addition, a high velocity component with an anisotropic distribution appears and is due to the photodissociation of I2, a product of cluster chemistry. Even with extensive clustering, the main features of the H+ flight‐time spectrum do not change and cage effects are not as pervasive as for the I atom fragments. However, there is also a broad H+ component peaked near zero velocity and exhibiting an isotropic spatial distribution. Inelastic H atom collisions involving excitation of internal modes in HI cage species likely play a role in...

Reactions of cobalt clusters in the 6 to 150 atom size range with water and ammonia are used to probe cluster geometrical structure. Clusters saturated with ammonia are found in several isomeric forms in much of this size region, including a structure based on icosahedral packing for clusters having 50 to 120 atoms. The variations with cluster size of the binding energy for a single water molecule provide evidence for icosahedral structure of the bare cobalt clusters in the same size range seen for the ammoniated clusters. However, both probes suggest that in many cases the bare clusters also have multiple isomers. The evidence for icosahedral structure in the bare and ammoniated clusters disappears above ∼120 atoms and is clearly not seen at the third icosahedral shell closing at 147 atoms. Comparison with earlier studies of hydrogenated cobalt clusters and with bare and hydrogenated nickel clusters is made. Reasons for the failure to form the closed‐shell 147 atom icosahedral cluster are discussed.

The first part of this paper deals with the enumeration of all possible stereoisomers (or polytypes) of a binary (two‐colored) icosahedron (either noncentered or centered) via Polya’s theorem. The numbers of achiral and chiral structures for each combination of the two types of atoms (or colors) A and B are determined. In cluster chemistry, A and B may represent two different kinds of atoms (e.g., main‐group elements vs transition metals) in a heteronuclear cluster, two different ligand environments in a homonuclear cluster, or atoms and holes (vacant sites) in an icosahedral cluster with missing vertices. The second part deals with the electronic requirements of binary icosahedral clusters where A and B may represent main‐group elements and transition metals, or, atoms and holes. Significantly different electron counts are observed for centered vs noncentered icosahedral clusters, depending upon the nature of the interstitial atom. Thus, icosahedral clusters can be classified into four broad categories according to their electron counts: Type I: the majority are those with 13 electron pairs as represented by noncentered icosahedral clusters, or those centered with main‐group elements or early transition metals as the interstitial atom; Type II: those with more than 13 electron pairs as exemplified by centered icosahedral clusters with late transition metals as the interstitial atom; and Types III and IV: those with less than 13 electron pairs as observed for centered Au icosahedral clusters. The atom and electron counting rules developed are rationalized in terms of molecular orbital theory. The utilities of the atom and electron counting rules in systematizing structural and spectroscopic data of icosahedral clusters (in both solid‐state and gas‐phase) as well as in rationalizing the variations of the electron counts in different icosahedral cluster systems are discussed.

Reactions with ammonia and with water are used to probe the geometrical structures of cobalt and nickel clusters that are saturated with hydrogen. Ammonia saturation experiments allow the determination of the number of primary NH{sub 3} binding sites on cluster surfaces, and this number shows a repeated minimization to values of 12 for many cluster sizes in the 50- to 200-atom size region. These sizes correspond to closed shells and subshells of icosahedra, suggesting that the ammoniated clusters have metal frameworks with icosahedral structure. The equilibrium reaction of the hydrogenated clusters with a single water molecule shows a pattern of local maxima in the cluster--water binding energy, with the maxima in most cases coming at clusters having one metal atom more than those showing minima in ammonia binding. This correlation suggests that nonammoniated clusters likewise have icosahedral structure, and is consistent with the nature of the metal--water interaction. Some of the larger clusters do not show clear evidence for icosahedral structure at room temperature, although they begin to do so at elevated temperature. Annealing experiments suggest that many of these clusters are icosahedral in their most stable configuration at room temperature, although the 147-atom nickel cluster is not. In general,more » hydrogenation enhances the icosahedral features in the ammonia and water binding patterns compared to those seen for bare clusters, and extends the cluster size region over which icosahedral structure is evident.« less

The reactions of nickel clusters with ammonia and with water are used to probe cluster geometrical structure. Ammonia uptake experiments allow the determination of the number of preferred binding sites on cluster surfaces. This number shows pronounced minima in the 50‐ to 116‐ atom size range for many of the cluster sizes that appear as magic numbers in mass spectra of rare gas clusters. Since these magic numbers arise from closings of shells and subshells of the Mackay icosahedra, the correlation suggests that ammoniated nickel clusters in this size region also have icosahedral structure. Similar structure is found for ammoniated clusters smaller than ∼30 atoms, but is not seen for room temperature clusters in the vicinity of the third shell closing at 147 atoms. Icosahedral features do appear for the larger clusters at elevated temperatures. For many clusters above 50 atoms, prolonged exposure to ammonia causes a conversion from the icosahedral structure to some other structure that binds more ammonia m...

