Vibrational Feshbach resonances in electron attachment to carbon dioxide clusters

Using a high resolution ( meV) laser photoelectron attachment method, we have studied the formation of (CO ₂) _q ⁻ ions (q = 4−22) in collisions of low energy electrons (1−180 meV) with (CO₂) _N () clusters. The previously reported “zero energy resonance”, observed at much larger electron bandwidths, actually consists of several narrow vibrational Feshbach resonances of the type [(CO ₂) _{N −1}CO which involve a vibrationally-excited molecular constituent ( denotes vibrational mode) and a diffuse electron weakly bound to the cluster by long range forces. The resonances occur at energies below those of the vibrational excitation energies of the neutral clusters [(CO ₂) _{N −1}CO ]; the redshift rises with increasing cluster ion size q by about 12 meV per unit; these findings are recovered by a simple model calculation for the size dependent binding energies. The size distribution in the cluster anion mass spectrum, resulting from attachment of very slow electrons, mainly reflects the amount of overlap of solvation-shifted vibrational resonances with zero energy; the cluster anion size q is identical with or close to that of the attaching neutral cluster. Received 11 January 2000 and Received in final form 10 April 2000     

Cold Ar 3 + generated by ionization of argon clusters in large helium clusters: Temperature dependence of the photofragmentation

Ionized argon clusters were generated by electron impact ionization of neutral argon clusters embedded in large neutral helium clusters. Photofragmentation spectroscopy of Ar ₃ ⁺ and Ar ₃ ⁺ He produced in this way demonstrates the strong influence of vibrational excitation on the photodissociation dynamics, and indicates the low internal energy of the latter cluster.     

Size-dependent light emission from mass-selected clusters

The emission of photons in the visible wavelength range from mass-selected Ag⁺ _n, Cu⁺ _n, Pt⁺ _n and Pd⁺ _n () clusters is observed. Photons are detected 10^-4 s after the cluster generation in a sputter source. The emission intensities display distinct variations with cluster size and material. The observations are interpreted in terms of the decay of metastable states which are excited during the high-energy sputtering process used for the generation of these clusters. Received: 28 October 1997 / Revised: 5 January 1998 / Accepted: 30 January 1998     

WnNim(n+m=8)团簇的极性和光谱性质的理论研究

采用密度泛函理论(DFT)中的B3LYP方法,在LANL2DZ基组水平上对WnNim(n+m=8)团簇的各种可能构型进行了几何参数全优化,得到了它们的基态构型;并对基态构型的偶极距、极化率、红外光谱和拉曼光谱性质进行了分析,结果表明:团簇WnNim(n+m=8)都具有极性,富W团簇非线性光学效应强,容易被外加场极化;振动频率主要分布在0-350 cm-1范围内,团簇W4Ni4因其振动方式的特殊性,红外光谱和拉曼光谱在频率421.971 cm-1处,都有明显强峰;团簇W5Ni3因其结构的高对称性在振动光谱中出现多处共振现象.     

Dynamical aspects and the role of IVR for the reactivity of noble metal clusters towards molecular oxygen

Here, we present the dynamical aspects and the role of internal energy redistribution (IVR) in the reactivity of noble metal clusters towards O₂. We show on the example of Ag₃O₂ ^- / Ag₃O₂ / Ag₃O₂ ⁺ that NeNePo spectroscopy carried out under zero electron kinetic energy (ZEKE) conditions can be a powerful tool to investigate the geometry relaxation and IVR induced by photodetachment in real time. Furthermore, we demonstrate that difference in the reactivity of Ag₆ ^- and Au₆ ^- towards O₂ can be attributed to different nature of the IVR process. Dissipative IVR in Ag₆ ^- favors fast complex stabilization, whereas resonant IVR found for Au₆ ^- might be an important factor determining the catalytic activity of Au₆ ^- cluster in the CO oxidation.     

Reactivity of anionic gold oxide clusters towards CO: experiment and theory

In this paper, we present results from our joint experimental and theoretical study of the reactivity of anionic gold oxide clusters Au_2,3O_1-4 ^- towards CO. We provide clear evidence that, although O–O bond weakening/dissociation is important to enable CO oxidation, the presence of atomic oxygen can be favorable but is not always sufficient. Furthermore, we show that with the addition of a single gold atom the reactivity channels can be changed. As a consequence, in contrast to CO oxidation in the case of anionic gold dimer oxides, association of CO or replacement of O₂ by CO become the dominant reaction channels for Au₃O_n ^-. This demonstrates the nonscalable properties of gold clusters in the size regime in which each atom counts.     

