Adsorption and decomposition of dimethyl methylphosphonate on size-selected (MoO3)3 clusters.

The adsorption and decomposition of dimethyl methylphosphonate (DMMP), a chemical warfare agent (CWA) simulant, on size-selected molybdenum oxide trimer clusters, i.e. (MoO3)3, was studied both experimentally and theoretically. X-ray photoelectron spectroscopy (XPS), temperature programmed reaction (TPR), and density functional theory (DFT)-based simulations were utilized in this study. The XPS and TPR results showed both, desorption of intact DMMP, and decomposition of DMMP through the elimination of methanol at elevated temperatures on (MoO3)3 clusters. Theoretical investigation of DMMP on (MoO3)3 clusters suggested that, in addition to pure (MoO3)3 clusters, reduced molybdenum oxide clusters and hydroxylated molybdenum oxide clusters also play an important role in decomposing DMMP via a "reverse Mars-van Krevelen mechanism". The present study, which focused on oxide clusters, underlines the importance of surface defects, e.g., the oxygen vacancies and surface hydroxyls, in determining the reaction pathway of DMMP, in agreement with previous studies on thin films. In addition, the structural fluxionality and the Lewis acidity of molybdenum oxide clusters, i.e. (MoO3)3, may make them good candidates for adsorption and decomposition of chemical warfare agents with similar structures to DMMP.

Low oxidation state aluminum-containing cluster anions: LAlH- and LAln- (n = 2-4, L = N[Si(Me)3]2).

Several low oxidation state aluminum-containing cluster anions, LAlH- and LAln- (n = 2-4, L = N[Si(Me)3]2), were produced via reactions between aluminum hydride cluster anions, AlxHy-, and hexamethyldisilazane (HMDS). These clusters were characterized by mass spectrometry, anion photoelectron spectroscopy, and density functional theory (DFT) based calculations. Agreement between the experimental and theoretical vertical detachment energies (VDEs) and adiabatic detachment energies (ADEs) validated the computed geometrical structures. Reactions between aluminum hydride cluster anions and ligands promise to be a new synthetic scheme for low oxidation state, ligated aluminum clusters.

Photoelectron spectroscopic and computational study of the PtMgH3,5(-) cluster anions.

The two cluster anions, PtMgH3(-) and PtMgH5(-), were studied by photoelectron spectroscopy and theoretical calculations. Experimentally-determined electron affinity (EA) and vertical detachment energy (VDE) values were compared with those predicted by our computations; excellent agreement was found. The calculated structures of PtMgH3(-) and PtMgH3 both exhibit η2-bonded H2 moieties. Activation of these H2 moieties is implied by the elongation of their bond lengths relative to the bond length of free H2. The calculated structures of PtMgH5(-) and PtMgH5 both exhibit all-hydrogen, five-member, σ-aromatic rings. These attributes are responsible for this anion's special stability.

Excited state nonadiabatic dynamics of bare and hydrated anionic gold clusters Au3(-)[H2O]n (n = 0-2).

We present a joint theoretical and experimental study of excited state dynamics in pure and hydrated anionic gold clusters Au3(-)[H2O]n (n = 0-2). We employ mixed quantum-classical dynamics combined with femtosecond time-resolved photoelectron spectroscopy in order to investigate the influence of hydration on excited state lifetimes and photo-dissociation dynamics. A gradual decrease of the excited state lifetime with the number of adsorbed water molecules as well as gold cluster fragmentation quenching by two or more water molecules are observed both in experiment and in simulations. Non-radiative relaxation and dissociation in excited states are found to be responsible for the excited state population depletion. Time constants of these two processes strongly depend on the number of water molecules leading to the possibility to modulate excited state dynamics and fragmentation of the anionic cluster by adsorption of water molecules.

Photoelectron spectroscopic study of carbon aluminum hydride cluster anions.

Numerous previously unknown carbon aluminum hydride cluster anions were generated in the gas phase, identified by time-of-flight mass spectrometry and characterized by anion photoelectron spectroscopy, revealing their electronic structure. Density functional theory calculations on the CAl5-9H- and CAl5-7H2- found that several of them possess unusually high carbon atom coordination numbers. These cluster compositions have potential as the basis for new energetic materials.

Low oxidation state aluminum-containing cluster anions: Cp(∗)AlnH(-), n = 1-3.

Three new, low oxidation state, aluminum-containing cluster anions, Cp*AlnH(-), n = 1-3, were prepared via reactions between aluminum hydride cluster anions, AlnHm (-), and Cp*H ligands. These were characterized by mass spectrometry, anion photoelectron spectroscopy, and density functional theory based calculations. Agreement between the experimentally and theoretically determined vertical detachment energies and adiabatic detachment energies validated the computed geometrical structures. Reactions between aluminum hydride cluster anions and ligands provide a new avenue for discovering low oxidation state, ligated aluminum clusters.

