Rayleigh instabilities in multiply charged sodium clusters

The stability of multiply charged sodium clusters Na(q+)(n) (q< or =10) produced in collisions between neutral clusters and multiply charged ions A(z+) ( z = 1 to 28) is experimentally investigated. Multiply charged clusters are formed within a large range of temperatures and fissilities. They are identified by means of a high-resolution reflectron-type time-of-flight mass spectrometer (m/deltam approximately 14 000). The maximum fissility of stable clusters is obtained for z = 28 and is X approximately 0.85+/-0.07, slightly below the Rayleigh limit (X = 1). It is mainly limited by the initial cluster temperature (T approximately 100 K).     

On the fragmentation of multiply charged sodium clusters

The fragmentation of multiply charged atomic sodium clusters of mass 200 is investigated using the Micro-canonical Metropolis Monte Carlo (MMMC) statistical technique for excitation energies up to 200 eV and for cluster charges up to +9e. In this work we present caloric curves and charged and uncharged fragment mass distributions for clusters with charges 0, 2, and 4. The caloric curves show a dip at the critical point implying a negative specific heat, as expected for finite systems, while the fragment mass distributions corroborate the picture of a phase transition from one dominant liquid-like cluster to complete vaporization. Received 7 November 2001 / Received in final form 4 April 2002 Published online 28 June 2002     

Features of the X-ray radiation of interacting multiply charged clusters

The features of coherent x-ray bremsstrahlung radiation arising from the interaction of nanoparticles, namely, a fast and an immobile cluster consisting of positively charged nuclei and electrons are analyzed. The polarized bremsstrahlung radiation (PBR) of fast charges on electrons of an immobile cluster and the conventional bremsstrahlung radiation (BR) of fast electrons on heavy charges of this cluster are analytically investigated.     

Production and investigation of multiply charged metal clusters in a Penning trap

Singly charged gold cluster ions from a laser-vaporization source are transferred into a Penning trap and subjected to electron bombardment. The charged reaction products are analyzed by time-of-flight mass spectrometry after axial ejection from the trap. They include singly charged cluster fragments, multiply charged clusters of the initial size and multiply charged cluster fragments. The multiply charged clusters are selected and further investigated by collision induced dissociation. Two types of reactions can be distinguished: Dissociation into several charged fragments and evaporation of neutrals. Several features of multiply charged clusters relevant for future investigations are reviewed.This work comprises part of the dissertation of J. Ziegler.     

Dissociation pathways of doubly and triply charged gold clusters

Collision induced dissociation is applied to study the fragmentation channels of multiply charged gold clusters, Au N2⁺, size N= 7 –35, and Au> _N3+}, N = 19–35, stored in an ion cyclotron resonance (Penning) trap. The main dissociation pathways are neutral monomer evaporation, Au> _NZ+to Au> _N-1Z+} + Au, for the larger and fission into a charged trimer plus the remaining cluster, Au> _NZ+to{}Au> _N-3(Z-1)+} +{}Au> ₃₊}, for the smaller clusters. In the intermediate cluster size region an odd–even alternation of the two competing decay pathways is observed. In addition, for some specific cluster sizes there are indications of neutral dimer evaporation, Au> _NZ+to{}Au> _N-2Z++Au> ₂}, and of extremely asymmetric fission of the form Au> _NZ+to Au> _N-1(Z-1)+} + Au⁺. This revised version was published online in July 2006 with corrections to the Cover Date.     

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     

Charge separation in CO oxidation involving supported gold clusters

The character of the catalytic oxidation of CO by supported gold cluster catalysts is analyzed with emphasis on the unique characteristics of this process. The scheme of this process used here has the reagent CO molecule captured in the interface between the cluster and support, with oxygen molecules or atoms located on the support surface to react with the CO. (Other models have also been presented.) The experimental data indicate that, together with configurational transitions that lead to the CO molecule joining an oxygen atom to form the CO₂ molecule, the charge separation due to capture of the CO molecule by the supported gold cluster is important. The process of release of the CO₂ molecule results in charge exchange; the time for this process is relatively long because of the large distance separating positive and negative charges, a distance exceeding the cluster radius. This provides a high efficiency of the oxidation of CO with this catalyst despite the relatively high activation energy for the configurational transition.     

中性和带电小钨团簇的第一性原理研究

采用基于密度泛函理论的第一性原理方法,计算了中性和带电小钨团簇W_n（n＝3—6）的构型,得到了一些能量较低的结构,它们都有可能是团簇的基态.研究发现,这些团簇大多具有较低的自旋多重度：中性小钨团簇为自旋单态或三重态;带电小钨团簇为自旋双重态或四重态.当n>3时,小钨团簇具有三维立体结构.通过拟合光电子谱、计算垂直离化能并与实验值比较,进一步讨论了带负电小钨团簇的基态结构. 关键词： 小钨团簇 构型 密度泛函理论