Kinetics of photo-induced dissociation of Na clusters deposited on Mica

Na clusters bound to mica surfaces have been irradiated with pulsed and cw visible laser light. Kinetic energy and angular distributions of the Na atoms desorbing from the clusters have been determined using cw two-photon laser-induced fluorescence detection. In addition the dependence of the desorption rate on laser power, wavelength and polarization has been measured. The most probable kinetic energyE _kin of the photodesorbed atoms at the surface temperatureT _S =300 K was found to beE _kin=18±5 meV, independent of laser irradiance (3 µJ/cm²...20 mJ/cm²) and wavelength (450 nm, 505 nm, 658 nm). With increasing orientation angle between detection axis and surface normal (0°≦Θ≦90°)E _kin was observed to decrease slightly, while it was nearly independent of surface temperature betweenT _S =30 K andT _S =300 K. Also, with increasing radius of the Na clusters the desorbing Na atoms slowed down. The angular distribution of the Na atoms was of cos²-type with respect to the surface normal. These observations suggest that laser-induced desorption of Na from Na clusters bound to mica surfaces involves an initial rate-limiting step of direct surface plasmon excitation followed by a final step of delayed thermal desorption.     

Molecular dynamics calculation of half-lives for thermal decay of Lennard-Jones clusters

Molecular dynamics has been used with a Lennard-Jones (6–12) potential in order to study the decay behavior of neutral Argon clusters containing between 12 and 14 atoms. The clusters were heated to temperatures well above their melting points and then tracked in time via molecular dynamics until evaporation of one or more atoms was observed. In each simulation, the mode of evaporation, energy released during evaporation, and cluster lifetime were recorded. Results from roughly 2000 simulation histories were combined in order to compute statistically significant values of cluster half-lives and decay energies. It was found that cluster half-life decreases with increasing energy and that for a given value of excess energy (defined asE=(E _tot ?E _gnd)/n), the 13 atom cluster is more stable against decay than clusters containing either 12 or 14 atoms. The dominant decay mechanism for all clusters was determined to be single atom emission.     

Electron energy loss spectroscopy of van-der-Waals clusters

A method to measure electron energy loss spectra (EELS) of clusters with a high resolution (30 meV) has been developed and has been applied to some van der Waals clusters (Ar _n , Kr _n ). Structures have been found which relate to the excitation of atoms on the surface and inside the cluster. An influence of the cluster size on the spectra has been observed.     

Structure and dynamics of Lennard-Jones clusters with impurities

Molecular dynamics simulations and Lennard-Jones potentials have been used to study binary mixed clusters. The low temperature structures, impurity solvation and the melting and freezing transitions for different values of the relative atomic size and interaction energy have been studied forA ₁₃ B,A ₁₂ B,A ₅₅ B andA ₁₃ B ₁₃ clusters. ForA ₁₃ B large impurities do not solvate even for high interaction energy. ForA ₅₅ B larger impurities remain on the surface for low interaction energies but solvate as the energy of interaction increases. The presence of the impurity very strongly affects the solid-liquid transition. Icosahedral structures remain as the minimum energy configurations forA ₁₃ B ₁₃ clusters with atoms of the same size and different interaction energies. ForA ₁₃ B ₁₃ clusters with atoms of the same interaction energy and different size, smaller atoms go inside surrounded by the bigger surface atoms.     

Fragmentation and reaction processes in cluster-surface collisions

We review the processes which have been observed from collisions between alkali-halide clusters and solid surfaces. Soft impact of nanocrystalline Na_nF _n?1 ⁺ clusters against solid surfaces causes them to cleave along the lowest energy (100) plane. At higher collision energies (E_i>1 eV/atom), an evaporative cascade occurs which is characteristic of a transformation of the nanocrystal to a molten state. Efficient F^? transfer from the cluster to the surface can occur for the larger clusters (>60 atoms) scattering from Si(111), in direct competition with the cleaving channel at low energies. In this regime, strong bonds can form between the F^? and silicon surface. The reaction probability increases with cluster size indicating that an impact-initiated shock wave is needed to enhance the reactive process.     

