bluuebunny/crossref_metadata_2025 huggingface dataset:
STRUCTURES OF ORGANOMETALLIC CLUSTERS: Vn(BENZENE)m AND Con(BENZENE)m
Authors: HOSHINO K. , KURIKAWA T. , TAKEDA H. , NAKAJIMA A. , KAYA K.
Published: October 2004

Metal-molecule clusters, V n (benzene) m, Co n (benzene) m, and their ions, were synthesized by the reaction of laser-vaporized vanadium atoms with benzene vapor. Both neutral and cationic V n (benzene) m clusters exhibit magic number behavior at m = n + 1(n = 1–5), which was explicable in terms of multiple-decker sandwich structure.
INITIAL PROCESS OF CLUSTER FORMATION OF LASER-ABLATED Si PARTICLES
Authors: OHYANAGI TAKASUMI , MIYASHITA ATSUMI , MURAKAMI KOUICHI , YODA OSAMU
Published: October 2004

We have performed x-ray absorption measurements of laser-ablated Si particles in vacuum and in Hegas environment. It is found that the spatial distributions of laser-ablated particles are very different between the two cases. This suggests that a higher-density region of helium gas is formed at the top of the plume of the ablated particles, and free expansion of particles is restrained by this helium cloud. Moreover, the spectrum obtained at 260 ns indicates that it takes more than 260 ns to form Si clusters. It is also demonstrated that Al particles can be sputtered in vacuum by laser-ablated Si particles if an Al plate is placed in front of the plume.
FORMATION OF C3N4 CLUSTERS BY LASER ABLATION OF NITRIDED GRAPHITE
Authors: MURRAY P.T. , CHEN M.Y.
Published: October 2004

Clusters of C 3 N 4 have been detected from laser ablation of nitrided graphite, The nitrided target was prepared by irradiating a graphite specimen with 248-nm laser radiation in the presence of N2. Auger electron spectroscopy of the irradiated graphite indicated the formation of a thin layer with a composition of CN 0.29. Ablation of this nitrided layer results in the ejection of carbon cluster as well as CN 4 and C 3 N 4. These results indicate that the C 3 N 4 cluster ion is stable. They also suggest that laser ablation within the presence of a reactive gas may be a generally applicable method of preparing targets for the subsequent formation of unique cluster species.
FINITE-TEMPERATURE SIMULATION OF ABSORPTION SPECTRA IN SMALL ARGON-CLUSTER IONS ${\rm Ar}_n^ + $(n=3, 4, 8, 13, AND 19)
Authors: GRIGOROV M. , SPIEGELMANN F.
Published: October 2004

We report finite-temperature simulations of the absorption spectra of argon-cluster ions using a diatomics in molecules (DIM) Hamiltonian (including spin-orbit coupling) for the determination of the potential-energy surfaces (PES), a point-charge approximation for the dipole transition moments and a Metropolis Monte-Carlo algorithm for the nuclear motion. The dependency of the absorption spectrum on cluster size (3≤n≤19) and on temperature (100 K≤T≤500 K) is analyzed.
PSEUDOPOTENTIAL AND SURFACE EFFECTS ON THE ELECTRONIC SHELL STRUCTURE OF TRIVALENT-METAL CLUSTERS
Authors: LERMÉ J. , PELLARIN M. , BAGUENARD B. , BORDAS C. , COTTANCIN E. , VIALLE J.L. , BROYER M.
Published: October 2004

Electronic shell and supershell structures in trivalent-metal clusters exhibit large discrepancies with the results of the conventional jellium model. For gallium the beat location in the supershell pattern is found around Ne=2500 valence electrons, in disagreement with the jellium model prediction Ne=1150. The discrepancies can be explained by a slight softness increase of the effective electronic potential at the surface. Theoretical calculations, taking into account ion pseudopotentials, within both local and nonlocal approaches, show that this softness arises to a large extent from the non-Coulombic short-range electron-ion interaction.
SHAPES AND FREE ENERGIES OF MOLTEN SODIUM CLUSTERS
Authors: FRAUENDORF S. , PASHKEVICH V.V.
Published: October 2004

The shell-correction method is formulated to calculate the shapes and free energies of hot alkali clusters. The equilibrium shapes of Na clusters with mass 50–700 are calculated by minimizing simultaneously with respect to two deformation parameters. For T=700°K strong deviations from spheroidal shape including reflection asymmetric shapes are found to survive in the center of the open shells. The second derivative of calculated free energy correlates with the derivative of the experimental cluster abundances, showing prominent spikes related to the change between spherical and deformed shape.
ENERGETICS AND DYNAMICS OF MOLECULAR CLUSTERS
Authors: JORTNER JOSHUA , EVEN UZI , GOLDBERG ALEXANDER , SCHEK ISRAEL , RAZ TAMAR , LEVINE RAPHAEL D.
Published: October 2004

We address some of the unique and basic features of molecular clusters, which involve (i) surface, interior, and site-selective energetics and dynamics, and (ii) the size dependence of the energetic, spectroscopic, electromagnetic, and dynamic attributes of large finite systems. Cluster-size equations provide a unified (but not universal) description of the “transition” of different attributes of clusters to those of the macroscopic bulk material. We explored fundamental issues, e.g., the physical origins of cluster-size effects, which originate either from cluster packing or from excluded volume contributions, and discussed some applications for the quantification of the size dependence of site-specific ionization potentials, extravalence and intravalence electronic spectroscopy, collective vibrational excitations, and dynamic effects. The quantification of dynamic cluster-size effects for energy acquisition in high-energy cluster-wall collisions opens avenues for the exploration of cluster-impact thermal femtosecond chemistry.
FIRST-PRINCIPLES STUDIES OF Cu CLUSTERS
Authors: MASSOBRIO C. , PASQUARELLO A. , CAR R.
Published: October 2004

