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Quantum confinement in small metal clusters leads to a bunching of states into electronic shells reminiscent of shells in atoms, enabling the classification of clusters as superatoms. The addition of ligands tunes the valence electron count of metal clusters and appears to serve as protecting groups preventing the etching of the metallic cores. Through a joint experimental and theoretical study of the reactivity of methanol with aluminum clusters ligated with iodine, we find that ligands enhance the stability of some clusters, however in some cases the electronegative ligand may perturb the charge density of the metallic core generating active sites that can lead to the etching of the cluster. The reactivity is driven by Lewis acid and Lewis base active sites that form through the selective positioning of the iodine and the structure of the aluminum core. This study enriches the general knowledge on clusters including offering insight into the stability of ligand protected clusters synthesized via wet chemistry.

Clusters composed of a few to a few hundred atoms are the ultimate nanoparticles where every atom and every electron count. Over the past 50 years clusters have evolved as a new field of matter intermediate between atoms and bulk. In this retrospective we trace the role mass spectrometry has played in this emerging field with emphasis on how extremely intense mass peaks indicating a high abundance of clusters led to the discovery of magic numbers in alkali metal clusters, the fullerenes in carbon clusters, and “Met-Cars” in transition-metal carbide clusters. Magic numbers in alkali metal clusters in turn helped bridge the gap between two disparate fields, atomic physics and nuclear physics. Studies of mass spectra combined with theory and a variety of other experimental techniques have since led to a fundamental understanding of the structure–property relationships of clusters, thus broadening the scope of cluster science. Clusters not only serve as a bridge between atoms and bulk and between atomic and nuclear physics, but also as a bridge across many disciplines. Although this retrospective is focused on atomic clusters, briefly discussed is the solvation phenomenon, a subject to which mass spectrometry has and continues to make a large contribution.

We have investigated the gas-phase reactivity of silver clusters with ethanethiol in a fast-flow tube reactor. The primary cluster products observed in this reaction are AgnSH(-) and AgnSH2(-), indicating C-S bond activation, together with interesting byproducts H3S(-) and (H3S)2(-). Agn(-) clusters with an odd number of valence electrons (n = even) were observed to be more reactive than those with an even number of electrons-a feature previously only observed in the reactivity of Agn(-) with triplet oxygen, indicating that radical active sites play a role in their reactivity. Furthermore, the reactivity dramatically increases with large flow rate of ethanethiol being introduced in the flow tube. Theoretical investigations on the reactivity of Ag13(-) and Ag8(-) with ethanethiol indicate that both Ag13(-) and Ag8(-) face significant barriers to reactivity with a single ethanethiol molecule. However, Ag8(-) reacts readily in a cooperative reaction with two ethanethiol molecules, consistent with the dramatic increase in reactivity with a large flow rate. Further hydrogen-transfer reactions may then release an ethylene molecule or an ethyl radical resulting in the observed AgnSH(-) species.

We have studied the size-selective reactivity of AlnBm(-) clusters m = 1,2 with O2 to investigate the effect of congener substitution in energetic aluminum clusters. Mixed-metal clusters offer an additional strategy for tuning the electronic and geometric structure of clusters and by substituting an atom with a congener; we may investigate the effect of structural changes in clusters with similar electronic structures. Using a fast-flow tube mass spectrometer, we formed aluminum boride cluster anions and exposed them to molecular oxygen. We found multiple stable species with Al12B(-) and Al11B2(-) being highly resistant to reactivity with oxygen. These clusters behave in a similar manner as Al13(-), which has previously been found to be stable in oxygen because of its icosahedral geometry and its filled electronic shell. Al13(-) and Al12B(-) have icosahedral structures, while Al11B2(-) forms a distorted icosahedron. All three of these clusters have filled electronic shells, and Al12B(-) has a larger HOMO-LUMO gap due to its compact geometry. Other cluster sizes are investigated, and the structures of the AlnB(-) series are found to have endohedrally doped B atoms, as do many of the AlnB2(-) clusters. The primary etching products are found to be a loss of two Al2O molecules, with boron likely to remain in the cluster.

Metal clusters featuring closed supershells or aromatic character usually exhibit remarkably enhanced stability in their cluster series. However, not all stable clusters are subject to these fundamental constraints. Here, by employing photoelectron imaging spectroscopy and ab initio calculations, we present experimental and theoretical evidence on the existence of unexpectedly stable open-shell clusters, which are more stable than their closed-shell and aromatic counterparts. The stabilization of these open-shell Al-Mg clusters is proposed to originate from the S-P molecular orbital coupling, leading to highly stable species with increased HOMO-LUMO gaps, akin to s-p hybridization in an organic carbon atom that is beneficial to form stable species. Introduction of the coupling effect highlighted here not only shows the limitations of the conventional closed-shell model and aromaticity but also provides the possibility to design valuable building blocks.

