Predictions of geometrical structures and ionization potentials for small barium clusters Ba

The geometrical structure of ground state Ba_n clusters (n =2-14) has been predicted from various types of calculations including two ab initio approaches used for the smaller sizes namely HF+MP2( n =2-6), DFT (LSDA)( n =2-6, 9) and one model approach HF+pairwise dispersion used for all sizes investigated here. The lowest energy configurations as well as some isomers have been investigated. The sizes n =4, 7 and 13 are predicted to be the relatively more stable ones and they correspond to the three compact structures: the tetrahedron, the pentagonal bipyramid and the icosahedron. The growth behavior from Ba₇ to Ba₁₃ appears to be characterized by the addition of atoms around a pentagonal bipyramid leading to the icosahedral structure of Ba₁₃ which is consistent with the observed size-distribution of barium clusters. Values for vertical ionization potentials calculated for n =2-5 at the CI level are seen to be in quite good agreement with recent measures. Received: 14 May 1997 / Received in final form: 2 February 1998 / Accepted: 27 February 1998     

Formation of small water cluster anions by attachment of very slow electrons at high resolution

Using a high resolution laser photoelectron attachment method, we have studied the formation of (H ₂ O) _q ^- (q = 2, 6, 7, 11, 15) cluster ions in collisions of slow free electrons (E = 1-80 meV) and Rydberg electrons (n = 12-300) with water clusters. Resonances at zero energy have been observed, the shapes of which are strongly dependent on cluster size. The results are discussed in terms of the formation of metastable negative ions. Received 8 March 1999     

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     

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     

Hydrogen adsorption and desorption on the Pt and Pd subnano clusters — a review

In this review, we present our recent first principles studies on the sequential H₂ dissociative chemisorption and H desorption on the Pt _n and Pd _n clusters (n=2–9, 13). Upon full saturation by H atoms, the calculated H2 dissociative chemisorption energy and H desorption energy on Pt₁₃ and Pd₁₃ clusters are similar to the corresponding values on smaller close-packed clusters. Indeed, the catalytic performances of these subnano clusters do not vary significantly with the particle sizes or shapes. Instead, they are dependent on the surface metal atoms which can be accessed by H atoms. In addition to the coverage dependency of the H₂ dissociative chemisorption and H sequential desorption energies, the phase transition of both Pt₁₃ and Pd₁₃ from the icosahedral to fcc-like structures at certain H coverage was also investigated.      

Charge transfer between alkali cluster ions and atoms in the 1 to 10 keV collisional energy range

The cross-sections for collisional charge transfer between singly charged free clusters M _n ⁺ (M = Li, Na; n=1...50) and atomic targets A (cesium, potassium) have been measured as a function of collisional relative velocity in laboratory energy range 1–10 keV. For each cluster size, the experimental values of the charge transfer cross-section are fitted with an universal parametric curve with two independent parameters and v_m, the maximum cross-section and the corresponding velocity. For small size clusters (), the characteristic parameters show strong variations with the number of atoms in the cluster. Abrupt dips observed for n=10 and n=22 are attributed to electronic properties. Charge transfer patterns observed for various collisional systems present similarities, which appear more sensitive to cluster quantum size effects than to collision energy defects. In their whole, the and v_m parameters show differences in both their size evolution and their absolute values discussed in term of projectile and target electronic structures. Received 13 April 2000 and Received in final form 29 June 2000     

利用Gupta势结合遗传算法研究ConCu55－n(n=0—55)混合团簇的结构演化及基态能量

采用半经验的Gupta多体势结合遗传算法对Co_nCu_55－n(n=0—55)混合团簇的基态结构和能量进行了研究,发现这些混合团簇的基态结构是在Co₅₅,Cu₅₅单质团簇(Mackay二十面体)的基础之上发生的畸变;从n=0(Cu₅₅)开始,Co原子从中心到表面,从棱到顶点依次、连续替换Cu原子;基态结构与键能较大键的数目及其平均键长有关;Co₁₃Cu₄₂具有最稳定的结构,13个Co原子全部位于团簇内部形成Mackay二十面体对整个团簇的稳定性有显著影响. 关键词： 团簇 结构和能量 Gupta势 遗传算法     

