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Experimental data on the uptake of ammonia by iron clusters in the polyicosahedral series Fe19, Fe23, Fe26, Fe29, Fe32, and Fe34 have been measured at a series of temperatures. The uptake data are analyzed with a statistical model that parametrizes both molecular adsorption and desorption in terms of collision cross sections, Rice–Ramsperger–Kassel (RRK) frequency factors, and cluster–ammonia binding energies. Fits to the data give a binding energy of 0.88±0.02 eV for the first NH3 molecule on all six clusters, and a linear decrease in binding energy with increasing number of molecules, with larger clusters showing a slower rate of decrease. When expressed as a function of coverage (the ratio of the number of molecules to the number of surface metal atoms), the binding energy is also linear and is essentially independent of cluster size. The observed behavior is discussed in terms of the probable nature of cluster–ammonia interactions. [ABSTRACT FROM AUTHOR]

Experimental evidence is presented that small (19- to 34-atom) clusters of iron, cobalt, and nickel tend to adopt polyicosahedral structure when saturated with ammonia. Clusters in the polyicosahedral series (having 19, 23, 26, 29, 32, and 34 atoms) often saturate with 12 NH3 molecules, the expected number for polyicosahedral structure. Other clusters in this size range, which do not have an obvious number of NH3 binding sites, generally do not show a clear saturation. Adsorbate-induced structural changes are also seen, and suggest that for larger clusters in the series there is a transformation to an alternate structure that binds more than 12 NH3 molecules. [ABSTRACT FROM AUTHOR]

Copper clusters in the 50- to 100-atom size range are found to exhibit electronic shell structure as well as icosahedral geometry. Clusters corresponding to filled shells have minimum intensity in near-threshold photoionization mass spectra, implying that they have locally higher ionization potentials than other cluster sizes. The chemical stability of these clusters is illustrated by a reduced reactivity towards O2. Cluster geometry is probed via the equilibrium reactions with H2O: Clusters having one copper atom more than closed icosahedral subshells show an enhanced binding of water. The relative importance of electronic and geometrical structure in determining cluster chemical properties is discussed. [ABSTRACT FROM AUTHOR]

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 molecules, and often the two structures are seen together. The equilibrium reaction of a single water molecule with the bare clusters probes the strength of the cluster–water bond. Enhanced water adsorption is often seen for clusters one atom larger than those showing minima in ammonia uptake, suggesting that these bare clusters likewise have icosahedral structure. The reasons for minima in ammonia uptake and maxima in water binding are discussed. [ABSTRACT FROM AUTHOR]

A procedure is described for the determination of metal cluster ionization potentials (IPs) using available excimer laser lines that gives error limits substantially smaller than traditional bracketing experiments. It is based on the observation that the adsorption of ammonia on cluster surfaces lowers cluster IPs, and that the IP lowering is linear in the number of adsorbed NH3 molecules. By determining the minimum number of NH3 molecules needed for ionization by the various excimer lasers, an approximation to the dependence of IP on coverage can be deduced. Extrapolation of this dependence to zero coverage gives the bare cluster IPs. Results are presented for clusters of iron, cobalt, and nickel having from 4 to 100 atoms. The effect of molecular adsorption on cluster IPs is analyzed theoretically, and the comparison with experimental results used to estimate the effective dipole moment of NH3 molecules adsorbed on these clusters. Comparison of the bare cluster IPs with the simple spherical drop model suggests that for transition metal clusters the Fermi level can be a significant function of cluster size. [ABSTRACT FROM AUTHOR]

Studies of two-reagent reactions of iron clusters with ammonia and deuterium are described. The compositions of clusters saturated with both ammonia and deuterium have been determined, and indicate that the ammonia binds nondissociatively to iron clusters and to different sites than the deuterium. Rate constants for the reaction of the first deuterium molecule with ammoniated clusters have been measured. In general, low ammonia coverage (one to three molecules) has either little effect on cluster reactivity, or in the case of unreactive bare clusters, increases reactivity dramatically. Clusters saturated with ammonia show general reactivity decreases, although again the least reactive bare clusters show dramatic increases in reactivity when covered with ammonia molecules. Interesting cluster activation and deactivation effects are seen. Possible mechanisms for these reactions are discussed. [ABSTRACT FROM AUTHOR]