Equilibrium geometries and electronic properties of neutral and anionic Cu n (n=2, 10) clusters are determined via first-principles calculations which treat s and d electrons on an equal footing. We find cluster shapes similar to those reported in the literature for Na n clusters, but the highly coordinated structures are energetically preferred. Electronic states with atomic s character are strongly hybridized with d states and located mostly at the band edges. Angular decomposition of the electronic wave functions shows that the predictions of the shell model are followed only approximately in Cu n clusters. Finally we interpret successfully the photoelectron spectrum of [Formula: see text] by accounting for final-state effects.
THEORY FOR THE ELECTRONIC SHELL STRUCTURE IN MESOSCOPIC SYSTEMS: SPHERES, DISCS, AND RINGS
Authors: STAMPFLI P. , BENNEMANN K.H. , TATIEVSKI B.
Published: October 2004

The electronic shell structure resulting from the interference of closed orbital paths is determined for spheres, discs, and rings using the semiclassical theory of Balian and Bloch. The effect of finite surface potentials and magnetic fields is discussed. We obtain analytical expressions for the oscillating structure in the density of states, which are in good agreement with numerical solutions of the Schrödinger equation. These results can be applied to the electronic shell structure of metallic clusters, including the asymptotic size dependence of the structure in the ionization potential and the cohesive energy. Dimensional quantum-mechanical effects are examined.
REINTRODUCING THE IONIC STRUCTURE IN THE JELLIUM MODEL FOR METAL CLUSTERS: PSEUDOPOTENTIAL PERTURBATION THEORY
Authors: PACHECO J.M. , EKARDT W. , SCHÖNE W.D.
Published: October 2004

Ionic structure effects in small sodium clusters are studied via second-order pseudopotential perturbation theory. It is found that this formulation not only leads to the same ionic structures found via ab-initio Car–Parrinello structure optimization, but also to a substantial improvement in the overall description of the optical response of the clusters.
THE DYNAMICAL JAHN–TELLER PROBLEM IN X32E ⊗ e SYSTEMS
Authors: KOIZUMI HIROYASU , SUGANO SATORU
Published: October 2004

We present some dynamical Jahn–Teller effects in X32E ⊗ e systems. Specific applications are made to Na3 and Cu3, where spin–orbit interaction is expected to be negligible for the former and significant for the latter. The emphasis is put on localized vibronic standing-wave states for the ground vibronic level, Zeeman splittings of vibronic levels, and level splittings due to spin–orbit interaction.
STRUCTURES OF SILICON CLUSTERS
Authors: PAN JUN , BAHEL ATUL , RAMAKRISHNA MUSHTI V.
Published: October 2004

We determined the structures of silicon clusters in the 11–14-atom size range using the tight-binding molecular dynamics method. These calculations reveal that Si11 is an icosahedron with one missing cap, Si12 is a complete icosahedron, Si13 is a surface-capped icosahedron, and Si14 is a 4-4-4 layer structure with two caps. The characteristic feature of these clusters is that they are all surface.
STRUCTURES AND DYNAMICS OF SMALL ALKALI-HALIDE CLUSTERS
Authors: CHENG V.K.W. , ROSE J.P. , BERRY R.S.
Published: October 2004

The structure and phase behavior of small alkali-halide clusters were studied by constant-energy molecular dynamics and optimization. Premelting was found for ( KCl )n, n=6, 9, 12. A correlation of the energy of a number of alkali-halide clusters and their relative ionic radius was made to establish the condition for the preference of a given type of isomers as the more stable form. The isomerization dynamics of large ring-like LiBr, i.e., folding of larger rings to smaller ones with cube-like or close-packed structure, is consistent with the correlation.
ELECTRONIC STRUCTURE AND STABILITY OF NH4(NH3)n AND NH4(NH3)m(H2O)n
Authors: TAKASU R. , FUKE K. , MISAIZU F.
Published: October 2004

The photoionization process of NH 4( NH 3)n and NH 4( NH 3)m( H 2 O )n radicals produced by an ArF excimer laser photolysis of ammonia clusters and ammonia–water mixed clusters are examined using time-of-flight mass spectroscopy. The ionization potentials (IPs) of NH 4( NH 3)n (n=0−35) and NH 4( NH 3)m( H 2 O )n (m=0−4, n=0−3) are determined by the photoionization threshold measurements. The binding energies of NH 4( NH 3)n−1− NH 3 (n=1−6) are estimated from IPs. The results indicate that the bonding between NH 4 and NH 3 is semi-ionic. The IPs for the large ammoniated NH 4 clusters decrease with increasing n up to 35. The limiting value (n→∞) is found to be 1.33 eV, which coincides with the photoemission threshold of liquid NH 3. This feature is similar to those found recently for alkali-atom–ammonia clusters. A clear trend is found for the IPs of NH 4( NH 3)m( H 2 O )n; the clusters containing more water molecules have higher IP. This trend is ascribed to the large binding energy of [Formula: see text] comparing with that of [Formula: see text].
ELECTRON-CORRELATION EFFECTS ON GEOMETRICAL AND ELECTRONIC STRUCTURES OF SinNa CLUSTERS
Authors: KISHI R. , NAKAJIMA A. , IWATA S. , KAYA K.
Published: October 2004

The geometries and energies of silicon-sodium binary clusters ( Si n Na , n=2–7) were investigated with the inclusion of electron-correlation effects using the second-order Møller–Plesset perturbation theory (MP2) with the 6-31G basis set. In addition, the fourth-order (MP4) energies were evaluated to determine the ground-state isomers. Si 4 Na cluster was calculated with the polarized 6-31G* basis set. For all of the Si n Na clusters, the most stable isomers have bridge-site type structures. Apart from the bridge-site type Si 6 Na , the structure of the most stable isomer of SinNa keeps the frame of the corresponding Si n cluster unchanged, and the electronic structure of Si n Na is similar to that of the corresponding negative ion [Formula: see text]. The silicon framework of the bridge-site type of Si 6 Na is distorted from the compressed octahedron to the face-capped trigonal bipyramid.
SIZE-SELECTIVE MEASUREMENTS OF SILICON-CLUSTER POLARIZABILITIES BY A CLUSTER-BEAM DEFLECTION TECHNIQUE
Authors: WOENCKHAUS J. , SCHÄFER R. , BECKER J.A.
Published: October 2004