Based on the method of laser-ablation in liquids (LAL) with a strategy of bubbling nitrogen through a custom-made chamber, we prepared chemically-pure gold clusters which were found to be metastable for an extensive period of time beyond months. A practical use of discrimination among different surfaces is demonstrated here by applying the gold clusters as surface coatings which result in surface-enhanced Raman scattering (SERS) due to the surface plasmon resonance (SPR). This technique identifies various documents from different printers/copiers and written with different pen-inks. The stable and additive-free gold clusters enable repetitive examinations without impurity interference.

The electronic and structural properties of small monomagnesium oxide clusters, MgO(x)(-) and MgO(x) (x = 1-4), have been investigated using a synergistic approach combining photoelectron imaging spectroscopy and first principles electronic structure calculations. The adiabatic detachment energy (ADE) and vertical detachment energy (VDE) of MgO(x)(-) clusters along with the photoelectron angular distributions (PADs) are determined experimentally. The measured PADs of the clusters are dependent on both the orbital symmetry and electron kinetic energies. Density-functional theory (DFT) calculations were performed to explore the optimized geometries of neutral and anionic MgO(x) clusters. The theoretical ADE and VDE values calculated according to the optimized geometries are in good agreement with our experimental measurements. In addition, MgO(-) and MgO4 clusters are found to have enhanced relative stability in the corresponding anionic and neutral series, based on both theoretical parameters and the experimental cluster distribution.

We have studied the size-selective reactivity of Aln– clusters with formaldehyde to determine if carbonyl bonds may be broken by complementary active sites. Gas phase experiments reveal that Aln–, where n = 8–12, react with formaldehyde to form Aln–2CH2–, which demonstrates that the carbonyl bond is broken in the reaction with the cluster, while Al13– is found to be resistant to reaction. The most likely leaving group is determined to be Al2O. We also found n = 15–19 to be reactive with the products being a mix of Aln–2CH2– and Aln(OCH2)m–. Theoretical investigations find that the adjacent Lewis acid and Lewis base sites stabilize the resonance structure in which the carbonyl is reduced to a single bond which encourages carbonyl cleavage. A transition state analysis of the cleavage of the carbonyl bond confirms the size selective cleavage of the carbonyl bond and supports the importance of complementary Lewis acid–Lewis base active sites in governing the reactivity.

A femtosecond visible ring dye laser centered at 624 nm is used to probe the ionization dynamics of small neodymium and europium oxide clusters. Semi-classical atomic tunneling theory using sequential ionization potential values is compared to experimental ion signal and the gaps between the saturation intensities of each charge state are quantified. Differences in saturation intensities for the onset of ion signal between atomic charge states were found to increase as the metal–oxygen bond strength of the cluster increases. Neodymium showed larger enhancements in ionization whereas larger gaps in ion signal between europium charge states indicate more sequential-like ionization.

We report a joint experimental and theoretical investigation into the geometry, stability, and reactivity with oxygen of alloy metal clusters Al(n)Mg(m)(-) (4 ≤ n+m ≤ 15; 0 ≤ m ≤ 3). Considering that Al and Mg possess three and two valence electrons, respectively, clusters with all possible valence electron counts from 11 to 46 are studied to probe the magic numbers predicted by the spherical jellium model, and to determine whether enhanced stability and reduced reactivity may be found for some Al(n)Mg(m)(-) at non-magic numbers. Al5Mg2(-) and Al11Mg3(-) exhibit enhanced stability corresponding to the expected magic numbers of 20 and 40 electrons, respectively; while Al7Mg3(-), Al11Mg(-), and Al11Mg2(-) turn out to be unexpectedly stable at electron counts of 28, 36, and 38, respectively. The enhanced stability at non-magic numbers is explained through a crystal-field-like splitting of degenerate shells by the geometrical distortions of the clusters. Al(n)Mg(m)(-) clusters appear to display higher oxidation than pure Al(n)(-) clusters, suggesting that the addition of Mg atoms enhances the combustion of pure aluminum clusters.

Strong-field ionization and concomitant Coulomb explosion studies are performed on small (

The electronic structure of the diatomic VO anion was explored by combining the photoelectron-imaging spectroscopy and high-level theoretical calculations. The electron affinity (EA) of VO is determined to be 1.244 \ifmmode\pm\else\textpm\fi{} 0.025 eV from the vibrationally resolved photoelectron spectrum acquired at 532 nm. The anisotropy parameter (\ensuremath{\beta}) for the EA defined peak is measured to be 1.59 \ifmmode\pm\else\textpm\fi{} 0.02, indicating that it is the 9\ensuremath{\sigma} electron attachment leading to the formation of the ground state of $\mathrm{V}{\mathrm{O}}^{\ensuremath{-}}$. The present imaging experiment provides direct evidence that the ground state of $\mathrm{V}{\mathrm{O}}^{\ensuremath{-}}$ is $X\phantom{\rule{0.16em}{0ex}}{}^{3}{\mathrm{\ensuremath{\Sigma}}}^{\ensuremath{-}}$ with a ${(3\ensuremath{\pi})}^{4}{(8\ensuremath{\sigma})}^{2}{(9\ensuremath{\sigma})}^{2}{(1\ensuremath{\delta})}^{2}$ electron configuration, which resolves the significant discrepancy in previous experiment and theory. In addition, the molecular orbitals and bonding involved in the anionic VO cluster are also examined based on the present high-level theoretical calculations.