The studies on the structure and electronic properties of hydrogenated metal embedded Al(12) cage clusters have been performed by density functional theory calculations. We have investigated aluminum cluster hydrides with 12 and 14 hydrogen atoms, respectively. Insertion of the Mg, Ca alkali metals remarkably enhances the stability of the aluminum clusters. The hydrogen atom prefers to occupy on-top sites along the surface of the clusters. Mulliken population analysis indicates that significant charge transfer occurs between the Mg and Ca atoms and the Al atoms. Our computations suggest that these clusters appear to be physically and chemically stable.

Though discovered more than a half century ago metallic glasses remain a scientific enigma. Unlike crystalline metals, characterized by short, medium, and long-range order, in metallic glasses short and medium-range order persist, though long-range order is absent. This fact has prompted research to develop structural descriptions of metallic glasses. Among these are cluster-based models that attribute amorphous structure to the existence of clusters that are incommensurate with crystalline periodicity. Not addressed, however, are the chemical factors stabilizing these clusters and promoting their interconnections. We have found that glass formers are characterized by a rich cluster chemistry that above the glass transformation temperature promotes exchange as well as static and vibronic sharing of atoms between clusters. The vibronic mechanism induces correlated motions between neighboring clusters and we hypothesize that the distance over which these motions are correlated mediates metallic glass stability and influences critical cooling rates.

Pulsed fast flow reactor techniques have been used to study the reactivity of CO toward clusters of many different transition metals; V, Fe, Co, Ni, Cu, Nb, Mo, Ru, Pd, W, Ir, and Pt as well as Al, for clusters containing up to 14 atoms. Clusters are produced by pulsed laser vaporization of metal substrates, injected into the reactor, formed into a molecular beam, and detected by photoionization time of flight mass spectrometry. Our results show that CO is readily chemisorbed on most transition metal clusters containing five or more metal atoms, and that the reactivity for larger clusters varies by a factor of 2 or 3, depending on both cluster size and metal type. Depending on the metal, certain atoms, dimers, trimers, and tetramers exhibit little evidence of reactivity toward CO. This observation is explained in terms of a competition between unimolecular decomposition and collisional stabilization, and leads to a prediction of the ordering of the metal–CO bond strengths.

We have performed a series of experiments in which the conditions necessary for synthesis and detection of carbon clusters have been systematically investigated. The effects of ionizing laser intensity and frequency, vaporization and condensation conditions, and graphite substrate composition (pure and metal loaded) on the carbon cluster ion signals show that ‘‘special’’ conditions are needed to observe locally enhanced ion signals on particular clusters (e.g., a large C+60 signal). We present evidence for two sources of carbon clusters, one source which produces mostly smaller (n 32) carbon clusters. In addition we find that several, not just one, metal atom can be attached to C60. Lastly we propose that large variations in ion signals may be explained in terms of the electronic properties of the carbon clusters.

FT‐ICR techniques were used to probe the surface chemistry of isolated silicon cluster ions in the 7–65 atom size range. Dissociative chemisorption reactions with NH3 were observed to proceed with rates which varied widely with cluster size. One particular cluster, Si+39, was found to be remarkably inert. Clusters with 20, 25, 33, and 45 atoms were found to be unreactive as well, while those with 18, 23, 30, 36, 43, or 46 atoms were quite reactive. Similarly oscillating reaction patterns were observed with CH3OH, whereas highly reactive free radical scavengers such as O2 and NO showed little selectivity. These results suggest the silicon clusters in this size range have well‐defined structures which vary in ability to catalyze dissociative chemisorption at the surface.

A method is presented for the measurement of the chemisorptive reactivity of transition metal cluster ions at near room temperature. Similar to a technique introduced previously for neutral clusters [Rev. Sci. Instrum. 56, 2123 (1985)], this cluster ion method utilizes a fast‐flow reactor attached to a supersonic, laser vaporization metal cluster source, followed by time‐of‐flight mass spectral analysis of the cluster ions as a function of reactant concentration. Results are presented for clusters of cobalt and niobium in the 1–22 atom size range for their chemisorptive reactions with CO, CO2, and N2. Both Nb+n and Co+n clusters displayed chemical reactivity that is remarkably similar to that of the corresponding neutral clusters. For both charge states of each metal, CO was found to chemisorb with a rate which varied in a slow, monotonically increasing fashion with cluster size. The chemisorption rate of N2 and CO2, on the other hand, was found to be significantly slower than that of CO and sharply dependent upon the cluster size, this dependency being roughly independent of whether the transition metal cluster had a net positive or neutral charge. Photodissociation measurements of the mass‐selected positive ion chemisorption products showed that the desorption energy of these products parallels the relative reaction rate.

A new procedure is described for the study of chemical reactions of metal clusters with simple gas phase molecules. It consists of determining the average number of reagent molecules taken up by the clusters as a function of reagent pressure over a very wide range of pressures. When the results of such experiments are appropriately analyzed, they provide information on cluster reaction kinetics, thermodynamics, and the composition of reaction products. An illustration of this procedure for the reaction of iron clusters with ammonia is presented.