Average static electric polarizabilities of small and middle silicon-cluster size ranges have been measured employing a mass-selective molecular beam deflection method. The largest studied clusters contained N=60 atoms of silicon. The polarizabilities of the semiconductor clusters are compared to the results obtained for metal clusters. Especially, the measurements of the midsized semiconductor clusters Si30–Si45 are discussed with regard to the predicted spherical cage-like structures which have been recently by Röthlisberger et al.1 The results for the larger clusters are analyzed in terms of recent experiments of photoluminescence of small silicon nanocrystallites.2
STUDIES OF METASTABLY EXCITED HELIUM CLUSTERS
Authors: JIANG T. , KIM C. , NORTHBY J.A.
Published: October 2004

Helium clusters containing long-lived metastably excited helium molecules are produced by electron impact and formed into a beam. The delayed release of excited molecules from clusters after a long flight path is measured. The release rate can be enhanced by an intersecting monochromatic infrared light beam. The frequency dependence of the light-induced signal allows us to identify the excitation as the ground-state triplet helium [Formula: see text] molecule [Formula: see text]. The rotational structure of the a→b transition is noticeably different from the same line in bulk helium, and may provide information about the nature of the cluster in the future.
FIRST-PRINCIPLES STUDY ON ELECTRIC-FIELD-INDUCED STATES OF SILICON MICROCLUSTERS
Authors: WATANABE KAZUYUKI , ABE TAKASHI , OGINO TSUYOSHI , TAKEDA SAKURA
Published: October 2004

The first-principles molecular dynamics (FPMD) method is applied to a Si dimer and a Si trimer in electrostatic fields to calculate the charge polarization, the stable structures, and the cohesive energies. It is found that the bond length of the dimer increases, and the bond length and the bond angle of the trimer decrease as the electric field increases. The obtained structural change due to electric fields is compatible with the change in the cohesive energy. The vibrational dynamics of the dimer and trimer are also studied. The bond-bending mode of the trimer is found to be more largely influenced by the electric field than the bond-stretching mode. The present simulation revealed a fundamental role of electric fields in manipulating microclusters.
CORRELATION ENERGY IN SMALL JELLIUM CLUSTERS
Authors: GUET C. , BLUNDELL S.A.
Published: October 2004

The correlation energy of finite systems of electrons in a neutralizing spherical positively charged background (jellium) is investigated within various approximations. The correlation energy is defined with respect to the Hartree–Fock energy (with exact treatment of exchange). On one hand, the exact second-order energy contribution is calculated from many-body perturbation theory. On the other hand, a quasi-boson approximation to harmonic order only allows one to estimate the RPA ground-state correlation energy and to emphasize the relative importance of Coulomb-interaction screening corrections (ring diagrams). One finds that the second-order contribution accounts for most of the correlation energy. Comparisons with other approaches, such as density-functional theory and CI calculations, confirm this assertion.
PHOTOELECTRON SPECTROSCOPY OF MASS-SELECTED METAL-WATER CLUSTER NEGATIVE IONS: Cu−(H2O)n AND Na−(H2O)n
Authors: MISAIZU F. , TSUKAMOTO K. , SANEKATA M. , FUKE K.
Published: October 2004

The electronic structures of hydrated metal-atom clusters have been investigated by negative-ion photoelectron spectroscopy. We have obtained the photoelectron spectra of Cu −( H 2 O )n with n=0−4 and Na −( H 2 O )n with n=0−12. For the former clusters, we also detected the electron detachment from the CuOH −( H 2 O )n−1 which coexists with Cu −( H 2 O )n. The observed bands for both Cu −( H 2 O )n and Na −( H 2 O )n were all assigned to the transitions to the states originating in those of the metal atoms, which are shifted as a result of hydration. This result implies that the ground states of the neutral clusters still have a one-center character at the size range examined. In contrast, for the Na–water clusters, increasing character of the Rydberg-type ion-pair state in the negative-ion state is suggested from the vertical-detachment-energy dependence on the solvent number.
PHOTOELECTRON SPECTROSCOPY OF BINARY-METAL CLUSTER ANIONS CONTAINING SULFUR ATOM
Authors: NAKAJIMA A. , TAGUWA T. , NAKAO K. , HOSHINO K. , IWATA S. , KAYA K.
Published: October 2004

Photoelectron spectra of [Formula: see text] (M= Al , Cu, Ag, and Au) cluster anions were measured at the photon energies of 3.49 and 4.66 eV using a magnetic-bottle electron spectrometer having ~60-meV resolution. The electron affinities of AlS−, CuS−, AgS−, and AuS− are 2.60(3), 2.10(3), 2.09(3), and 2.44(3) eV, respectively. The large EA of AlS− is attributed to large stability by six bonding electrons, and a new electronic state of A ′ 2Π can be found 0.4 eV above the AlS ground state, which is in agreement with a theoretical calculation. In the photoelectron spectrum of AuS−, two peaks due to a splitting by the L–S coupling could be observed, and the amount of the splitting is 0.18(2) eV. The results of aluminum clusters containing sulfur atom will also be presented.
MAGNETISM IN TRANSITION-METAL CLUSTERS FROM THE ATOM TO THE BULK
Authors: BILLAS I.M.L. , CHÂTELAIN A. , de HEER W.A.
Published: October 2004