The local electronic and magnetic properties of Fe impurity in Al clusters

The local magnetic property,d electronic structure and the charge transfer effect of Fe impurity in Al clusters have been studied by using a tight-binding model Hamiltonian in the unrestricted Hartree-Fock approximation, which includes intra-atomic and interatomic Coulomb interactions. We have obtained that local magnetic moment of Fe impurity in FeAl _N clusters decreases with increasing cluster size and convergences to zero (that of bulk given by Anderson) withN larger than 12, meanwhile, the local magnetic moment for smaller clusters depends on the clusters size and it is a monotonous descent function of cluster size. We have also found that the spin splitting of the localizedd states decreases as the cluster size increases, which mainly results from the interaction between the localized electrons of Fe atom and the delocalized electrons of Al atoms.     

Study of water clusters in the n = 2–34 size regime, based on the ABEEM/MM model

Various properties of water clusters in the n = 2–34 size regime with the change of cluster size have been systemically explored based on the newly developed flexible-body and charge-fluctuating ABEEM/MM water potential model. The ABEEM/MM water model is to take ABEEM charges of all atoms, bonds, and lone-pairs of water molecules into the intermolecular electrostatic interaction term in molecular mechanics. The computed correlating properties characterizing water clusters (H₂O) _n (n = 2–34) include optimal structures, structural parameters, ABEEM charge distributions, binding energies, hydrogen bonds, dipole moments, and so on. The study of optimal structures shows that the ABEEM/MM model can correctly predict the following important structural features, such as the transition from two-dimensional (from dimer to pentamer) to three-dimensional (for clusters larger than the hexamer) structures at hexamer region, the transition from cubes to cages at dodecamer (H₂O)₁₂, the transition from all-surface (all water molecules on the surface of the cluster) to one water-centered (one water molecule at the center of the cluster, fully solvated) structures at (H₂O)₁₇, the transition from one to two internal molecules in the cage at (H₂O)₃₃, and so on. The first three structural transitions are in good agreement with those obtained from previous work, while the fourth transition is different from that identified by Hartke. Subsequently, a systematic investigation of structural parameters, ABEEM charges, energetic properties, and dipole moments of water clusters with increasing cluster size can provide important reference for describing the objective trait of hydrogen bonds in water cluster system, and also provide a strong impetus toward understanding how the water clusters approach the bulk limit.     

Adsorbate binding energies for ammonia on cobalt and nickel clusters

Equilibrium reactions of ammonia with cobalt and nickel clusters are analyzed to determine cluster-adsorbate binding energies. The temperature dependence of reaction equilibrium constantsK _eq are measured and ?ΔH ⁰ values obtained from plots of lnK _eq vs 1/T. We find that binding energies generally decrease with increasing ammonia coverage, and that for a given number of NH₃ molecules binding energies increase with increasing cluster size. The pattern of binding energies is found to be consistent with proposed geometrical structures for Co₁₉ and for clusters in the 55-atom size range. Cluster-ammonia binding energies are generally somewhat higher than for bulk metal surfaces, an expected result considering the character of the cluster surface and the nature of the NH₃-metal interaction.     

The size of neutral free clusters as manifested in the relative bulk-to-surface intensity in core level photoelectron spectroscopy

A new approach for obtaining an estimate of the effective size of the free neutral clusters is proposed. The approach relies on an experimental measure of the surface and interior or "bulk" cluster atoms provided by the x-ray photoelectron spectroscopy and on a model for the attenuation of photoelectrons ejected from the bulk of the cluster as the result of the ionizing irradiation. The experimental part gives the ratio of the electron signal from the bulk cluster atoms to that from the cluster surface atoms for a wide range of cluster sizes and electron kinetic energies. The attenuated response of the bulk atoms is modeled using an exponential law with the cluster size and kinetic-energy-dependent electron escape depth as parameters. For the experimental size range, model-based calculations for Ar, Kr, and Xe clusters are presented. The cluster size estimates obtained from comparison of the model calculations and experimental results agree well with those determined from the parameters of the cluster creation process. The combination of experiment and modeling also makes it possible to estimate the effective escape depth for electron propagation in free clusters. For Ar, Kr, and Xe clusters of varying mean size, absolute determination of the surface and bulk electron binding energies of the core levels used in the experiments has also been made.     