Studies are described of the chemisorption of ammonia on isolated neutral iron clusters Fen for 2≤n≤165. Clusters are generated by laser vaporization in a continuous-flow-tube reactor, and reaction products are detected by laser-ionization time-of-flight mass spectrometry. Ammonia is found to chemisorb nondissociatively on cluster surfaces on the 1 ms time scale of these experiments. Measurements of ammonia uptake provide information on adsorption kinetics and on the number and nature of the binding sites. The ammonia binding energy is found to decrease with increasing cluster coverage. For chemically saturated clusters, the ratio of adsorbed NH3 molecules to surface iron atoms is found to decrease with increasing cluster size, going from >1/3 for small clusters to <1/3 for n>100. Ammonia chemisorption is accompanied by a large decrease in cluster ionization potentials, as much as 2 eV for saturated clusters. At sufficiently high exposure the beginning of the formation of a second, physisorbed layer of molecules is seen. Detailed measurements of product composition under different exposure conditions give evidence for numerous changes in cluster structure throughout the growth sequence from small to large clusters. Often these structural changes involve particularly stable reaction products. Evidence for the existence of metastable structures is presented. Several possibilities for cluster structure are suggested. [ABSTRACT FROM AUTHOR]

Evidence is presented for structural changes in iron clusters in the Fe13 to Fe23 size range. Abrupt changes with cluster size are found for several chemical properties, including reactivity with hydrogen and binding energies of ammonia and water. These changes often come at the same cluster sizes, pointing to a common origin—fundamental changes in the structure of the bare iron clusters. In addition, changes in structure as a consequence of adsorbate binding are suggested. The experimental observations leading to these conclusions are detailed, and possible structures for clusters in this size range are proposed. [ABSTRACT FROM AUTHOR]

The gas-phase reactions of nitrogen, hydrogen, and carbon monoxide with Ni38 are studied as a function of reagent pressure at several reaction temperatures. Saturation coverage of the cluster is found at Ni38(N2)24, Ni38H36, and Ni38(CO)36. These saturation levels are consistent with the metal core of the ligated cluster having the structure of a truncated octahedron in each case. An alternate fcc structure derived from a 40-atom truncated tetrahedron is consistent with the nitrogen data, but not with the hydrogen or carbon monoxide results. In addition, the nitrogen uptake data indicate that the bare Ni38 cluster also has the structure of a truncated octahedron or possibly a deformed truncated octahedron. There is no indication that Ni38 has an icosahedral or polyicosahedral structure. The nature of the binding of the three reagents to the cluster is discussed. Evidence is presented that CO initially binds to atop sites, but following saturation of these sites a local rearrangement to bridge sites occurs that allows an increase in coverage to the observed saturation at Ni38(CO)36. At high reagent pressures all three reagents cause adsorbate-induced structural changes to isomers that bind more ligands and whose structures have yet to be determined. © 1997 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Multiphoton absorption and ionization of hydrogenated (or deuterated) iron clusters generally leads to desorption of a specific number of H2 (or D2) molecules for a given cluster size and a given number of photons absorbed. Ionization via single photon absorption occurs without desorption. Experimental results demonstrate that the fragmentation pattern resulting from multiphoton absorption is independent of whether desorption precedes or follows ionization. From the number of desorbed molecules and the number and energy of absorbed photons an estimate of 1.3 eV for the desorption energy can be made without the necessity of modeling the desorption process. A simple statistical model of the process provides similar estimates of the desorption energies, and indicates that the energy has some dependence on cluster coverage. [ABSTRACT FROM AUTHOR]

