Observation of multiply charged silver-cluster anions

Singly charged silver-cluster anions are produced in a laser vaporization source and transferred into a Penning trap. After size selection the clusters are subjected to an electron bath in the trap, which results in the attachment of further electrons. The relative abundance of dianions or trianions as a function of the clusters' size is analyzed by time-of-flight mass spectrometry. Silver-cluster dianions are observed for sizes n≥ 24 and trianions for n > 100. In addition, a detailed study of the cluster sizes 24 ?n? 60 shows a pronounced resistance to electron attachment for singly charged anions Ag_n ^- with a closed electronic shell, in particular Ag₂₉ ^-, Ag₃₃ ^-, and Ag₃₉ ^-. Both the threshold size for the observation of dianionic silver clusters and the shell effects in the production yield correlate favorably with previous theoretical investigations of the respective electron affinities. Received 24 November 2000     

Vibrational wave packet dynamics in the silver tetramer probed by NeNePo femtosecond pump-probe spectroscopy

We present results on the ultrafast dynamics of mass-selected neutral Ag₄ clusters using NeNePo (negative ion - neutral - positive ion) femtosecond pump-probe spectroscopy. One-color pump-probe spectra of the Ag₄ ^-/Ag₄/Ag₄ ⁺ system measured at 385 nm and an internal cluster temperature of 20 K display a complex beat structure over more than 60 ps. The oscillatory structure is attributed to vibrational wave packet dynamics in an excited “dark" state of neutral Ag₄. A dominant 740 fs wave packet period as well as wave packet dephasing and rephasing are observed in the spectra. Fourier analysis of the spectra yields a group of frequencies centered around 45 cm^-1 and an anharmonicity χ _e2νχ _eχ _e of 2.65 cm^-1 for the active vibrational mode. Received 30 November 2000     

Implantation of size-selected silver clusters into graphite

We report on a systematic study of the implantation of size-selected Ag_N ⁺ clusters on a graphite sample, for different cluster sizes (N = 1,3,7,9,13) and different impact energies (E = 1-30 keV). Results show that the implantation depth scales linearly with the momentum of the cluster, with a stopping power which depends on cluster size. We have particularly investigated the effects of the size and the geometry of the cluster on the implantation into the graphite substrate. A sort of universal behavior, which unifies different elements and different cluster geometries, can be recognized by scaling the momentum with the cluster projected surface. The stopping power of the cluster while penetrating the HOPG surface has been investigated for each cluster size, and a molecular effect is recognized, meaning that the stopping power is not additive in the number of atoms of the cluster.Received: 24 November 2003, Published online: 10 February 2004PACS: 61.46. + w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc. - 79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces - 81.05.Uw Carbon, diamond, graphite     

Large-scale molecular dynamics simulations of high energy cluster impact on diamond surface

Large-scale molecular dynamics simulations with high acceleration energy on a diamond surface were performed in order to investigate the surface erosion process. Accelerated argon or CO₂ clusters (∼960 atoms, 100 keV/cluster) impacted on the (111) surface of diamond which consisted of more than 1,000,000 carbon atoms. A typical hemispherical crater appeared about 0.7 ps after the impact, and two or three-layered shockwaves were formed and propagated to certain directions, but the crater was immediately filled up with the fluidized hot carbon material due to the collective elastic recovery before the reflection of the shockwave. The impact energy of the cluster was at first transferred mainly as kinetic energy of the diamond surface in a short time, and the potential energy was activated later. The activated carbon and oxygen atoms from the impact cluster stimulated the evaporation from the diamond surface for the CO₂ cluster impact while the evaporation seemed to be suppressed by the argon atoms themselves for the argon cluster impact. Received 22 November 2000     

Scaling relations for implantation of size-selected Au,Ag, and Si clusters into graphite

The deposition of size-selected clusters represents a new route to the fabrication of truly nanometer-scale surface architectures, e.g., nanopores. We report a systematic experimental study, coupled with molecular dynamics simulations, of the implantation depths of size-selected Au7, Ag7, and Si7 clusters in the model graphite substrate. For impact energies between 1.0 and 5.5 keV, we find that the implantation depth scales linearly with the momentum of the clusters for all three types of cluster. This "universal" behavior is consistent with a (viscous) retarding force proportional to the velocity of the cluster, akin to Stokes's law.     