CNDO calculations of nickel atom clusters

CNDO molecular orbital calculations for nickel atom clusters containing from one to thirteen atoms in various geometric arrangements are presented. The parameters were selected so that an octahedral Ni₆ cluster had an equilibrium inter-nuclear distance, d band occupancy, binding energy, Fermi level, and d band width approximating those of bulk nickel. For clusters with a given number of atoms the stability always increased in the order linear < planar < three-dimensional cluster. The general assumption that the binding energy per atom in metal clusters is proportional to the number of nearest neighbours is supported by these CNDO calculations although this relation is certainly not exact for small clusters. Examination of the calculated orbitals does not indicate a separation of the d band into one part made up from atomic t_2g orbitals and another from e_g orbitals. Overall the CNDO method appears to present a reasonable approach to calculating properties of small metal clusters.     

The use of low energy sims to study the chemisorption of CO on Ni(100) and polycrystalline nickel

Adsorption of CO on Ni(100) has been investigated using secondary ion mass spectrometry (SIMS) and Auger electron spectroscopy at 175 and 295 K. Interaction with polycrystalline nickel was examined at 295, 325 and 365 K. All the secondary ions, Ni⁺, Ni₂⁺, NiCO⁺ and Ni₂CO⁺ show large increases in intensity as CO is adsorbed but there is no simple correlation of the secondary ion species with the sequence of linear and bridge-bonded CO species expected from electron energy loss spectroscopy. Adsorption of CO at 175 K on a hydrogen saturated Ni(100) surface, which is thought to permit only bridge-bonded adsorbed CO, does not result in any enhancement of Ni₂CO⁺. The extent of increases in secondary ion yields after CO adsorption on the nickel surfaces are primarily related to the variations in the heat of adsorption as a function of surface coverage. The presence of more weakly-held species is important in enhancing secondary ion yields.     

Sputtering of clusters from nickel-aluminium

A NiAl(1 1 1) single crystal was bombarded with 15 keV Ar⁺, and the resulting secondary neutrals were analysed by laser postionisation secondary neutral mass spectrometry. By measuring the individual cluster photoion intensity as a function of laser power, the sputter yields of 33 individual clusters were determined. The yield of Al_n clusters sputtered from NiAl falls with increasing cluster nuclearity as n^−8.7 while Ni_n and Al_m−nNi_n yields are proportional to n^−5.9 and n^−5.2, respectively. The distribution of thee yields of mixed Al_m−nNi_n clusters with n and m is found to diverge significantly from the expected distribution based on a random combinatorial approach, indicating that the energetics due to the chemical bonding in the clusters plays a significant role during cluster formation in the sputtering process.     

Predicting experimental signatures for the oxidation of magnesia supported palladium clusters by density functional theory

We showed in a recent density functional study that small palladium cluster on a MgO surface with F-centers can be oxidized to epitaxial Pd_xO_y nano-oxides below room temperature [1]. Here, we employ density functional theory in order to explore different methods for an experimental verification of the Pd_xO_y formation. The electronic density of states (DOS) of bare, O₂-decorated and of oxidized palladium cluster was calculated. For many cluster sizes a clear difference in the DOS could be observed allowing for a detection of the oxidation with surface sensitive spectroscopic methods. In addition, adsorption sites and stretch frequencies of a single CO molecule on bare and oxidized Pd₄ clusters were calculated. While CO prefers hollow sites on Pd₄, top adsorption sites are found for Pd₄O₂. Markedly different CO stretch frequencies indicate a possible discrimination of bare clusters and oxides by Fourier transform infrared spectroscopy.     

Stabilities,electronic and magnetic properties of Cu-doped nickel clusters: a DFT investigation

In this paper, we investigate the electronic and magnetic properties of Cu-doped nickel clusters by means of density functional theory. The stabilities of these clusters have also been studied in terms of the binding energies, second-order difference of energies, fragmentation energies and HOMO–LUMO energy gaps. The obtained results reveal that the N₄Cu, N₅Cu and Ni₇Cu clusters are found to be more stable that than all other clusters. Higher HOMO–LUMO gap was observed for Ni₅Cu cluster (2.265 eV), indicating its higher chemical stability. A half-metallic behaviour has also been observed for the Ni_nCu clusters, which suggests that these clusters can be employed as nanocatalysts for several catalytic processes, particularly for hydrogenation and dehydrogenation reactions. The magnetism calculations show that the magnetic moment is mostly located on the Ni atoms, and the contribution of the Cu atom to the total magnetic moment in the Ni_nCu clusters is very small. Furthermore, partial density of states analysis indicates that the 3d orbitals in Ni atoms are mostly responsible for the magnetic behaviour of these clusters, and the s orbitals have a very little contribution to the total magnetic moment.     