In this gas-phase photoelectron spectroscopy study, we present the electron binding energy spectrum and photoelectron angular distributions of NbN- by the velocity-map imaging technique. The electron binding energy of NbN- is measured to be 1.42 +/- 0.02 eV from the X band maximum which defines the 0-0 transition between ground states of anion and neutral. Theoretical binding energies which are the vertical and adiabatic detachment energies are computed by density functional theory to compare them with experiment. The ground state of NbN- is assigned to the (2)Delta(3/2) state and then the electronic transitions originating from this state into X-3 Delta(Omega) (Omega = 1-3), a(1)Delta(2), A(3)Sigma(-)(1), and b(1)Sigma(+)(0) states of NbN are reported to interpret the spectral features. As a prospective study for catalytic materials, spectral features of NbN- are compared with those of isovalent ZrO- and Pd-. Published by AIP Publishing.

Small methane clusters (CH4)n are irradiated using intense femtosecond laser excitation at 624 nm. The ionized species and those resulting from their fragmentation are detected via time-of-flight mass spectrometry (TOF MS). We find evidence of bound, multiply charged methane molecules and clusters resulting from Coulomb explosion upon exposure to highly energetic, ultrafast radiation. The assignment of the mass spectra is done after first-principles calculations (at the (R)MP2/aug-cc-pVXZ (X = D,T) level) on the charged (CH4)n(q+) clusters (n = 1-4, q = 1-4). We also considered the cluster stabilities and fragments that may result from intracluster molecular reactivity. Complex intracluster ion-molecule reactions induced by photoionization are expected to occur. Interestingly, we show that multi charged small methane clusters undergo intracluster reactions and fragmentations which are different from those observed for isolated methane ions or for large ionized methane clusters.

Spin accommodation is demonstrated to play a determining role in the reactivity of silver cluster anions with oxygen. Odd-electron silver clusters are found to be especially reactive, while the anionic 13-atom cluster exhibits unexpected stability against reactivity with oxygen. Theoretical studies show that the odd-even selective behavior is correlated with the excitation needed to activate the O-O bond in O(2). Furthermore, by comparison with the reactivity of proximate even-electron clusters, we demonstrate that the inactivity of Ag(13)(-) is associated with its large spin excitation energy, ascribed to a crystal-field-like splitting of the orbitals caused by the bilayer atomic structure, which induces a large gap despite not having a magic number of valence electrons.

Growth and ionization patterns of small silicon clusters are studied using ultrafast pulses centered at 624 nm by varying the metal electron source for cluster formation using group 10 transition metals. The silicon-cluster size was observed to change as the electron source was varied from Pd

Strong-field ionization and subsequent Coulomb explosion of small group 5 metal carbide clusters are explored using ultrashort pulses centered at 624 nm. More efficient Coulomb explosion was observed according to: Ta > Nb > V due to the larger mass of tantalum, the slower cluster expansion times, and lower ionization potentials of large atoms. Minimum laser intensities required for the onset of each atomic charge state of V, Nb, and Ta were found to be nearly identical between metal carbides and previously observed group 5 metal oxide clusters indicating electron delocalization within the cluster. Ionization enhancement is explored by comparison to semi-classical tunneling theory.

Understanding the emergence of properties from the size-selective cluster regime to larger nanoparticles is one of the principal goals of nanoscience. We have measured the size-selective reactivity of aluminum cluster anions with alcohols. All clusters with more than 20 atoms are found to be reactive, while Al11(-), Al13(-), and Al20(-) show enhanced resistance to oxidation at smaller sizes. The reactivity of aluminum cluster anions with water, methanol, and tert-butyl alcohol all exhibit patterns that require complementary active sites (Lewis acid, Lewis base) on adjacent atoms. Theoretical investigations reveal that at small sizes, the location of reactive pairs occurs on specific active sites, but at larger sizes the reactive pairs begin to accumulate on the edges between facets, marking the transition from the nonscalable size-dependent regime to the scalable regime where the nanoparticles are universally reactive.

We report a surface-enhanced Raman spectroscopy (SERS) investigation to probe the adsorption and dynamic behavior of rhodamine 6 G (Rh6G) molecules on spherical Ag nanoparticles which were produced via laser ablation in liquid. Assembly of the colloidal Ag nanoparticles on a cover glass was used to work as SERS substrates on which high-quality SERS spectra of Rh6G were obtained with interesting time dependence when using low and ultralow concentrations, respectively. The variation of SERS spectra over time was identified with the adsorption behavior of multiple and individual molecules on the Ag nanoparticles. Analysis indicates that the adsorbed Rh6G molecules can desorb away from the initial locations on the substrate under continuous laser excitation; simultaneously, some individual molecules can move and become trapped in the gap between the aggregated Ag nanoparticles. These investigations help to clarify the origins of forming ‘hot-spots’ which host probe molecules and hence improve the understanding of mechanisms for single-molecule SERS spectroscopy. Copyright © 2012 John Wiley & Sons, Ltd.