Tight-binding study of structural and electronic properties of silver clusters

Tight-binding model is developed to study the structural and electronic properties of silver clusters. The ground state structures of Ag clusters up to 21 atoms are optimized by molecular dynamics-based genetic algorithm. The results on small Ag_n clusters (n = 3-9) are comparable to ab initio calculations. The size dependence of electronic properties such as density of states, s-d band separation, HOMO-LUMO gap, and ionization potentials are discussed. Magic number behavior at Ag₂, Ag₈, Ag₁₄, Ag₁₈, Ag₂₀ is obtained, in agreement with the prediction of electronic ellipsoid shell model. We suggest that both the electronic and geometrical effect play significant role in the coinage metal clusters. Received 7 August 2000     

Collision-induced fragmentation and neutralization of methanol cluster cations

This contribution addresses the inelastic interaction of positively charged molecular cluster ions with a solid surface at kinetic energies up to 30 eV/molecule. We report experimental results on the scattering of mass-selected, protonated methanol cluster cations (CH₃OH)_nH⁺, n = 4-32, off a diamond-coated silicon surface. In particular we provide fragment size distributions of methanol cluster ions following their impact on the target, as well as surface-induced neutralization probabilities of methanol cluster ions as a function of the size and the kinetic energy of the parent clusters. Received 30 November 2000     

Pinning of size-selected Co clusters on highly ordered pyrolytic graphite

Deposition and implantation of size-selected Co⁺ _50±5 cluster ions on/in highly ordered pyrolytic graphite (HOPG) have been performed. Cobalt clusters were produced by laser ablation using the second harmonic (532 nm) of a Nd:YAG laser. They were deposited/implanted with energies of 250–4850 eV/cluster and examined using scanning tunneling microscopy (STM). For the highest energies the clusters created craters and wells with residual clusters at the bottom of the wells. Decrease of the impact energy led to formation of bumps whichconsist of damaged graphite areas mixed with fragmented cobalt clusters. Further decrease of the impact energy to 250–450 eV/cluster probably corresponds to the so-called pinning regime, when the impacting cluster creates defects in the surface layer and becomes bound to them. The transition from implantation to pinning with a decrease of impact energy was confirmed by etching experiments showing the depth of the damage introduced by the cluster collisions with HOPG.     

Shallow implantation of "Size-Selected" Ag clusters into graphite

We have investigated the implantation of Ag(N) (N = 20-200) clusters into a graphite substrate over the range of energies (E) 0.75-6 keV using molecular dynamics simulations. We find that after implantation the silver clusters remain coherent, albeit amorphous, and rest at the bottom of an open tunnel in the graphite created by the impact. It is found that the implantation depth of the clusters varies linearly as E/N2/3. We conclude that the cluster is decelerated by a constant force proportional to its cross-sectional area. We also identify a threshold energy for surface penetration associated with elastic compression of the graphite substrate.     

Electronic properties of assembled islands of hydrogen-saturated silicon clusters on Si(111)-(7 x 7) surfaces studied by scanning tunneling spectroscopy

We present results of scanning tunneling spectroscopy (STS) measurements of hydrogen-saturated silicon clusters islands formed on Si(111)-( 7×7) surfaces. Nanometer-size islands of Si₆H₁₂ with a height of 0.2-4 nm were assembled with a scanning tunneling microscope (STM) using a tip-to-sample voltage larger than 3 V. STS spectra of Si₆H₁₂ cluster islands show characteristic peaks originating in resonance tunneling through discrete states of the clusters. The peak positions change little with island height, while the peak width shows a tendency of narrowing for the tall islands. The peak narrowing is interpreted as increase of lifetime of electron trapped at the cluster states. The lifetime was as short as 10^-13 s resulting from interaction with the dangling bonds of surface atoms, which prevents charge accumulation at the cluster islands. Received 30 November 2000     

Structures and stability of Al<Subscript><Emphasis Type="Bold">7</Emphasis></Subscript>C and Al<Subscript><Emphasis Type="Bold">7</Emphasis></Subscript>N clusters