Structure,energetic and phase transition of small nickel-palladium heterogeneous clusters

Molecular dynamics simulation (MD) with Sutton-Chen potential for palladium-palladium, nickel-nickel and palladium-nickel interactions has been used to generate the minimum energy structures and to study the thermodynamic and dynamic properties of mixed transition metal cluster motifs of Ni _n Pd_(13?n) for n ≤ 13. Thirteen particle icosahedral clusters of neat palladium and nickel atoms were first reproduced accordingly with the results in literature. Then in the palladium icosahedra, each palladium atom has been successively replaced by nickel atom. Calculation is repeated for both palladium-centered and nickel-centered clusters. It is found that the nickel-centered clusters are more stable than the palladium-centered clusters and cohesive energy increases along the palladium end to nickel end. Phase transition of each cluster from one end-species to the other end-species is studied by means of caloric curve, root mean square bond fluctuation and heat capacity. Trend in variation of melting temperature is opposite to the energy trend. Palladium-centered cluster shows a premelting at low temperature due to the solid-solid structural transition. Species-centric order parameters developed by Hewage and Amar is used to understand the dynamic behavior in the solid-solid transition of palladium-centered cluster to more stable nickel-centered cluster (premelting). This species-centric order parameter calculation further confirmed the stability of nickel-centered species over those of palladium-centered species and solid-solid structural transition at low temperature.     

Electronic structure and magnetism in 4d-transition metal clusters

We analyze the stability of magnetic states obtained within the tight-binding model for cubooctahedral (O_h) and icosahedral (I_h) clusters of early 4d (Y, Zr, Nb, Mo, and Tc) transition metals. Several metastable magnetic clusters are identified which suggests the existence of multiple magnetic solutions in realistic systems. A bulk-like parabolic behavior is observed for the binding energy of O_h and I_h clusters as a function of the atomic number along the 4 d-series. The charge transfer on the central atom changes sign, while the average magnetic moments present an oscillatory behavior as a function of the number of d electrons in the cluster. Our results are in agreement with other theoretical calculations. Received: 20 November 1997 / Received in final form: 9 March 1998 / Accepted: 30 March 1998     

Deposition of Ni<Subscript> 13</Subscript> and Cu<Subscript> 13</Subscript> clusters on Ni(111) and Cu(111) surfaces

The soft deposition of Ni₁₃ and Cu₁₃ clusters on Ni(111) and Cu(111) surfaces is studied by means of constant-energy molecular-dynamics simulations. The atomic interactions are described by the Embedded Atom Method. It is shown that the shape of the nickel clusters deposited on Cu(111) surfaces remains rather intact, while the copper clusters impacting on Ni(111) surfaces collapse forming double and triple layered products. Furthermore, it is found that for an impact energy of 0.5 eV/atom the structures of all investigated clusters show the lowest similarity to the original structures, except for the case of nickel clusters deposited on a Cu(111) surface. Finally, it is demonstrated that when cluster and substrate are of different materials, it is possible to control whether the deposition results in largely intact clusters on the substrate or in a spreading of the clusters. This separation into hard and soft clusters can be related to the relative cohesive energy of the crystalline materials.     

Structural properties and reactivity of bimetallic silver-gold clusters

Bimetallic silver-gold clusters are well suited to study changes in metallic versus ionic properties involving charge transfer as a function of the size and the composition. We present structures, ionization potentials (IP) and vertical detachment energies (VDE) for neutral and charged bimetallic Au_nAg_m ( 2(n + m)5) clusters obtained from density functional level of theory. In the stable structures of these clusters Au atoms assume positions which favor charge transfer from Ag atoms. In clusters with equal numbers of hetero atoms (n = m = 1- 4) heteronuclear bonding is preferred to homonuclear bonding, giving rise to large values of ionization potentials. For larger clusters (n=m=5, 10) stable structures do not favor neither hetero bonding nor segregation into the single components, although they exhibit more metallic than ionic features. This remains valid also for Au₈Ag₁₂ cluster characterized by strong charge transfer to gold subunit. The influence of doping of pure gold clusters with silver atoms on VDE and IP values is discussed in context of their reactivity towards O₂ and CO molecules. As a starting point we consider reactivity towards CO and O₂ molecules on the example of AgAu^- dimer. The results show that the catalytic cycle can be fullfilled.     