Reported herein are strong-field ionization studies of small, neutral Pd(x)O(y) and Zr(x)O(y) clusters made using ultrafast laser pulses (~100 fs) centered at 624 nm. An enhancement in ionization of nearly 1.5 orders of magnitude lower in laser intensity than predicted from literature values is observed for both systems due to clustering. The change in enhancement upon addition of carbon monoxide at different pressures was also studied. Enhancement of high charge states of palladium was found to decrease upon CO addition, whereas in the case of the zirconium system, high charge states of zirconium were observed to increase. Pd and ZrO showed similar reactivity trends with CO and were found to have similar reactivity ratios in accord with their isovalent nature.

We investigated the halogenation reactivity of copper cluster anions produced via a magnetron-sputter source after introduction into a fast-flow tube reaction apparatus simultaneously with chlorine gas. Interesting cluster products corresponding to [Cu(n)Cl(n+1)](-) (n = 1-6) were observed with notable stability, and the mass distribution of these clusters exhibits an exponential decay with increasing values of n. Reaction kinetics analysis is provided on the gas-phase reactivity of copper cluster anions with chlorine. First-principle calculations suggest a series of cubic-like structures for these species similar to the structure of alkali halide clusters due to their similar electronic configurations. These structures act as a starting point in the formation of ionic crystals.

A magnetron-sputtering (MagS) cluster source was used to produce metal clusters of different size distributions by varying individual source parameters. Selectivity of the size of the Al clusters was observed and the mass distribution presents wide-range controllability by the MagS-source, which enables experimental determination of the growth process of Al clusters. By implementing the extended Smoluchowski rate equation, here we propose an interpretation of the cluster-growth kinetics in the gas phase environment. A collision-dependent growing time domain is demonstrated.

The structure and reactivity properties of a series of clusters ZrxO2x+1+ (x = 1–3) are investigated using DFT calculations and guided-ion-beam mass spectrometry. Calculations determine the cluster...

Although early transition metals are too reactive to serve as effective combustion catalysts, it has been proposed that combining them with nonmetallic elements may moderate the reactivity and allow them to replace more expensive catalysts. To test this hypothesis, we use ZrO+ as our replacement and compare the reactivity with Pd+, in a synergistic study employing gas-phase guided-ion-beam mass spectroscopy in conjunction with theoretical investigations. The findings reveal similar reactivity patterns for both ethane and propane with the scission of a C–C bond being the primary reaction mechanism for the interaction of both gases with ZrO+ and Pd+. Hydrogen abstraction is also observed with similar branching ratios for both ions. It is proposed that the similar reactivity is due to ZrO+ having molecular orbitals with 4d components that participate in the reaction in a similar way as the 4d orbitals of Pd+.

Recently, it was discovered that specific aluminum clusters (e.g., Al13–) that demonstrate enhanced resistance to reactivity with oxygen may do so not only because of a closed electronic jellium shell as originally supposed but also because of a forbidden spin-flip in the transition state of the reaction. Herein, we discuss an experiment using a multiple-species laminar flow reaction vessel coupled to a singlet oxygen generator. The present results suggest that all clusters react with singlet oxygen. Additionally, we observe Al9–, a cluster previously unidentified as having any notable stability, as being resistant to reaction with triplet oxygen. Furthermore, we discuss a means of estimating rate constants in a multiple-species flow tube where the products and reactants do not allow the use of traditional methods.

In this work, we report experimental results from the strong-field ionization and subsequent Coulomb explosion of narrow distributions of small (40 atoms) heteronuclear clusters composed of transition metal (Ti, V, Cr, Nb, or Ta) and carbon atoms. Analysis of the resulting multiply charged ions was performed through time-of-flight mass spectrometry, and evidence regarding ionization dynamics was obtained. The data reveal the presence of enhanced ionization during exposure to the ultrashort (∼100 fs) pulse resulting in the formation of ions possessing significantly higher charge states than those predicted from atomic species. Regardless of the transition metal species, we observe the absorption of similar amounts of energy from the external field, as indicated by the maximum observed charge states in each experiment. These results are compared to our previously reported study on the strong-field ionization of transition metal oxide clusters. We observe identical maximum observable charge states for each of the transition metal species resulting from both metal oxide and metal carbide clusters.

Cluster science has given rise to the possibility of forming “superatoms”, species that mimic elements of the periodic table but often display multiple functionalities. The ability to tailor the properties of these species opens up a new approach to forming nanoscale materials from the bottom up via cluster assembly. Recent success in designing these superatoms composing a “3D periodic table” and understanding the fundamentals governing their properties and stability are discussed, as well as prospects for the future of this field.