Stability and fragmentation of multiply ionized clusters

The stability of neutral, singly and multiply ionized silicon clusters, (N = 2-7, M = 0, , , ), has been investigated using an ab initio density functional method. We show that the fragmentation effect significantly affects the structure of mass-spectra of multiply ionized silicon clusters. For clusters, the clusters with a large fragmentation energy are found to correspond to the high peaks at N = 4 and 6 in mass-spectra. For clusters, a peak at N = 5 in mass-spectra has been predicted to be especially high. Received: 9 June 1997 / Revised: 8 January 1998 / Accepted: 25 February 1998     

Systematic study of small BN clusters

We performed a systematic investigation of the small B_xN_y (x + y? 6) clusters using the ab initio Hartree-Fock scheme plus second-order perturbation theory. The nature of the potential energy surface extrema are analyzed through analytical total energy second derivatives. Ionization potentials, binding energies and the stability against some possible reaction mechanisms are calculated. Based on these results we propose that the growing process for these clusters is mainly due to the successive incorporation of BN molecules. A discussion of some mass spectrometry experimental results is also presented. Received 2 October 2000     

Hindered Coulomb explosion of embedded Na clusters — stopping,shape dynamics and energy transport

We investigate the dynamical evolution of a Na₈ cluster embedded in Ar matrices of various sizes from N=30 to 1048. The system is excited by an intense short laser pulse leading to high ionization stages.We analyze the subsequent highly non-linear motion of cluster and Ar environment in terms of trajectories, shapes, and energy flow. The most prominent effects are: temporary stabilization of high charge states for several ps, sudden stopping of the Coulomb explosion of the embedded Na₈ clusters associated with an extremely fast energy transfer to the Ar matrix, fast distribution of energy throughout the Ar layers by a sound wave. Other ionic-atomic transfer and relaxation processes proceed at slower scale of few ps. The electron cloud is almost thermally decoupled from ions and thermalizes far beyond the ps scale.     

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     

Ag309团簇升温过程中结构转变的分子动力学模拟研究

采用分子动力学模拟研究了具有面心立方（fcc）晶格结构的截断八面体Ag309团簇升温过程中结构演变。对团簇的能量曲线变化、快照图演变和键对分析表明：无缺陷截断八面体Ag309团簇在410 K时转变为二十面体,在840 K时熔化;不同缺陷诱导二十面体结构转变温度异常变化,沿晶面滑移缺陷使二十面体转变温度升高,沿晶面旋转缺陷使二十面体结构转变温度降低;不同缺陷对团簇键型和势能产生的影响是使二十面体结构转变温度异常变化的主要诱导因素。这种通过缺陷控制团簇结构转变的研究为新型纳米结构的可控制备提供理论基础。     

密度泛函理论研究Zrn(n=2—16)团簇的基态结构及其稳定性

从第一性原理出发，利用密度泛函理论中的广义梯度近似(GGA)对Zr_n(n=2—16)团簇进行了结构优化、能量和频率计算.在充分考虑自旋多重度的前提下，对每一具体尺寸的团簇，得到了多个平衡构型，并根据能量高低确定了团簇的基态结构.综合团簇的结合能、离解能、二阶能量差分以及团簇的最高占据轨道(HOMO)和最低未占据轨道(LUMO)间的能隙可知Zr₂，Zr₅，Zr₇，Zr₁₃ 关键词： n团簇')" href="#">Zr_n团簇 密度泛函理论(DFT) 基态结构 自旋多重度     

Static electric dipole polarizabilities of Na clusters

The static electric dipole polarizability of Na _N clusters with even N has been calculated in a collective, axially averaged and a three-dimensional, finite-field approach for , including the ionic structure of the clusters. The validity of a collective model for the static response of small systems is demonstrated. Our density functional calculations verify the trends and fine structure seen in a recent experiment. A pseudopotential that reproduces the experimental bulk bond length and atomic energy levels leads to a substantial increase in the calculated polarizabilities, in better agreement with experiment. We relate remaining differences in the magnitude of the theoretical and experimental polarizabilities to the finite temperature present in the experiments. Received 8 November 1999     

Non-crystalline structures in the growth of silver nanoclusters

The growth of nanometer-size free Ag clusters is studied by Molecular Dynamics simulations. The morphology transition from the icosahedron at the magic size of N = 55 atoms to the Marks truncated decahedron at N = 75 is analyzed in details, in order to single out kinetic trapping and entropic effects. At very low T, the cluster is kinetically trapped in an icosahedral structure. At intermediate T the transition takes place sharply at N≃ 65. At higher T, the transition is smeared out and finally, around 550 K no transition is found because the 75 decahedron is melted. Received 28 November 2000     