Electronic properties of isolated GaNAsM clusters: photoionization-, photodissociation- and photoluminescence quantum yields

Photoionization quantum yields of Ga_NAs_M clusters with N + M = 85 ± 5 atoms in the spectral range of ω = 3.5–6.4 eV have been investigated by measuring their total photoabsorption and photoionization cross sections. It is found that the photoionization quantum yields of these clusters are strongly increased by about two orders of magnitude against the values of bulk GaAs. Photodissociation of the cluster provides another efficient channel whereas photoluminescence plays a minor role for the observed cluster size distributions with N + M typically between 30 and 130 atoms.     

Effect of hydrogen chemisorption on the photoionization threshold of isolated transition metal clusters

Large increases in the photoionization threshold energies of small V_x, NB_x, and Fe_x clusters (x = 3–25) induced by chemisorption of H₂ have been observed using photoionization time-of-flight mass spectrometry of a molecular beam. These shifts exhibit a definite dependence both on the number of atoms constituting the bare metal cluster and on the number of chemisorbed hydrogens, and are particularly large (≳ 0.8 eV) for multiple-H₂ chemisorption on small clusters. A simple frontier orbital model for the chemisorption process predicts the direction of adsorbate-induced shifts in cluster ionization threshold for both H₂ and NH₃ as adsorbates.     

Unimolecular dissociation of trivalent metal cluster ions (Al n + , Ga n + , In n + ): evidence for a transition from covalent to metallic bonding

Unimolecular dissociation of aluminum, gallium and indium clusters is investigated. Small sizes dissociate into two channels: either the evaporation of a neutral or a charged monomer. Above a given size n _c, only dissociation of a neutral atom subsists. The evaporation of a charged monomer is characteristic of trivalent metal clusters and is consistent with the size evolution of the ionization potential towards the atomic value. The experiments are interpreted in the framework of the statistical R.R.K. model. For smaller sizes (n < n _c), as two evaporation processes are in competition, we have evaluated cluster relative dissociation energies and ionization potentials. The competition between the two evaporation channels is well mirrored by the evolution of the ionization potentials independently measured by near-threshold photoionization experiments. For gallium, our measurements have revealed that the covalent to metal transition occurs for larger sizes (n = 30–50 atoms) than for aluminum clusters.     

Structural,Relative Stable,and Electronic Properties of Pb<Subscript>n</Subscript>Sn<Subscript>n</Subscript> (n = 2–12) Clusters were Investigated Using Density Functional Theory

The structural, relative stable and electronic properties of Pb_nSn_n (n = 2–12) alloy clusters were systematically studied using density functional theory. The isomers of Pb_nSn_n alloy clusters were generated and determined by ab initio molecular dynamics. By comparing the calculated parameters of Pb₂ dimer and Sn₂ dimers with the parameters from experiments, our calculations are reasonable. With the lowest-energy structures for Pb_nSn_n clusters, the average binding energies, fragmentation energies, second- order energy differences, vertical ionization potentials, vertical electron affinities, HOMO–LUMO gaps, and density of states were calculated and analyzed. The results indicate that the Sn atoms have a tendency to bond together, the average binding energies tend to be stable up to n = 8, Pb₈Sn₈ cluster is a good candidate to calculate the molecular interaction energy parameter in Wilson equation, the clusters become less chemical stable and show an insulator-to-metallic transition, 3, 6, 8 and 11 are magic numbers of Pb_nSn_n (n = 2–12) clusters, the charges always transfer from Sn atoms to Pb atoms in Pb_nSn_n clusters except for Pb₁₀Sn₁₀ cluster, and density of states of Pb_nSn_n clusters becoming continuous and shifting toward negative with the increasing size n.     

Atomic and electronic structure of Na10K10Cs n clusters

Density functional theory is used to study the atomic and electronic structures of Na₁₀K₁₀Cs _n clusters with up to sixty atoms. The simplifying approximation has been made of replacing the external potential of the ionic background by its spherical average about the cluster centre in the iterative process of solving the Kohn-Sham equations for each geometry tested. The search for the equilibrium geometry is performed by employing the technique of simulated annealing. We have always found segregation of Cs atoms to the surface as well as a rather neat separation of different species in different (radial) regions of the cluster. This layering effect appears to be consistent with a tendency to maximize electronegativity differences. When the cluster is big enough, Cs atoms begin to appear also inside the cluster. Those geometrical effects do not perturb the electronic magic numbers well known for pure alkali metal clusters.     