Small aluminum clusters (

We present the results of a systematic experimental approach utilizing time-of-flight mass spectrometry (TOF-MS) to investigate changes in maximum ionization states as the result of femtosecond pulse ionization. Strong-field gas-phase ionization experiments were performed on well characterized distributions of small early transition metal oxide (MxOy) clusters, where M = Ti, V, Cr, Nb, or Ta, created as a molecular beam via supersonic expansion. Utilizing high resolution TOF-MS enabled the observation of highly charged ions resulting from ionization and subsequent fragmentation via Coulomb explosion. Maximum charge states for each cluster distribution are reported as observed under various ionization conditions. Evidence for direct correlation with previously reported ionization energies of the component species is presented. Regardless of metal composition, it is observed that every cluster distribution gains approximately the same amount of energy from the external field. The extreme ionization of the targeted clusters produced ions of Ti10+, V9+, Cr8+, Nb11+, Ta11+, and O6+, requiring up to 216 eV of energy to create these maximum charge states via sequential ionization. Several enhanced ionization models are discussed with relation to our experimental data. Systematic investigations on the influence of cluster size and ionization laser pulse width indicate enhanced ionization contributions from the ionization ignition and charge-resonance enhanced ionization mechanisms.

The electronic structure and stability of three gas phase heteroatomic clusters, Ga 2 Bi 3 - , In 2 Bi 3 - , and In 2 Sb 3 - , which are isoelectronic with the Zintl ion Sn 5 2 - , are examined using photoelectron spectroscopy and first-principles theoretical investigations. While all the isoelectronic clusters are stable with high adiabatic detachment energies, the HOMO–LUMO gap, absolute stability and the relative stability of the isomers depend on the atomic size and point of substitution. Theoretical analysis reveals the variations are attributable to atomic size differences, which affect covalent bonding within the cluster. The studies offer a strategy for controlling properties of clusters that might be incorporated into cluster-assembled materials.

The electronic structure of two particularly stable lead–indium clusters, namely PbIn3- and Pb2In2, has been studied by photoelectron spectroscopy and first-principles electronic structure calculations. Our studies reveal an enhanced stability of both PbIn3- and Pb2In2 clusters that can be classified within ‘all-metal aromaticity’. These clusters possess large gaps between the highest occupied and lowest unoccupied molecular orbitals (1.34 and 1.45 eV, respectively). Further, we show that despite the difference in electronegativities for lead and indium, the aromatic behavior is retained, and in the case of Pb2In2, aromaticity remains even after changes to the atomic arrangement of the planar cluster.

The stability and electronic properties of anionic and neutral PbxIny clusters containing up to 5 Pb and up to 7 In atoms have been investigated using negative ion photodetachment spectroscopy along with first-principles electronic structure studies within a gradient corrected density functional approach. Through studies of the detachment energies, gaps in the electronic spectrum, variations in binding energy, and nature of the electronic states, two families of stable species are identified. PbIn3−, Pb2In2, and Pb3In2 exhibit enhanced stability compared to their neighbors and the stability is linked to the aromatic character identified in their molecular orbitals. On the other hand, PbIn5− and Pb2In4 exhibit enhanced stability associated with filled electronic shells within a confined nearly free electron gas.

We present the results of photoelectron velocity-map imaging experiments for the photodetachment of small negatively charged ammonia solvated Bi(n) and Pb(n) (n = 1, 2) clusters at 527 nm. The vertical detachment energies of the observed multiple electronic bands and their respective anisotropy parameters for the solvated Bi and Pb anions and clusters derived from the photoelectron images are reported. The electronic bands of Bi(NH(3))(n=1,2) are distinct from the Bi metal ion in exhibiting a perpendicular distribution whereas the electronic bands in Pb(NH(3))(n=1,2), unlike the Pb anion, show an isotropic distribution with respect to the laser polarization. Density-functional theory calculations with a generalized gradient approximation for the exchange-correlation potential were performed on these clusters to determine their atomic and electronic structures. Calculated geometries show a dramatic change between anionic and neutral ammonia solvated Bi and Pb species. Anionic clusters exhibit van der Waals interactions between the hydrogen atoms of ammonia and the metal core, where it was determined that the negative charge is localized. Neutral clusters, on the other hand, present a covalent bond between the nitrogen atom of ammonia and the metal core. Calculated binding energies show an enhancement in the bonding of the (NH(3))(2) dimer in the presence of the anionic Bi(1,2)(-) and Pb(1,2)(-) metal ions. This is rationalized by the electrostatic interaction between the negative charged metal core and the hydrogen atoms of the ammonia molecule.

The electronic structure and stability of neutral and negatively charged BixIny (x = 1−4, y = 1−6) clusters are investigated through anionic photoelectron spectroscopy employing magnetic bottle and photoelectron velocity map imaging experiments. Experimental and theoretical adiabatic and vertical detachment energies of the anionic species containing up to 4 Bi and 4 In atoms are deduced from first principles calculations. Among the BixIny series, many clusters are found to exhibit special stability in the mass spectra, exhibit a large gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO−LUMO gap), and a large formation energy. This stability is rationalized by different mechanisms. Bi2In− is classified as a gas phase Zintl species despite only having three atoms, making it the smallest possible case. Bi3In2−, with 12 valence electrons and a closo structure in agreement with Wade’s rule, is similar to Bi3Ga2−, a gas phase Zintl analogue of Sn52−. Bi4In− and Bi4In2 are both found ...