Molecular beam deflection measurements of small iron, cobalt, and nickel clusters show how magnetism develops as the cluster size is increased from several tens to several hundreds of atoms for temperatures from 80 and 1000 K. Cluster magnetization is found to be superparamagnetic for rotationally warm clusters, where it follows the Langevin function. The magnetization of rotationally cold clusters is anomalous: it is strongly reduced and nonlinear with the applied field. For superparamagnetic clusters, the magnetic moments can be determined from the magnetization. We find that ferromagnetism occurs even for the smallest sizes: for clusters with less than about 30 atoms the magnetic moments are atom-like and as the size is increased up to 700 atoms they approach the bulk limit, with oscillations probably caused by surface-induced spin-density waves. The trends are explained in a magnetic shell model. The magnetic properties of iron cluster show anomalies, suggesting that a high moment to low moment crystallographic phase transition in Fe clusters occurs at relatively low temperatures.
THE POSSIBILITY OF TWO MAGNETIC STATES IN Co19 CLUSTER
Authors: LEE KEEYUNG
Published: October 2004

The possibility of more than one magnetic state has been explored for the face-centered-cubic Co19 cluster using the local spin-density linear combination of the Gaussian orbital method. Fixed moment calculations were done and the converged states were used as an input potential to obtain self-consistent solutions. Two different magnetic states with the magnetic moment per atom of 1.00 μB and 2.05 μB were obtained. The high-spin moment state has lower energy and is considered the ground state, and the low spin moment a metastable state. The local magnetic moment of the center, first- and second-shell atoms are found to be 1.90, 1.64, and −0.72 μB respectively for the low spin state, and 1.87, 1.92, and 2.01 μB respectively for the high spin state. Considering that the center-site atom has shown slow convergence to bulk moment for other transition-metal clusters in previous studies, Co clusters are found to be exceptional as far as this property is concerned. Density of states (DOS) of the cluster and bulk have been compared after the cluster levels were Gaussian-broadened. The DOS similarity between cluster and bulk is also found to be reasonably good.
LONG-RANGE CORRELATIONS IN SMALL ATOMIC CLUSTERS
Authors: NAYAK SAROJ K. , RAMASWAMY R.
Published: October 2004

We study the power spectrum of fluctuations in the potential energy of atoms in small rare-gas clusters. At temperatures when the cluster is in a liquid-like state the spectra have a “1/f” dependence over a wide range of frequency f. This behavior is distinctly different from both the solid phase of clusters or bulk liquid, and is indicative of long-range temporal correlations. The origins of this phenomenon is explored by studying the individual potential-energy distributions in pure and mixed rare-gas clusters, Xe55 and ArXe54, via molecular dynamics simulations. Substitution of atomic impurities acts as an effective probe of the dynamics, and we observe that long-lived memory effects have their origins in hierarchical relaxation processes arising in the motion of the atoms from the surface to the core and vice-versa.
MAGNETISM OF SMALL TRANSITION-METAL CLUSTERS AND EFFECTS OF ISOMERIZATION
Authors: BORRMANN PETER , DIEKMANN BERND , HILF EBERHARD R. , TOMÁNEK DAVID
Published: October 2004

We investigate the magnetic properties of small transition-metal clusters using a simple statistical model, which requires some input data from ab-initio spin-density-functional calculations. In our study, we consider a thermodynamically equilibrated ensemble of clusters with different structures, spin multiplicities, and ground-state energies. We calculate the physical properties of this system by weighting the individual configurations according to the Boltzmann statistics. We find that presence of isomers with very similar ground-state energies, yet very different magnetic properties, gives rise to a rich magnetic behavior of the system which differs significantly from what would be expected for single configurations. We apply the present model to determine the magnetic susceptibility of a cluster ensemble of Langevin paramagnets. Our results show that some of the anomalies in the magnetic behavior of transition-metal clusters might be understood in the framework of our model which is, of course, limited by the extremely high computational effort needed to obtain the input data.
He-ATOM SCATTERING AS A PROBE OF CLUSTER VIBRATIONAL DYNAMICS
Authors: BUCK UDO
Published: October 2004

In a crossed molecular beam arrangement helium atoms are scattered from argon, ammonia, and water clusters which are generated in the size range of n=25 to 4600 by adiabatic expansion through conical nozzles. The inelastic energy transfer is detected by time-of-flight analysis of the scattered helium atoms with a resolution of better than 1 meV. For Ar the peak of the transferred energy is interpreted as compressional mode of a dense solid sphere. The positions of the maximum decrease as a function of the increasing average cluster size and converge to the value of the surface phonons of solid argon. The molecular clusters behave differently; the excitation of librational modes and of more than one peak in the transferred energy are observed.
FEMTOSECOND VIBRATIONAL DYNAMICS OF POTASSIUM DIMER
Authors: RUTZ S. , SCHREIBER E. , WÖSTE L.
Published: October 2004

Combining the femtosecond pump & probe technique with isotope-selective detection, the dynamics of the [Formula: see text] state could be investigated. The three-photon-ionization (3PI) signal was recorded over a delay time range of 220 ps. A strong oscillation around the zero-of-time dephases within 10 ps. Fourier analysis of the spectrum yields a group of frequencies settled between 63 and 68 cm−1 which can be assigned to the energy spacings in the A state being excited by a 90-fs laser pulse. The corresponding vibrational periods could be estimated to lie between 499 and 520 fs.
SPHEROIDAL VIBRATIONS OF Ar-CLUSTER ISOMERS ANALYZED BY MOLECULAR DYNAMICS CALCULATION
Authors: OZAKI YASUSHI , SHINODA YASUJI , ICHIHASHI MASAHIKO , KONDOW TAMOTSU
Published: October 2004

The vibrational motion of Ar clusters ( Ar n, n=20, 23, 25, 27, and 30) having isomers in a variety of shapes was simulated by the molecular dynamics method. The intensities of the breathing and quadruple spheroidal vibrations following the breathing-mode excitation of the isomers were calculated. It was found that only the breathing vibration was excited in nearly spherical isomers, while significant excitation of the quadruple spheroidal vibration as well as the breathing vibration was observed in nonspherical isomers. The degrees of sphericality of the isomers were assessed by use of their principal moments of inertia. The excitation of the quadruple spheroidal vibration was found to be closely related to the degrees of sphericality of the isomers.
VIBRATIONALLY RESOLVED PHOTOELECTRON SPECTRA OF ANNEALED ${\rm C}_n^-$ AND ${\rm Si}_n^-$ CLUSTERS
Authors: ICKING-KONERT G. SCHULZE , HANDSCHUH H. , BECHTHOLD P.S. , GANTEFÖR G. , KESSLER B. , EBERHARDT W.
Published: October 2004