Size-dependent barriers for reaction of aluminum cluster ions with oxygen

Reactions of cooled, size-selected aluminum cluster ions (Al_n⁺, n = 1–8) with oxygen have been studied at collision energies from 0.15 to 10.0 eV (center-of-mass) under single-collision conditions. With the exception of the atomic ion, all size clusters undergo exoergic reactions which result in extensive fragmentation of the metal cluster framework. Significant energy barriers are found for reaction of all clusters except the dimer. The barrier height increases with cluster size from Al₃⁺ to Al₇⁺, then drops for Al₈⁺.     

Photoionization studies of free sodium ammonia clusters

Free sodium ammonia clusters Na(NH₃) _n up ton=45 were generated in a pickup source by injecting a beam of neutral sodium atoms into the expansion zone of a piezo driven pulsed nozzle. The clusters thus formed are studied by one-photon ionisation in the region of 266 nm to 520 nm, time-of-flight mass spectrometry as well as photoelectron spectrometry. Ionisation thresholds for clusters up ton=18 and dissociation energies for the neutral Na(NH₃) _n up ton=6 are reported.     

A Systematic Search for Structures and Stabilities of Asymmetric Clusters (HfInN<Subscript>3</Subscript>)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> (<Emphasis Type="Italic">n</Emphasis> = 1-6)

In order to find single source precursors (SSP), the structures, relative stabilities, and IR spectra of small asymmetric clusters (HFInN₃) _n (n = 1–6) are systematically investigated by means of the density functional theory at the B3LYP level. The obtained geometries show that the frameworks of clusters (HFInN₃) _n (n = 2–6) prefer to be 2n-membered ring with alternating indium and α-nitrogen atoms. The averaged binding energies reveal that all of asymmetric clusters (HFInN₃) _n (n = 1–6) can continue to gain energy as the cluster size n increasing. The second-order difference of energy (Δ₂E) and the HOMO-LUMO energy gap (E_gap) as a function of the cluster size n both exhibit a pronounced even-odd alternation phenomenon. The influences of cluster size n and temperature T on the thermodynamic properties of clusters are discussed. Judged by enthalpies and Gibbs free energies, the formations of the most stable clusters (HFInN₃) _n (n = 2–6) from the monomer are thermodynamically favorable in the range of 200–800 K.     

Threshold photoionization behaviour of (Li2O)Lin clusters produced by a laser vaporization source

We report on the production of small and medium size lithium and lithium oxide clusters by a laser vaporization cluster source. The isotopomeric distribution of natural lithium allowed to identify Li_kO clusters as the most abundant components in the mass spectrum. Photoionization efficiency curves of Li_kO clusters with photon energies from 3.4 to 4.7 eV were measured for 8 ≤ k ≤ 27. Using linear extrapolation of the increase in photoionization efficiency with photon energy, ionization potentials were extracted. With the chemical bond of the O^2- anion to two Li atoms, leaving n = k-2 valence electrons in the (Li₂O)Li_n clusters, clear shell closure effects are present at n = 8 and n = 20.     

Structure and Magnetism of Hybrid Fe and Co Nanoclusters up to N?≤?7 Atoms

The structural and magnetic properties of small gas-phase Fe _m Co _n clusters with m?+?n ranging between 2 and 7 atoms are investigated using spin-polarized density functional theory. For a given cluster size possible compositions are subject to optimization using a variety of initial structures. The geometry, bond lengths, binding energies and magnetization are reported for the lowest energy structures. The results show that a magnetization peak occurs for Fe₄, while for hybrid clusters, switching a cobalt atom with an iron atom increases the cluster’s total magnetization by 1?μ _B . Our structural predictions are generally in agreement with other theoretical results; the origin of the discrepancies arising in some cases is discussed.     

Plasmon profiles and shapes of sodium cluster ions

Photo-absorption cross-sections for charged sodium clusters (14 to 48 atoms) have been measured for photon energies from 2.0 eV to 3.5 eV. The spectra are dominated by surface plasma oscillations of the valence electrons exhausting 70–100% of the dipole sum rule. The mean resonance energy of ?2.75 eV is nearly independent of cluster size. A splitting of the resonance peaks is observed for non-“magic” clusters and discussed in terms of a deformation picture involving prolate and oblate shapes.