The reactivity of aluminum anion clusters with water was found to exhibit variations with size, with some clusters exhibiting negligible reactivity, others absorbing one or more water, while even others releasing H(2) with addition of multiple waters. (Roach, P.J., Woodward, W.H. et al. Science, 2009, 323, 492). Herein, we provide further details on the role of complementary active sites in the breaking of the O-H bond on the cluster. We examine the reactions of Al(n)(-) + H(2)O where n = 7-18, and show how the complementary active sites may be best identified. The clusters with active sites are found to be reactive, and clusters with barriers to reactivity have an absence of paired active sites. The role of charge in the reactivity is considered, which could account for the observed increase in reactivity at large sizes. The H(2) release in the reactivity of Al(17)(-) with multiple water molecules is also studied by comparing multiple reaction pathways, and the selective H(2) production is explained by the first water inducing a new active site. A mechanism for transferring hydroxyl groups on the surface of the cluster is also discussed.

Photoelectron imaging experiments for the negative ions, Pt − and WC − at a photon energy of 2.33 eV, are highlighted. In the Pt − photoelectron angular distribution (PAD), intense signatures from the electron affinity (EA) transition 3 D 3 ← 2 D 5/2 , as well as several features from the excited states of Pt − , 2 D 3/2 and 2 S 1/2 , were observed. Present in the PAD of the valence isoelectronic diatomic molecule WC − , are several transitions originating from the 2 Δ 3/2 ground and higher energy 2 Δ 5/2 and 4 Σ − excited states. Significantly, the spectroscopic transitions arising from WC − exhibit similarities with those in Pt − , providing insight into the reactivity of WC and Pt surfaces. Discussion regarding the photoelectron spectra of several other isoelectronic diatomic-elemental species further reveals quantitative aspects of the superatom concept. The recent surprising findings in this field are opening a new realm of studies pertaining to the behavior of matter governed by its unified atom/superatomic character.

Results are presented of a high-resolution time-of-flight mass spectrometric study on large silicon hydride cluster anions, Si n H x − (where n = 8–12, 0 ≤ x ≤ 25). The clusters were produced via the reactions between a dilute silane gas mixture and a laser-induced transition metal plasma. Extensive degrees of hydrogenation were observed for larger Si n − clusters, with an onset appearing around Si 8 − and continuing on into the Si 12 − series. The relative abundances of the majority isotopes constituting a precise mass were completely analyzed allowing qualitative assessments of the stability for a particular Si n H x − cluster to be ascertained. The Si n H x − clusters are observed to possess varying abundances depending on the extent of hydrogenation and the number of silicon atoms constituting the cluster. Specific clusters, which exhibited enhanced stability in the molecular beam, are Si 8 H 6 − , Si 8 H 8 − , Si 9 H 5 − , Si 11 H 2 − and Si 12 H 2 − , respectively. In general, the lighter Si n H x − ( n = 8–10) clusters were dominated by high relative abundances of the pure silicon cluster parent, Si 8 – 10 − , and the singly hydrogenated cluster, Si 8 – 10 H − . A pronounced decrease in intensity was then noted with one additional H adsorption, forming the di-hydride species, Si 8 – 10 H 2 − . The exact converse was observed in the heavier silicon hydride series, Si n H x − ( n = 11–12), where the pure silicon cluster parent, Si 11 – 12 − , and single hydrogenation species, Si 11–12 H − are suppressed by a substantial production of an especially stable Si 11 – 12 H 2 − di-hydride moiety.

The electronic structure of Al(n)X (n = 1-6; X = As, Sb) clusters has been investigated using a synergistic approach combining negative ion photoelectron spectroscopy and first principles electronic structure calculations. It is shown that Al(3)X and Al(5)X exhibit enhanced energetic stability, as evidenced from calculated removal and embedding energies as well as chemical stability manifested through a large gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). However, the stabilities of these species are derived from different mechanisms. Al(3)As and Al(3)Sb, with HOMO-LUMO gaps of 1.86 and 1.73 eV, respectively, are shown to have planar geometries where the p orbitals combine to form one pi and two sigma aromatic orbitals reminiscent of conventional all-metal aromatic species. Al(5)As and Al(5)Sb, with 20 valence electrons, possess a closed electronic shell (1s(2), 1p(6), 1d(10), 2s(2)) within a jellium framework and have HOMO-LUMO gaps of 1.12 and 1.17 eV, respectively.