CO2 Activation and Hydrogenation by PtHn (-) Cluster Anions.

Gas phase reactions between PtHn (-) cluster anions and CO2 were investigated by mass spectrometry, anion photoelectron spectroscopy, and computations. Two major products, PtCO2 H(-) and PtCO2 H3 (-) , were observed. The atomic connectivity in PtCO2 H(-) can be depicted as HPtCO2 (-) , where the platinum atom is bonded to a bent CO2 moiety on one side and a hydrogen atom on the other. The atomic connectivity of PtCO2 H3 (-) can be described as H2 Pt(HCO2 )(-) , where the platinum atom is bound to a formate moiety on one side and two hydrogen atoms on the other. Computational studies of the reaction pathway revealed that the hydrogenation of CO2 by PtH3 (-) is highly energetically favorable.

Thin films of size-selected Mo clusters: growth modes and structures.

Thin films of MoO3 were prepared by deposition of size-selected ligand-free Mo clusters under high vacuum conditions and subsequent exposure to air. The growth pattern is highly dependent on the cluster size. At low coverage, small clusters (Mo51) form a continuous monolayer of fused particles. On top of this monolayer, additional clusters survive as individual entities. Medium sized clusters (Mo251 and Mo1253) do not coalesce and form a monolayer of clusters. Close examination using in situ scanning tunneling microscopy reveals a local order of the particles. At higher coverage a new pattern of large 3-dimensional aggregations of clusters (pylons) appears. The pylons are not formed under high vacuum conditions. Their formation is most likely caused by the air exposure. For the largest clusters (Mo3349) studied here, no monolayer is formed. Instead, the clusters are randomly distributed as expected for particles with zero mobility. These results demonstrate the high potential of cluster deposition for the production of new types of nanostructured surfaces, thin films and nanomaterials.

Photoelectron spectroscopic and theoretical study of the [HPd(η(2)-H2)](-) cluster anion.

Anion photoelectron spectroscopic and theoretical studies were conducted for the PdH(-) and PdH3 (-) cluster anions. Experimentally observed electron affinities and vertical detachment energies agree well with theoretical predictions. The PdH3 (-) anionic complex is made up of a PdH(-) sub-anion ligated by a H2 molecule, in which the H-H bond is lengthened compared to free H2. Detailed molecular orbital analysis of PdH(-), H2, and PdH3 (-) reveals that back donation from a d-type orbital of PdH(-) to the σ* orbital of H2 causes the H-H elongation, and hence, its activation. The H2 binding energy to PdH(-) is calculated to be 89.2 kJ/mol, which is even higher than that between CO and Pd. The unusually high binding energy as well as the H-H bond activation may have practical applications, e.g., hydrogen storage and catalysis.

Parallel deposition of size-selected clusters: a novel technique for studying size-selectivity on the atomic scale.

A new size-selected cluster deposition technique referred to as "parallel-deposition" is presented. An ion beam of multi-sized Aun clusters was spatially separated into individual cluster sizes by utilizing a Wien filter and the clusters spatially separated based on their atomic sizes were simultaneously deposited on a SiO2/Si(100) substrate. Parallel-deposited Aun clusters (n = 6, 7, and 8) on the SiO2/Si(100) substrate showed even-odd oxidation behaviour upon exposure to an atomic oxygen atmosphere, demonstrating the potential of this new technique to study the size-dependent properties of deposited clusters in various research fields.

The PtAl(-) and PtAl2(-) anions: theoretical and photoelectron spectroscopic characterization.

We report a joint photoelectron spectroscopic and theoretical study of the PtAl(-) and PtAl2(-) anions. The ground state structures and electronic configurations of these species were identified to be C∞v, (1)Σ(+) for PtAl(-), and C2v, (2)B1 for PtAl2(-). Structured anion photoelectron spectra of these clusters were recorded and interpreted using ab initio calculations. Good agreement between theory and experiment was found. All experimental features were successfully assigned to one-electron transitions from the ground state of the anions to the ground or excited states of the corresponding neutral species.

Aluminum Zintl anion moieties within sodium aluminum clusters.

Through a synergetic combination of anion photoelectron spectroscopy and density functional theory based calculations, we have established that aluminum moieties within selected sodium-aluminum clusters are Zintl anions. Sodium-aluminum cluster anions, Na(m)Al(n)(-), were generated in a pulsed arc discharge source. After mass selection, their photoelectron spectra were measured by a magnetic bottle, electron energy analyzer. Calculations on a select sub-set of stoichiometries provided geometric structures and full charge analyses for both cluster anions and their neutral cluster counterparts, as well as photodetachment transition energies (stick spectra), and fragment molecular orbital based correlation diagrams.

Hydrogen mimicking the properties of coinage metal atoms in Cu and Ag monohydride clusters.