Photoelectron spectra of annealed carbon-cluster anions [Formula: see text]n=5–70, are presented which show vibrational fine structure. From the vibrational frequencies, theoretical results, and ion mobility measurements, the geometry of the clusters can be determined: clusters with n=5, 7, and 9 are linear chains; n=10, 12, 14, 16, and 18 build monocyclic rings; for n=20, 24, and 28 the vibrational features correspond best to bicyclic rings; and for even n>30 fullerenes are found. In contrast to carbon, the mass distribution of silicon-cluster anions (up to n≈30) does not change dramatically upon annealing. An enhanced abundance of typical fragmentation products [Formula: see text] with n=4, 6, 7, and 10 is found. The vibrational fine structure for [Formula: see text] agrees with that of a pentagonal bipyramid.
ELECTRON–PHONON INTERACTION AND ELECTRONIC STRUCTURE OF SMALL METAL CLUSTERS
Authors: ZHAO JIJUN , CHEN XIAOSHUANG , LIU FENGQI , WANG GUANGHOU
Published: October 2004

The Su–Schrieffer–Heeger (SSH) Hamiltonian has been extended to study the electron–phonon interaction and the electronic structures of the alkali-like metal clusters. The eigen-energy levels of s valence electrons are obtained from a Hückel-like Hamiltonian including the correction of the electron–phonon interaction in the hopping integral, which is proportional to the variable of bond length. The self-consistent equations for electrons and phonons are solved adiabatically through an iteration process. The energy-level structures of an octahedral Cu6 cluster are calculated with variable electron–phonon coupling constant λ to investigate the influence of electron–phonon interaction on the lattice distortion and electronic structures of metal clusters. The size-dependent ionization potential for small Cun clusters are calculated and compared with the experimental results.
LIGHT EMISSION DURING EVAPORATION OF Ge IN AN Ar-GAS ATMOSPHERE
Authors: SASAKI Y. , SATO J. , KASUYA A. , NISHINA Y.
Published: October 2004

We have observed light emission from Ar atoms [Ar(I)], Ge atoms [Ge(I)]/clusters[Ge2], and Ge ions [Ge(II)] during the evaporation process of Ge in an Ar-gas atmosphere of a few Torr. The Ar(I) lines are observed regardless of whether the crucible is empty or filled with Ge. Ar atoms are excited through discharge initiated by electrons thermionically emitted from the hot W wire heating the crucible and accelerated by the potential applied to the wire. Since the emission due to Ge is observed only when the Ar(I) lines are observed, it is concluded that Ge atoms are excited by excited Ar atoms and possibly ions. Intensities of Ar(I) lines decrease by transferring energy to Ge atoms/clusters if the density of Ge vapor is high. Emission intensity diminishes with distance from the crucible surrounded by a heat reflector and a water-cooled shroud, dropping most rapidly for Ge(II), and progressively less rapidly for Ge(I), Ar(I), and Ge2. The Ge(II) lines are stronger for lower Ar pressures and for lower Ge vapor density, while the Ge2 line is stronger for higher Ar pressures. The relative intensities of Ar(I), Ge(I), GE(II), and Ge2 also depend on the structure of the evaporation source. Reabsorption of emitted light by Ge clusters modifies the profile of the emission spectrum at ~3.7 and 4.3 eV.
STATIC POLARIZABILITIES OF CHARGED SILVER METAL CLUSTERS EXTRACTED FROM THE OPTICAL SPECTRA
Authors: TIGGESBÄUMKER JOSEF , KÖLLER LARS , MEIWES-BROER KARL-HEINZ
Published: October 2004

The use of sum rules appears to be a powerful tool for obtaining experimental data on static properties of charged metal clusters which are currently not available. For this purpose the optical spectra of the clusters must be known. The measured absorption profiles of sputtered, i.e. hot, [Formula: see text] and [Formula: see text] which have been measured in our group give information about the desired properties. The extracted static polarizabilities show a distinct shell structure which reflects the overall electronic shape of the cluster. A comparison is made with AgN and positive alkali metal clusters. The polarizabilities of closed-shell [Formula: see text] and [Formula: see text] are in reasonable agreement with SIC-LDA corrected RPA polarizabilities.
PSEUDOPOTENTIAL EFFECTS ON THE OPTICAL RESPONSE OF ALKALI-METAL CLUSTERS
Authors: BLUNDELL S.A. , GUET C.
Published: October 2004

We discuss the size dependency of the optical response of Li, Na, and K clusters. The effects of the ionic structure and of the pseudopotential describing the electron–ion interaction are considered. The pseudopotential effect is found to be quite significant, especially in its nonlocal formulation important for Li clusters.
OPTICAL SPECTRA OF FREE SEMICONDUCTOR MICROCRYSTALS PRODUCED BY THERMAL EVAPORATION IN NOBLE-GAS STREAM
Authors: MOCHIZUKI S. , NAKATA H. , ASANUMA M.
Published: October 2004

A simple apparatus for measuring the optical spectra of free semiconductor microcrystals has developed. Well-directional microcrystal beams were produced and their extinction spectra show the quantum confinement of carriers or excitons in free semiconductor microcrystals. Experimental details and optical spectra of free and quasifree Ge, CuBr, ZnSe, and PbS microcrystals are presented.
MULTIPLE IONIZATION OF SODIUM CLUSTERS BY ION IMPACT
Authors: CHANDEZON F. , GUET C. , HUBER B.A. , JALABERT D. , MAUREL M. , MONNAND E. , RISTORI C. , ROCCO J.C.
Published: October 2004