We report results of the atomic and electronic structures of Al₇C cluster using ab initio molecular dynamics with ultrasoft pseudopotentials and generalized gradient approximation. The lowest energy structure is found to be the one in which carbon atom occupies an interstitial position in Al₇ cluster. The electronic structure shows that the recent observation [Chem. Phys. Lett. 316, 31 (2000)] of magic behavior of Al₇C^- cluster is due to a large highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) gap which makes Al₇C^- chemically inert. These results have further led us to the finding of a new neutral magic cluster Al₇N which has the same number of valence electrons as in Al₇C^- and a large HOMO-LUMO gap of 1.99 eV. Further, calculations have been carried out on (Al₇N)₂ to study interaction between magic clusters. Received 28 July 2001     

Theoretical investigations of rotational phenomena and dielectric properties in a nematic liquid crystal

The molecular dynamics (MD) simulation, based on a realistic atom-atom interaction potential, was performed on 4-n-pentyl-4'-cyanobiphenyl (5CB) in the nematic phase. The rotational viscosity coefficients (RVCs) γ _i, (i = 1, 2) and the ratio of the RVCs λ = - γ ₂/γ ₁ were investigated. Furthermore, static and frequency-dependent dielectric constants and ε were calculated using parameters obtained from the MD simulation. Time correlation functions were computed and used to determine the rotational diffusion coefficient, D _⊥. The RVCs and λ were evaluated using the existing statistical-mechanical approach (SMA), based on a rotational diffusion model. The SMA rests on a model in which it is assumed that the reorientation of an individual molecule is a stochastic Brownian motion in a certain potential of mean torque. According to the SMA, γ _i are dependent on the orientational order and rotational diffusion coefficients. The former was characterized using: i) orientational distribution function (ODF), and ii) a set of order parameters, both derived from analyses of the MD trajectory. A reasonable agreement between the calculated and experimental values of γ _i and λ was obtained. Received 22 March 2000 and Received in final form 8 October 2000     

Growth of tellurium clusters in presence of metal impurities

The growth of small tellurium clusters in helium and the influence of a metal impurity (dysprosium atoms) on the cluster size distribution are investigated in a double laser vaporization source. A model describing the role of the carrier gas as collision partner is presented, emphasizing the crucial influence of the gas pressure on cluster formation. Changes in cluster reactivity due to dysprosium addition are discussed in terms of ionic structures Dy ^{3 +}(Te _N)^{3 -} containing a radical electron. Received 28 November 2000     

Decay pathways of small gold clusters

The decay pathway competition between monomer and dimer evaporation of photoexcited cluster ions Au ⁺ _n, n = 2-27, has been investigated by photodissociation of size-selected gold clusters stored in a Penning trap. For n > 6 the two decay pathways are distinguished by their experimental signature in time-resolved measurements of the dissociation. For the smaller clusters, simple fragment spectra were used. As in the case of the other copper-group elements, even-numbered gold cluster ions decay exclusively by monomer evaporation, irrespective of their size. For small odd-size gold clusters, dimer evaporation is a competitive alternative, and the smaller the odd-sized clusters, the more likely they decay by dimer evaporation. In this respect, Au ⁺ ₉ shows an anomalous behavior, as it is less likely to evaporate dimers than its two odd-numbered neighbors, Au ⁺ ₇ and Au ⁺ ₁₁. This nonamer anomaly is typical for copper-group cluster ions M ⁺ ₉ (M = Cu, Ag, Au) and a similar behavior is found in the anionic heptamers M ^- ₇. It is discussed in terms of the well-known electronic shell closing at n _e = 8 atomic valence electrons. Received 2 November 2000     

Ultraviolet photoelectron spectroscopy of Si 4 - to Si 1000 -

Using a new experimental setup we have measured UV (h ν = 6.4 eV) photoelectron spectra of cold silicon cluster anions Si _n ^- in a very broad size range. For sizes up to n = 46 the spectra exhibit rich structures. For larger sizes only smooth spectra have been obtained. No trace of a bandgap has been found even for clusters with more than 1000 atoms. Received 30 November 2000     

Structure determination of mixed clusters by surface scattering

In this paper, we investigate the global structure of mixed clusters created by coexpansion. To determine the relative dopant sites within the mixed clusters, we take advantage of the strong dependence of the cluster/surface collision dynamics on the incident mixed cluster structure. Using both experiments and molecular dynamic simulations, we show that the coexpansion process leads to the most stable cluster structure for Ar _<880> Kr _<120> clusters. This structure corresponds to an annealed structure and can be characterized as a nearly homogeneous mixture throughout the cluster with a thin argon coating. Received 4 October 2000     