Novel metal-encapsulated caged clusters of silicon and germanium

We report the recent findings of metal (M) encapsulated clusters of silicon from computer experiments based on ab initio total energy calculations and a cage shrinkage and atom removal approach. Our results show that using a guest atom, it is possible to wrap silicon in fullerenelike (f) structures, as sp² bonding is not favorable to produce empty cages unlike for carbon. Transition M atoms have a strong bonding with the silicon cage that are responsible for the compact structures. The size and structure of the cage change from 14 to 20 Si atoms depending upon the size and valence of the M atom. Fewer Si atoms lead to relatively open structures. We find cubic, f, Frank-Kasper (FK) polyheral type, decahedral, icosahedral and hexagonal structures for M@Si_n with n = 12-16 and several different M atoms. The magic behavior of 15 and 16 atom Si cages is in agreement with experiments. The FK polyhedral cluster, M@Si₁₆ has an exceptionally large density functional gap of about 2.35 eV calculated within the generalized gradient approximation. It is likely to give rise to visible luminescence in these clusters. The cluster-cluster interaction is weak that makes such clusters attractive for cluster assembled materials. Further studies to stabilize Si₂₀ cage with M = Zr, Ba, Sr, and Pb show that in all cases there is a distortion of the f cage. Similar studies on M encapsulated germanium clusters show FK polyhedral and decahedral isomers to be more favorable. Also perfect icosahedral M@Ge₁₂ and M@Sn₁₂ clusters have been obtained with large gaps by doping with divalent M atoms. Recent results of the H interaction with these clusters, hydrogenated silicon fullerenes as well as assemblies of clusters such as nanowires and nanotubes are briefly presented.     

Temperature-dependent ionization potential of sodium clusters

The ionization potential of sodium clusters () at a finite temperature is studied using density functional theory and ab initio molecular dynamics. The threshold regions of the photoionization efficiency curves are deduced from the integrated IP distributions, which are obtained from the energy eigenvalues of the highest occupied Kohn-Sham states during molecular dynamics by applying a theoretically well-defined shift. The calculated ionization potentials are directly compared to the experimental values. The energetically best geometry of Na₅₅ is found to be a slightly distorted icosahedron. Received 16 April 1999 and Received in final form 6 July 1999     

Simulation of small positively charged MgO clusters and study of their electronic densities

With the help of ab initio methods the clusters [(MgO)₁₃Mg] ^Q+ are simulated for Q = 0, 1, 2. Then, vacancy clusters [(MgO)₁₂Mg₂] ^Q+ obtained by removing one oxygen atom are computed for Q running from 0 to 4. These clusters exhibit a slight sphericity and generally shorter interatomic distances than in the crystal. The electronic densities variations are studied in function of Q. In particular, it is observed that the electronic density in the oxygen vacancy goes to a maximum when Q = 2. The ionisation potentials vary from approximately 4 to 14 eV when Q varies from 0 to 3, with a more rapid increase from Q = 1 to Q = 2. The stability study of vacancy clusters show that they experience a phase transition when their charge becomes equal to 2, in accordance with the features mentioned above. Received 14 September 1999 and Received in final form 2 December 1999     

Influence of covalence and anion symmetry on the structure of small metal hydroxide clusters: Sodium versus silver hydroxide

An ab initio study of the Na_n(OH)_n, Na_n(OH)_n-1 ⁺, Ag_n(OH)_n, and Ag_n(OH)_n-1 ⁺ clusters with n up to four is presented. The results of this study show that, in accordance with experimental observations, the sodium hydroxide clusters are almost purely ionic, while the Ag-O bond exhibits a significant covalent character. The perturbation caused by the non-spherical OH^- group relatively to an atomic anion, as well as the influence on structures and energies of the covalent character of the metal-oxygen bond are determined. The appearance of metal-metal bonds in the silver hydroxide clusters is also discussed. Finally, the theoretical results obtained on the Na-OH clusters are compared to experimental results available on the dissociation of the Na_n(OH)_n-1 ⁺ clusters. Received 9 August 1999 and Received in final form 1st December 1999     

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