The catalytic activity of chemisorbed oxygen and carbon monoxide species on palladium and related metals investigated using SCF-Xα-SW calculations

Theoretical calculations of the electronic structure of small clusters modelling the adsorption of carbon monoxide on nickel and palladium and the adsorption of oxygen on palladium and platinum have been carried out using the SCF-Xα-SW method. Our results for the carbon monoxide-nickel clusters are in good agreement with earlier work. Comparing the carbon monoxide-nickel with the carbon monoxide-palladium results, suggests that the ordering of CO-derived cluster orbitals is the same in both cases but that the relative shifts are much different. In addition, the oxygen atom participates more significantly in the cluster containing palladium. An analysis of optical transition energies for Ni₅CO and Pd₅CO clusters is given and discussed in terms of experimental data regarding photodesorption of carbon monoxide. In the case of oxygen atoms in platinum and palldium clusters, we have used two different M₅O geometries: one in which the five metal atoms are in a single plane and oxygen is directly over a single metal atom (Type A) and a second in which the oxygen atom is coordinated to four sur face metal atoms and is directly over a metal atom in the second layer (Type B). The levels calculated for the Pd50 type B cluster are in good agreement with available UPS data. Significant differences in type A and type B clusters are noted for palladium. The results can be correlated with experimental Auger spectroscopic and kinetic data. In particular, the type A oxygen cor relates well with an experimentally observed very reactive species, while type B oxygen cor relates well with a quite unreactive species. These two types also correlate with two species observed by Auger spectroscopy.     

Deposition dynamics and chemical properties of size-selected Ir clusters on TiO2

We report a study of Ir_n/TiO₂ samples prepared by size and energy-selected deposition of Ir_n⁺ (n=1, 2, 5, 10, 15) on rutile TiO₂(1 1 0) at room temperatures. The Ir clusters are found to be formally in the zero oxidation state, and there are no significant shifts in Ir 4f binding energy with cluster size. Over a wide range of impact energies, both Ir XPS intensity and peak position are constant, indicating constant sticking coefficient, and no impact-driven redox chemistry. Low energy ion scattering spectroscopy (ISS) suggests that the deposited Ir clusters remain largely intact, neither fragmenting nor agglomerating, and retaining 3-D structures for the larger sizes. For impact energies above 10 eV/atom, comparison of ISS and XPS data show that the Ir clusters are penetrating into the TiO₂ surface, with the extent of penetration increasing with both per atom energy and cluster size. Temperature programmed desorption (TPD) of CO is used to further characterize the deposited Ir_n. This system shows pronounced substrate-mediated adsorption (SMA) in low CO exposures, with strong dependence on cluster size. ISS and sputtering experiments indicate that CO adsorbed via SMA is bound differently than CO adsorbed in high dose experiments. In experiments with sequential C¹⁶O and C¹⁸O doses, facile C¹⁶O → C¹⁸O exchange is observed for Ir₅ and larger clusters, but not for Ir₂. The peak CO desorption temperature is found to decrease with cluster size. The cycle of CO adsorption and heating comprising a TPD experiment have a dramatic effect on the sample morphology, leading to encapsulation of Ir by a thin TiO_x layer.     

Structure, dynamic and energetic of mixed transition metal clusters

Classical molecular dynamics simulation (MD) with Sutton-Chen potential has been used to generate the minimum energy and to study the thermodynamic and dynamic properties of mixed transition metal cluster motifs of Ag _n Ni_(13?n) for n ?? 13. Literature results of thirteen particle clusters of neat silver and nickel atoms were first reproduced before the successive replacement of the silver atom by nickel. Calculation was repeated for both silver-centred and nickel-centred clusters. It was found that the nickel-centred clusters were more stable than the silver-centred clusters. Heat capacities and hence the melting points of silver and nickel-centred clusters were determined by using the Histogram method. Species-centric order parameters developed by Hewage and Amar were used to understand the dynamic behaviour in the transition of silver-centred clusters to more stable nickel-centred clusters. This species-centric order parameter calculation further confirmed the stability of nickel-centred clusters over those of silver-centred species.     

Optical properties of mixed clusters: comparative study of Ni/Ag and Pt/Ag clusters

The optical properties of mixed (Ni_0.5Ag_0.5)_n and Pt/Ag clusters are investigated in the size range 2-5 nm. Low Energy Ion Spectroscopy (LEIS) experiments show that the cluster surface is entirely covered by silver atoms for the two systems. The optical spectra of Ni/Ag clusters exhibit a large Surface Plasmon Resonance (SPR), damped and widened when the cluster size decreases, in agreement with a classical model assuming a core-shell geometry and including the reduction of the conduction electron mean-free path in the silver shell. For Pt/Ag clusters, no SPR emerges in the size range 2-5 nm, although it is predicted within a classical model, a pronounced SPR appearing only for clusters larger than 10 nm in diameter.