Reactions of iron clusters with an excess of hydrogen are found to yield fully hydrogenated products FenHm whose compositions remain fixed over a wide range of hydrogen pressures. For n=6 to 131, the observed m values are always even, have narrow ranges, and for many clusters are unique. Up to n=30, nearly stoichiometric 1:1 ratios of m to n are found. Above 30, cluster hydride compositions are consistent with a monolayer of chemisorbed hydrogen on the cluster surfaces. At sufficiently high hydrogen pressures additional hydrogens bind to the clusters, most likely as a second, physisorbed layer. The experimental results are discussed in terms of cluster structure and the relation to bulk iron behavior. [ABSTRACT FROM AUTHOR]

The kinetics of the gas phase reactions of hydrogen and deuterium with iron clusters in the range Fe6 to Fe68 have been investigated. It is found that reaction rate constants are a strong function of cluster size, varying by more than five orders of magnitude in this size range. The largest rate constants correspond to approximately 3% of a hard sphere cross section. Abrupt changes in the rate constant from one cluster to the next are seen. Qualitative temperature dependencies of cluster reactivity have been determined. The more reactive clusters show decreased reactivity with increased tempeature, while the least reactive clusters become more reactive. Strong isotope effects are seen only in the Fe10 to Fe14 size range. Mechanisms for the reactions of H2 and D2 with iron clusters are discussed in light of these observations. [ABSTRACT FROM AUTHOR]

The kinetics of the reactions of cobalt clusters (Con, n=9–21) with deuterium are studied over the temperature range 133–373 K. The measured rate constants for the dissociative addition of the first D2 molecule to Con, and their temperature dependences, are found to be strongly dependent on cluster size. Co10–13 and Co15–17 are relatively reactive, with their reaction rates essentially independent of temperature, suggesting no significant reaction barrier for deuterium chemisorption. The reaction rate of Co20 is nearly constant above 213 K, but increases at lower temperature. For Co9 and Co14, the rate constants increase both at high and low temperatures. An increase at higher temperatures suggests the presence of activation barriers, while increases at low temperatures are interpreted as due to an increase in the initial trapping of D2 onto the clusters’ surfaces. For Co18, Co19, and Co21, evidence for more than one isomer at low temperature is found. The experimental results are discussed in terms of cluster geometrical structure and the detailed reaction mechanism. [ABSTRACT FROM AUTHOR]

The kinetics of reactions of D2 with Ni9 through Ni14 are studied in a flow-tube reactor over the temperature range 133–413 K. Rate constants for the dissociative chemisorption of the first D2 molecule have been determined. Reaction probabilities are near unity for Ni10–Ni14 and are essentially independent of temperature. For Ni9, the reaction probability is about 5% between 213 and 413 K, but increases below 213 K to ∼50% at 133 K. The possibility that these reactions are mediated by a physisorbed precursor state is discussed. The increase in Ni9 reactivity below 213 K is interpreted as due to an increase in the initial trapping of the D2 molecule on the cluster’s surface. [ABSTRACT FROM AUTHOR]

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 NH3 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, 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. [ABSTRACT FROM AUTHOR]

The reactions of neutral iron clusters Fe7–27 with water are studied in a laser-vaporization cluster source coupled to a continuous-flow reactor. Reaction products are detected via laser ionization and time-of-flight mass spectrometry. The reactions of room-temperature clusters with H2O show adsorbate decomposition and hydrogen desorption, as do the reactions with D2O at elevated temperatures. The room-temperature reaction with D2O appears not to involve any decomposition, and is at equilibrium under the conditions of these experiments. The dependence of reaction extent on D2O pressure yields equilibrium constants for the addition of the first and second D2O molecules. The analysis is complicated by the presence of two-photon ionization processes that are treated quantitatively with a rate-equation model. This treatment also yields estimates for cluster photoabsorption cross sections, which are found to be approximately linear in cluster size, having a magnitude of 2.3×10-17 cm2 per iron atom. From the derived equilibrium constants and estimated adsorption entropies, approximate D2O–cluster binding energies are determined. They range from 0.42 to 0.59 eV, and their dependence on cluster size shows a remarkable similarity to the dependence of the rate constants for reaction of iron clusters with H2. The implications of this similarity are discussed. [ABSTRACT FROM AUTHOR]