The electronic excitation of alkali-metal clusters by low-energy ions is investigated experimentally for the first time. Multi-ionized clusters possibly undergoing Coulomb dissociation are formed and detected. Free sodium clusters of a few hundred atoms have been bombarded by different ion beams ( H +, O 5+, Ar 8+) of velocity ranging from 0.2 to 0.9v0. The mass/charge ratio of charged outgoing clusters is measured by a time-of-flight spectrometer of high resolution [Formula: see text]. Critical sizes for stability against charge excess have been deduced for cluster charges up to 6. Temperature effects on critical sizes are observed, and they reveal two distinct regimes: electron transfer in peripheral collisions with multicharged ions leading to “cold” highly ionized clusters and strong electronic excitation in central collisions subsequently followed by multielectron emission.
DELAYED ELECTRON EMISSION OF NEGATIVELY CHARGED TUNGSTEN CLUSTERS
Authors: WEIDELE H. , BECKER ST. , KLUGE H.-J. , LINDINGER M. , SCHWEIKHARD L. , WALTHER C. , ZIEGLER J. , KREISLE D.
Published: October 2004

The delayed electron emission of negatively charged tungsten clusters has been investigated on a time scale from 1 to 500 ms. After being stored in a Penning trap clusters ions [Formula: see text] were heated via multiphoton absorption (hν=1.81 eV). In contrast to alkali and coinage metals no photofragmentation could be detected. Instead, for all cluster sizes studied so far only a decrease in the initial ion intensity as a function of time after excitation was observed. This decrease is not caused by ion loss from the trap, but has to be attributed to neutralization via delayed electron emission. The presented results strongly suggest that this process can be viewed as “thermionic emission” known from bulk-metal surfaces.
DRAMATIC EFFECT ON LITHIUM-CLUSTER EVAPORATION: THE EXAMPLE OF${\rm Li}_{11}^+$
Authors: ANTOINE R. , RAYANE D. , DUGOURD PH. , VEZIN B. , TRIBOLLET B. , BROYER M.
Published: October 2004

We have studied the unimolecular evaporation of ionized lithium clusters for different conditions of ionization. In this article we present the total evaporative rates and the monomer–dimer branching ratios for [Formula: see text]. We observe nonstatistical effects, associated with the dissociation along a less energetically favorable channel. Even after a few microseconds, the internal energy is probably not statistically distributed in all the vibrational modes.
CORE EXCITATION IN FREE CLUSTERS: IONIZATION, RELAXATION, AND FRAGMENTATION
Authors: RÜHL E. , KNOP A. , HITCHCOCK A.P. , DOWBEN P.A. , MCILROY D.N.
Published: October 2004

Cluster-size-dependent spectroscopic changes are reported for the Ar(2p) core ionization, electronic relaxation via Auger decay, and subsequent fragmentation of free argon clusters with sizes up to 700 atoms. Excitation spectra recorded using zero kinetic energy (ZEKE) photoelectrons, Auger electrons, as well as multicoincidence ion mass spectra, are reported. It is found that core ionization energies decrease as the cluster size increases from the atom to the condensed phase. Evidence for thermalization of photoelectrons via intracluster scattering is found, indicating that ZEKE photoelectrons are sensitive to the bulk of clusters. In contrast, Auger electron yield spectra are more sensitive to the surface of the clusters. Ion pair and triplet signals from dissociative double and triple ionization are used to investigate the final stabilization step of core-excited clusters. Fragmentation mechanisms are deduced for different cluster sizes.
MASS SPECTRA AND DISSOCIATION PATTERNS OF TELLURIUM CLUSTERS USING POSITIVE AND NEGATIVE SECONDARY-ION MASS SPECTROMETRY
Authors: ITO H. , SAKURAI T. , MATSUO T. , ICHIHARA T. , KATAKUSE I.
Published: October 2004

Size distribution of positive and negative tellurium clusters in the size range from 2 to 56 atoms was investigated by secondary-ion mass spectrometry (SIMS). Cluster ions were produced by the 12-keV Xe+ ions bombardment of a sample tellurium sheet and were mass-analyzed using sector-type double-focusing mass spectrometers. It was found that a discontinuous variation of cluster-ion intensity appeared at specific numbers of n. These numbers were 5, 8, 12, 15, 19, and 23 for positive clusters and 6, 10, 13, and 16 for negative clusters. The dissociation pattern was also investigated by an acceleration-voltage scanning method. It was found that Te2, Te5, and Te6 fragmentation events occurred at a large probability. Observation of specific fragmentation patterns suggested the existence of nonsequential fragment channels.
FRAGMENTATION PROBABILITIES OF EXCITED ALKALI CLUSTERS
Authors: RUTZ S. , KOBE K. , KÜHLING H. , RUFF A. , SCHREIBER E. , SOMMERER G. , WÖSTE L.
Published: October 2004

Employing transient resonant two-color two-photon ionization spectroscopy in the picosecond and femtosecond regime, we have investigated the fragmentation dynamics of several excited electronic states of alkali clusters. In the case of the Na 3 C-state a simple energy-level model well describes the single exponential decay found for the temporal evolution of the transient spectra. This allowed to estimate fragmentation probabilities of the C-state vibrational bands as well as the onset of predissociation. For larger clusters, Na n as well as K n with n≥3, the ultrafast transients could be measured. By means of an improved fragmentation model the fragmentation probabilities are determined as a function of the cluster size. Significant differences between. Na n and K n clusters are discussed.
FRAGMENTATION OF ALUMINUM-CLUSTER ANIONS IN COLLISION WITH A SOLID SURFACE
Authors: TERASAKI A. , TSUKUDA T. , YASUMATSU H. , SUGAI T. , KONDOW T.
Published: October 2004