Detailed in the present investigation are results pertaining to the photoelectron spectroscopy of negatively charged atomic ions and their isoelectronic molecular counterparts. Experiments utilizing the photoelectron imaging technique are performed on the negative ions of the group 10 noble metal block (i.e. Ni - , Pd - , and Pt - ) of the periodic table at a photon energy of 2.33 eV (532 nm). The accessible electronic transitions, term energies, and orbital angular momentum components of the bound electronic states in the atom are then compared with photoelectron images collected for isoelectronic early transition metal heterogeneous diatomic molecules, M-X - (M = Ti,Zr,W; X = O or C). A superposition principle connecting the spectroscopy between the atomic and molecular species is observed, wherein the electronic structure of the diatomic is observed to mimic that present in the isoelectronic atom. The molecular ions studied in this work, TiO - , ZrO - , and WC - can then be interpreted as possessing superatomic electronic structures reminiscent of the isoelectronic elements appearing on the periodic table, thereby quantifying the superatom concept.

Titanium carbide nanoparticles were synthesized by flowing methane through a plasma generated from an arc discharge between two titanium electrodes. Different methane concentrations were employed in studies made to investigate the effects of carbon concentration on particle morphology. Transmission electron microscopy and X-ray diffraction were used to investigate the synthesized TiC nanopowders, whereupon it was found that nanocrystalline TiC nanoparticles prefer a cubic morphology at low concentrations of methane and a cuboctahedron morphology at high concentration of methane. The change in particle morphology is attributed to carbon affecting the relative growth rates of the {111} and {100} facets on a TiC seed crystal.

Evidence obtained by experiments in the gas phase and density functional theory calculations indicates that differences in the CO binding energy to the transition metal site and the overall change in energy and spin multiplicity from reactants to products, underlie the variation in relative reactivity observed for the oxidation of CO by anionic transition metal oxide clusters. Experimental reactivity studies of MO2− and M2O3− (M = Fe, Co, Ni, and Cu) reveal that anionic oxide clusters with the same number of atoms and stoichiometry but different elemental composition exhibit specific trends in relative oxidation reactivity with CO. The anionic MO2− and M2O3− clusters are found to be most reactive for M = Fe and Cu and relatively less reactive for M = Co and Ni. Theoretical electronic structure studies within the density functional theory framework indicate that the most reactive clusters either have relatively large initial binding energies of CO to the cluster which provide sufficient energy to overcome ...

In this report, we apply the concept of delocalization of electrons in a chain structure to an all-metal system using experimental (photoelectron spectroscopy, velocity map imaging) and first principles theoretical investigations. We show that In 2 Bi − is a triatomic cluster anion with a bent structure, where bismuth is the bridging atom between the two indium atoms. It is found that 6p-electrons (2π and 4σ) are delocalized over the cluster. The In–Bi bond lengths in In 2 Bi − are identical to bond lengths of In 3 Bi, which is an all-metal aromatic system analogue to Al 3 Bi, and are shorter than the rest of In x Bi (−) clusters, showing the presence of a higher bond order.

Dinuclear gold(III) clusters with a rhombic Au(2)O(2) core and 2,2'-bipyridyl ligands substituted in the 6-position (bipy(R)) are examined by tandem mass spectrometry. Electrospray ionization of the hexafluorophosphate salts affords the complexes [(bipy(R))Au(mu-O)(2)Au(bipy(R))](2+) as free dications in the gas phase. The fragmentation behavior of the mass-selected dications is probed by means of collision-induced dissociation experiments which reveal an exceptionally pronounced effect of substitution. Thus, for the parent compound with R = H, i.e., [(bipy)Au(mu-O)(2)Au(bipy)](2+), fragmentation at the dicationic stage prevails to result in a loss of neutral H(2)O concomitant with an assumed rollover cyclometalation of the bipyridine ligands. In marked contrast, all complexes with alkyl substituents in the 6-position of the ligands (bipy(R) with R = CH(3), CH(CH(3))(2), CH(2)C(CH(3))(3), and 2,6-C(6)H(3)(CH(3))(2)) as well as the corresponding complex with 6,6'-dimethyl-2,2'-dipyridyl as a ligand exclusively undergo Coulomb explosion to produce two monocationic fragments. It is proposed that the additional steric strain introduced to the central Au(2)O(2) core by the substituents on the bipyridine ligand, in conjunction with the presence of oxidizable C-H bonds in the substituents, crucially affects the subtle balance between dication dissociation under maintenance of the 2-fold charge and Coulomb explosion into two singly charged fragments.

During the course of investigating the solvation and photochemical dynamics of HI(H 2 O) n clusters employing a pump–probe technique, we observed unusual behavior associated with iodine atoms dissociating from the HI(H 2 O) n clusters, displaying different time responses than for the case of the HI monomer or HI clusters. Further experimentation provided experimental evidence for a theoretically predicted biradical species which consists of a halo-acid clustered to water molecules. In addition, as a side issue, we discuss our results in conjunction with others that point to a possible alternative explanation compared to the traditionally accepted one for the phenomena known as the solvated electron.

Gas-phase cluster reactivity experiments, combined with theoretical calculations, provide insight into how factors such as size, stoichiometry and charging effects influence reactions promoted by metal and metal oxide catalysts. In this Letter, we describe the use of clusters to investigate the role of different types of oxygen centers in catalytic oxidation reactions. First, we focus on recent results from our laboratory concerning the oxidation of carbon monoxide by gold clusters and how the charge state of the cluster determines the mechanism of oxidation. We then discuss how radical oxygen centers in zirconium oxide clusters promote the oxidation of carbon monoxide, ethylene and acetylene and how charge state may be used to tune the selectivity of these reactions. An overview of future directions for the research is given in conclusion.