A systematic study of the electronic structure and equilibrium geometries of Cun, Cun-1H, Agn, and Agn-1H; n = 2-5 clusters is carried out using photoelectron spectroscopy (PES) experiments and density functional theory based calculations. Our objective is to see if the substitution of a coinage metal atom by hydrogen would retain the electronic structure of the parent metal cluster since both systems are isoelectronic. For clusters with n ≥ 3, we find that the measured PES and vertical detachment energies (VDEs) (i.e. energies necessary to remove an electron from the anionic Mn(-) (M = Cu, Ag) clusters without changing their geometries) are close to those of Mn-1H(-) clusters, suggesting that substitution of a metal atom with hydrogen does not perturb the electronic structure of the parent cluster anion significantly. Calculated VDEs agree very well with experiment validating the theoretical methods used as well as the geometries of the neutral and anionic clusters.

Photoelectron spectroscopy of the aluminum hydride anions: AlH2(-), AlH3(-), Al2H6(-), Al3H9(-), and Al4H12(-).

We report measurements of the negative ion photoelectron spectra of the simple aluminum hydride anions: AlH2(-), AlH3(-), Al2H6(-), Al3H9(-), and Al4H12(-). From these spectra, we measured the vertical detachment energies of the anions, and we estimated the electron affinities of their neutral counterparts. Our results for AlH2(-), AlH3(-), and Al2H6(-) were also compared with previous predictions by theory.

Origin of the unusual properties of Au(n)(BO2) clusters.

We report the discovery of a new class of clusters consisting of Au(n)(BO(2)) that forms during the oxygenation of gold clusters when boron nitride is used as insulation in a pulsed-arc cluster ion source (PACIS). Photoelectron and mass spectroscopy of these clusters further revealed some remarkable properties: instead of the expected Au(n)O(m) peaks, the mass spectra contain intense peaks corresponding to Au(n)(BO(2)) composition. Some of the most predominant features of the electronic structure of the bare Au clusters, namely even-odd alternation in the electron affinity, are preserved in the Au(n)(BO(2)) species. Most importantly, Au(n)(BO(2)) [odd n] clusters possess unusually large electron affinity values for a closed-shell cluster, ranging from 2.8-3.5 eV. The open-shell Au(n)(BO(2)) [even n] clusters on the other hand, possess electron affinities exceeding that of F, the most electronegative element in the periodic table. Using calculations based on density functional theory, we trace the origin of these species to the unusual stability and high electron affinity of the BO(2) moiety. The resulting bond formed between BO(2) and Au(n) clusters preserves the geometric and electronic structure of the bare Au(n) clusters. The large electron affinity of these clusters is due to the delocalization of the extra electron over the Au(n) cluster.

Model catalysts of supported Au nanoparticles and mass-selected clusters.

In surface science, much effort has gone into obtaining a deeper understanding of the size-selectivity of nanocatalysts. In this article, electronic and chemical properties of various model catalysts consisting of Au are reported. Au supported by oxide surfaces becomes inert towards chemisorption and oxidation as the particle size became smaller than a critical size (2-3 nm). The inertness of these small Au nanoparticles is due to the electron-deficient nature of smaller Au nanoparticles, which is a result of metal-substrate charge transfer. Properties of Au clusters smaller than ∼20 atoms were shown to be non-scalable, i.e., every atom can drastically change the chemical properties of the clusters. Moreover, clusters with the same size can show dissimilar properties on various substrates. These recent endeavours show that the activity of a catalyst can be tuned by varying the substrate or by varying the cluster size on an atom-by-atom basis.

Electronic structure and properties of isoelectronic magic clusters: Al13X (X=H,Au,Li,Na,K,Rb,Cs).

The equilibrium structure, stability, and electronic properties of the Al(13)X (X=H,Au,Li,Na,K,Rb,Cs) clusters have been studied using a combination of photoelectron spectroscopy experiment and density functional theory. All these clusters constitute 40 electron systems with 39 electrons contributed by the 13 Al atoms and 1 electron contributed by each of the X (X=H,Au,Li,Na,K,Rb,Cs) atom. A systematic study allows us to investigate whether all electrons contributed by the X atoms are alike and whether the structure, stability, and properties of all the magic clusters are similar. Furthermore, quantitative agreement between the calculated and the measured electron affinities and vertical detachment energies enable us to identify the ground state geometries of these clusters both in neutral and anionic configurations.

PtZnH5(-), A σ-Aromatic Cluster.

We report a joint photoelectron spectroscopic and theoretical study of the PtZnH5(-) cluster anion. This cluster exhibited an unprecedented planar pentagonal coordination for Pt and an unusual stability and high intensity in the mass spectrum. Both are due to the σ-aromaticity found in the H5-cycle supported by the 5d orbitals on the Pt atom. σ-Aromaticity in all-H systems has been predicted in the past but never found in experimentally observed species. Besides fundamental importance, mixed transition-metal hydrides can be found as intermediates in catalytic processes, and thus, the unexpected stability facilitated by σ-aromaticity can be appreciated also in practical applications.