Dynamical processes involved in the scattering of aluminum-cluster anions, [Formula: see text](4≤N≤25), from a silicon surface were investigated. The scattered anions observed were fragment cluster anions, [Formula: see text](n≤N). The recoil velocities of these fragment anions were evaluated by analyzing both the angular dependence of their intensity and the collision-energy dependence of their time-of-flight. The recoil energy of each fragment anion thus obtained suggests that the fragmentation process involves not only sequential evaporation of aluminum atoms but also fission into fragment clusters. Comparison of the present result with that of the collision-induced dissociation with a rare-gas atom indicates that the electron transfer between the cluster and the surface plays an important role.
SPECTROSCOPY AND EXCITED-STATE DYNAMICS OF BENZENE-TRIMER ION
Authors: OHASHI K. , NAKAI Y. , SHIBATA T. , NISHI N.
Published: October 2004

The photodissociation of ( C 6 H 6)3+ is studied in the range hν=0.9–3.0 eV . The electronic spectrum of ( C 6 H 6)3+ shows the main band at hν≈1.29 eV and a side band at ≈1.02 eV. The ( C 6 H 6)3+ ion retains spectroscopic features characteristic of ( C 6 H 6)2+, suggesting that the positive charge is carried by a dimer subunit in ( C 6 H 6)3+. The major fragment ion changes smoothly from ( C 6 H 6)2+ to C 6 H 6+ with increasing photon energy from 0.9 to 1.8 eV. The product branching ratios and the kinetic energy of the C 6 H 6+ fragment are consistent with a two-step model for the formation of C 6 H 6+.
THEORETICAL STUDY OF FISSION OF SIMPLE METAL CLUSTERS USING A JELLIUM MODEL
Authors: KOIZUMI HIROYASU , SUGANO SATORU
Published: October 2004

Fission of simple metal clusters is investigated using a jellium model which allows axial symmetrical deformations and fission. The Kohn-Sham equations for the jellium model are solved by the discrete-variable representation method where the zeros of the sine functions and the zeros of the 0th order Bessel functions are used for the basis for z-coordinate and ρ-coordinate, respectively. The comparison between the jellium model results and the shell-correction method results is made.
ON THE FISSION OF CHARGED ALKALI-METAL CLUSTERS
Authors: RIGO A. , GARCIAS F. , ALONSO J.A. , LÓPEZ J.M. , BARRANCO M. , MAÑANES A. , NÉMETH J.
Published: October 2004

A deformed jellium model is used to calculate the fission barrier height of [Formula: see text] within the Kohn-Sham density-functional theory (i.e., including shell effects). Although the shape of the barrier obviously depends on the parametrization of the fission path, we have found that the saddle point corresponds to a configuration in which the emerging fragments are already formed and rather well apart. From this result, we offer a simple way to calculate the barrier height for charged trimer emission from alkali-metal clusters of (almost) arbitrarily large size and charge state.
FISSION OF NUCLEI AND METAL CLUSTERS
Authors: BJØRNHOLM SVEN
Published: October 2004

The fission process is very revealing in the way it illustrates the interplay between average (i.e., bulk) properties of the fissioning medium and specific quantum-size effects. In the nuclear quantum liquid, the charge (i.e., proton) density is everywhere confined to be about one-half of the total nucleon density, and the polarizability is extremely small. In metals, on the other hand, the net charge-to-mass ratio is virtually a free parameter and the excess charge resides on the surface, where it is easily polarized by external fields. Because of this, there is a very wide scope for fission studies with metal clusters. So far, such fission experiments have relied on thermally activated fission in competition with the evaporation of neutral atoms. The high polarizability and the sensitivity to initial- and final-state quantum effects is brought to light in this way. On the other hand, fission of large and highly charged clusters, which should be truly symmetric and proceed through a relatively compact transition configuration, remains to be explored.
REACTION PROCESSES INVOLVING GAS-PHASE CLUSTERS
Authors: KONDOW TAMOTSU
Published: October 2004

Gas-phase clusters exhibit specific reactivities which mainly arise from their unique geometric and electronic structures, and collective vibrations due to a limited number of the constituent atoms and molecules. In order to elucidate the specificities of the reaction processes involving the clusters, we describe several examples including our studies on collisional dissociation of size-selected sodium-cluster ions with rare-gas atoms, and impact of size-selected aluminum-cluster anions and [Formula: see text] with a silicon surface. In particular, the essential features of the cluster-anion-surface collision are elucidated. A dynamic solvent effect on the dissociation of [Formula: see text] is observed in the [Formula: see text] system.
THE STRUCTURE OF REACTED METAL CLUSTERS: PHOTOELECTRON SPECTRA OF ${\rm{Pt}}_n^-$ CLUSTERS REACTED WITH CO
Authors: HANDSCHUH H. , ICKING-KONERT G. SCHULZE , GANTEFÖR G. , BECHTHOLD P.S. , EBERHARDT W.
Published: October 2004

[Formula: see text] clusters produced by a laser-vaporization source are reacted with CO. The various reaction products [Formula: see text] are studied systematically using photoelectron spectroscopy of negatively charged clusters. First results are interpreted in terms of the Blyholder model. The spectra indicate an open electronic shell structure of the unsaturated and a closed shell structure of the saturated Pt carbonyls. For [Formula: see text] emission from the 2π/5d hybrid orbital is directly observed. The assignment is supported by the measured vibrational fine structure.
IR STUDIES OF CHEMISORPTION ON MOLECULAR SURFACES: AMMONIA ON SILVER CLUSTERS
Authors: RAYNER D.M. , LIAN L. , ATHANASSENAS K. , COLLINGS B.A. , FOURNIER R. , MITCHELL S.A. , HACKETT P.A.
Published: October 2004