Evidence obtained by guided-ion-beam mass spectrometry experiments and density functional theory calculations indicates that by adding one oxygen atom with a full octet of valence electrons (O(2-)) to stoichiometric cationic zirconium oxide clusters (ZrO(2))(x)(+) (x = 1-4), a series of anionic clusters (Zr(x)O(2x+1))(-) (x = 1-4) are formed which contain radical oxygen centers with elongated (elongation approximately 0.24 +/- 0.02 A) metal-oxygen bonds. These anionic clusters oxidize carbon monoxide, strongly associate acetylene, and weakly associate ethylene, in contrast to the cationic species which were found previously to be highly active toward the oxidation of all three molecules. Theoretical investigations indicate that a critical hydrogen transfer step necessary for the oxidation of ethylene and acetylene at metal oxide clusters containing radical oxygen centers is energetically favorable for cationic clusters but unfavorable for the corresponding anionic species. The calculated electrostatic potential of the cluster reveals that in the case of cations, a favorable interaction with nucleophilic molecules takes place over the whole surface of the (ZrO(2))(x)(+) (x = 1-4) clusters, compared to a restricted interaction of ethylene and acetylene with the less coordinated zirconium atom in the case of the anionic (Zr(x)O(2x+1))(-) (x = 1-4) species. Therefore, in spite of the common presence of a radical oxygen center in specific anionic and cationic stoichiometries, the extent to which various classes of reactions are promoted is influenced by charge state. Moreover, the (Zr(x)O(2x+1))(-) (x = 1-4) series of anionic clusters may be regenerated by reacting oxygen deficient clusters with a strong oxidizer. This indicates that not only cationic species, as shown previously, but also anionic clusters may promote multiple cycles of carbon monoxide oxidation.

The collision induced fragmentation and reactivity of cationic and anionic nickel oxide clusters with carbon monoxide were studied experimentally using guided-ion-beam mass spectrometry. Anionic clusters with a stoichiometry containing one more oxygen atom than nickel atom (NiO 2 − , Ni 2 O 3 − , Ni 3 O 4 − and Ni 4 O 5 − ) were found to exhibit dominant products resulting from the transfer of a single oxygen atom to CO, suggesting the formation of CO 2 . Of these four species, Ni 2 O 3 − and Ni 4 O 5 − were observed to be the most reactive having oxygen transfer products accounting for approximately 5% and 10% of the total ion intensity at a maximum pressure of 15 mTorr of CO. Our findings, therefore, indicate that anionic nickel oxide clusters containing an even number of nickel atoms and an odd number of oxygen atoms are more reactive than those with an odd number of nickel atoms and an even number of oxygen atoms. The majority of cationic nickel oxides, in contrast to anionic species, reacted preferentially through the adsorption of CO onto the cluster accompanied by the loss of either molecular O 2 or nickel oxide units. The adsorption of CO onto positively charged nickel oxides, therefore, is exothermic enough to break apart the gas-phase clusters. Collision induced dissociation experiments, employing inert xenon gas, were also conducted to gain insight into the structural properties of nickel oxide clusters. The fragmentation products were found to vary considerably with size and stoichiometry as well as ionic charge state. In general, cationic clusters favored the collisional loss of molecular O 2 while anionic clusters fragmented through the loss of both atomic oxygen and nickel oxide units. Our results provide insight into the effect of ionic charge state on the structure of nickel oxide clusters. Furthermore, we establish how the size and stoichiometry of nickel oxide clusters influences their ability to oxidize CO, an important reaction for environmental pollution abatement.

The reactions of metal clusters with small molecules often depend on cluster size. The selectivity of oxygen reactions with aluminum cluster anions can be well described within an electronic shell model; however, not all reactions are subject to the same fundamental constraints. We observed the size selectivity of aluminum cluster anion reactions with water, which can be attributed to the dissociative chemisorption of water at specific surface sites. The reactivity depends on geometric rather than electronic shell structure. Identical arrangements of multiple active sites in Al 16 – , Al 17 – , and Al 18 – result in the production of H 2 from water.

The physical and chemical properties of cluster systems at the subnano and nanoscale are often found to differ from those of the bulk and display a unique dependence on size, geometry, and composition. Indeed, most interesting are systems which have properties that vary discontinuously with the number of atoms and composition, rather than scale linearly with size. This realm of cluster science where “one atom makes a difference” is undergoing an explosive growth in activity, and as a result of extensive collaborative activities through theory at VCU and experiment at PSU, our groups are recognized as pioneers in this area in which we have been active for many years. Herein we provide an overview of the field with primary focus on our joint undertakings which have spawned the superatom concept, giving rise to a 3-D periodic table of cluster elements and the prospect of using these as building blocks of new nanoscale materials with tailored properties.