Nuclear spins, magnetic moments and α-decay spectroscopy of long-lived isomeric states in 185Pb

Alpha-decay properties of the neutron-deficient isotope ¹⁸⁵Pb were studied at the PSB-ISOLDE (CERN) on-line mass separator using the resonance ionisation laser ion source (RILIS). The nuclei of interest were produced in a 1.4 GeV proton-induced spallation reaction of a uranium graphite target. In contrast to previous studies, two α-decaying isomeric states were identified in ¹⁸⁵Pb. The relative production of the isomers, monitored by their α-counting rates, could be significantly changed when a narrow-bandwidth laser at the RILIS setup was used to scan through the atomic hyperfine structure. Based on the atomic hyperfine structure measurements, along with the systematics for heavier odd-mass lead isotopes, the spin and the parity of these states were interpreted as 3/2^- and 13/2⁺ and their nuclear magnetic moments were deduced. The α-decay energy and half-life value for the I ^π = 13/2⁺ isomer are E _α = 6408(5) keV, T _1/2 = 4.3(2) s, respectively; while for the I ^π = 3/2^- isomer ( T _1/2 = 6.3(4) s) two α-decays with E _α1 = 6288(5) keV, I _α1 = 56(2)% and E _α2 = 6486(5) keV, I _α2 = 44(2)% were observed. By observing prompt α-γ coincidences new information on the low-lying states in the daughter isotope ¹⁸¹Hg was obtained. Received: 7 February 2002 / Accepted: 19 February 2002     

Landau fragmentation and deformation effects in dipoSe response of sodium clusters

Recent experimental data on the dipole plasmon in axial sodium clusters Na _N ⁺ with 11 ≤ N ≤ 57 are analyzed within a self-consistent separable random-phase approximation (SRPA) based on the deformed Konh-Sham functional. Good agreement with the data is achieved. The calculations show that, while in light clusters plasmon properties (gross structure and width) are determined mainly by deformation splitting, in medium clusters with N τ 50 the Landau fragmentation becomes decisive. Moreover, in medium clusters shape isomers come to play with contributions to the plasmon comparable with the ground state one. As a result, commonly used methods of the experimental analysis of cluster deformation become useless and correct treatment of cluster shape requires microscopic calculations.     

Effect of melting on ionization potential of sodium clusters

The effect of melting transition on the ionization potential has been studied for sodium clusters with 40, 55, 142, and 147 atoms, using ab initio and classical molecular dynamics. Classical and ab initio simulations were performed to determine the ionization potential of Na₁₄₂ and Na₁₄₇ for solid, partly melted, and liquid structures. The results reveal no correlation between the vertical ionization potential and the degree of surface disorder, melting, or the total energy of the cluster obtained with the ab initio method. However, in the case of 40 and 55 atom clusters, the ionization potential seems to decrease when the cluster melts. Received 1st November 2002 Published online 24 April 2003 RID="a" ID="a"e-mail: ar@phys.jyu.fi     

Resonant coupling of small size-controlled lead clusters with an intense laser field

We exposed small size-controlled lead clusters with a few hundreds of atoms to laser pulses with peak intensities up to 10¹⁵ W cm^-2 and durations between 60 fs to 2.5 ps. We measured kinetic energies and ionic charge of fragments as a function of the laser intensity and pulse duration. Highly charged Pbⁿ⁺ ions up to n = 26 have been detected presenting kinetic energies up to 15 keV. For comparison with our experimental results, we have performed simulations of the laser coupling with a cluster-sized lead nanoplasma using a qualitative model that was initially proposed by Ditmire and co-workers at LLNL for the case of rare gas clusters. From these simulations we conclude that two mechanisms are responsible for the explosion dynamics of small lead clusters. As already observed for large rare gas clusters (n = 10⁶), fragments with charge states below +10 are driven by Coulomb forces, whereas the higher charged fragments are accelerated by hydrodynamic forces. The latter mechanism is a direct consequence of the strong laser heating of the electron cloud in the nanoplasma arising from a plasmon-like resonance occurring at n _e = 3n _c. In order to obtain an optimized laser-nanoplasma coupling, our results suggest that the plasma resonance should occur at the peak intensity of the laser pulse. Due to inertial effects, even for such small-sized clusters, the observed optimum pulse duration is in the order of 1 ps which is in good agreement with our theoretical results. Received 18 March 2002 Published online 19 July 2002