Metal clusters are proposed as models to develop a molecular perspective on surface processes and to increase our understanding in important areas such as heterogeneous catalysis. This goal requires the characterization of small metal-cluster-ligand complexes in as great detail as possible. One avenue is to apply IR spectroscopy to cluster-ligand complexes. Problems of number density and mass selectivity prohibit the use of direct optical absorption methods. We report on progress towards applying infrared multiphoton dissociation, IRMPD, to obtain spectra by direct photodepletion. In this regard the silver-cluster-ammonia system is promising. The action spectrum for IRMPD of Ag 2 NH 3, monitored via the Ag 2 product in a fast-flow reactor, reveals an absorption peak at 1065 cm −1 which is assigned to the −NH3 umbrella mode of the complex. Molecular structure calculations using density-functional theory predict an end-on geometry and are in good agreement with this value. Preliminary results on complexes of larger Ag clusters studied by monitoring the IRMPD depletion of a Ag n NH 3 molecular beam by time-of-flight mass spectrometry are presented. The promise of these systems as models for NH 3 bound to Ag surfaces is discussed.
RECOMBINATION OF OPPOSITELY CHARGED AQUEOUS CLUSTER IONS USING ELECTROSTATICALLY MERGED ION BEAMS
Authors: PLASTRIDGE B. , COWEN K.A. , WOOD D.A. , COHEN M.H. , COE J.V.
Published: October 2004

A new method for studying cluster-cluster interactions is introduced which involves merging mass-selected beams of oppositely charged cluster ions with an electrostatic quadrupole deflector. Recombination is monitored by measuring the rate of fast neutral production. Relative rate constants have been measured for the reaction of H 3O+( H 2 O )n+ OH −( H 2 O )m as a function of cluster size (m=n=0–3), which display a pronounced enhancement with clustering. Relative rate constants have also been measured as a function of center-of-mass collision energy for a heavily clustered reaction (n=3, m=3) and a lightly clustered reaction (n=1, m=0) revealing that clustering produces a dramatic change in the reaction mechanism.
REACTIONS OF CARBON-SILICON BINARY CLUSTER IONS
Authors: NEGISHI Y. , WATANABE N. , SHIROMARU H. , ACHIBA Y. , KOBAYASHI N.
Published: October 2004

Ion-molecule reactions of silicon-carbon binary cluster ions [Formula: see text](n=1–2, m=1–2) with O 2 molecules were studied by using an rf ion trap. The rate constants and branching ratios were determined for these cluster ions. For [Formula: see text], the rate constants for two isomers were obtained.
PHOTOELECTRON SPECTROSCOPY OF ${\rm{Co}}_n^-$ AND PRODUCT ANIONS OF ${\rm{Co}}_n^-$ WITH O2 AND N2
Authors: YOSHIDA HIROYUKI , TERASAKI AKIRA , KONDOW TAMOTSU
Published: October 2004

The photoelectron spectra of [Formula: see text](3≤n≤70), Co m N − and Co m O −(3≤m≤6) were measured by a XeCl excimer laser (4.025 eV). Also the reactivities of [Formula: see text] with O 2 and CO were measured. A clear distinction between the cluster size of n≤6 and that of n≥7 was observed both in the photoelectron spectra and in the reactivities. (i) In the size range above n=7, the electron affinities of the cobalt clusters determined by the photoelectron spectra are linearly dependent on the reciprocal of the cluster radius, but below this size they deviate from this relation. (ii) The reactivity of [Formula: see text] with O 2 in the size range below n=6 is higher than that above n=7. However, the reactivity with CO in the size range below n=6 is extremely lower than that above n=7. These findings suggest that a transition of electronic structure in size dependence, which is correlated with the reactivity, occurs at n≃7.
REACTIONS OF MONOVALENT METAL IONS (M+) WITH MIXED MOLECULAR CLUSTERS (NH3)m(H2O)n AS STUDIED BY LASER-ABLATION MOLECULAR BEAM (LAMB) METHOD: PREFERRED OR NONSPECIFIC COORDINATION
Authors: SATO H. , MATSUZAKI A. , NISHIO S. , HORIKI Y. , ITO O.
Published: October 2004

Three distinct types of competitive solvation around monopositive metal ions in the gas phase have been revealed by the laser-ablation molecular beam method using binary clusters in the molecular beam: (i) preferred solvation of one of the two components, (ii) essentially nonselective solvation, and (iii) magic-number-like behavior of one component in the presence of the other component in the outer solvation sphere.
GAS-PHASE CHEMISTRY OF MOLYBDENUM CLUSTERS
Authors: RAYNER D.M. , LIAN L. , MITCHELL S.A. , HACKETT P.A.
Published: October 2004

The kinetics of reactions of molybdenum clusters, Mo n, n=1–25, in the pressure range 0.4–4 Torr, and temperature range 270–380 K, have been investigated using a large-bore, He-buffered, fast-flow reactor equipped with a laser-vaporization source for the production of clusters. The reactor is designed to make kinetic measurements on neutral metal clusters in the gas phase under well-defined pressures and temperatures. We discuss a new version of the instrument in which LIF techniques, used previously to monitor atoms and dimers, are replaced by laser ionization, time-of-flight mass spectrometry (TOFMS) in order to monitor larger clusters. The new version of the reactor has been tested against known reactions of Ti atoms. Examples of the reactor’s performance are taken from studies performed on Mon cluster reactivity. In particular we summarize some results on the dissociative chemisorption of molecular nitrogen, where large cluster-size effects are found. In some cases a negative-temperature dependence of the kinetics indicates the involvement of a precursor bound state and leads to conclusions concerning the shape of the potential-energy surface and how subtle changes associated with the cluster’s geometric structure might profoundly alter reaction rates.
COLLISIONAL PROCESSES INVOLVING SIZE-SELECTED SODIUM-CLUSTER IONS
Authors: TANAKA H. , MIZUNO T. , HIROKAWA J. , NONOSE S. , KONDOW T.
Published: October 2004

Collision-induced reactions of [Formula: see text](n=2–9) with Ne and Kr were investigated by means of a Tandem mass spectrometer equipped with octapole ion guide. The dominant processes were evaporation of a Na atom from [Formula: see text] with even sizes or Na 2 molecules from those with odd sizes. The absolute cross section for the reaction was measured as functions of the parent-cluster size and the collision energy. The branching fractions of the daughter-ion production were also determined. These results are interpreted in a scheme of a pair interaction in jellium background.