Small rare gas clusters in XUV laser pulses

Semi-classical molecular dynamics simulations of small rare gas clusters in short laser pulses of 100 nm wavelength were performed. For comparison, the cluster response to 800 nm laser pulses was investigated as well. The inner ionization dynamics of multi-electron atoms inside the cluster was treated explicitly. The simulation results underpin the belief that at XUV (extreme ultraviolet) wavelengths collisions play an important role in the energy absorption. The generation of the surprisingly high charge states of Xe atoms inside clusters, as they were observed in a free-electron laser experiment at DESY, Hamburg, Germany by Wabnitz et al. is due to the reduced ionization potential of atoms inside charged clusters, the ionization ignition mechanism, and collisions. PACS 36.40.Gk; 36.40.Vz; 31.15.Gy     

载能团簇离子诱发的非线性二次离子发射

载能团簇离子进入固体时, 由于集体相互作用,在入射路径上产生非常高的能量沉积密度。 实验发现, 载能团簇离子的作用结果并不等于团簇中各原子独立作用的总和, 而是具有非线性效应。 就二次离子发射而言, 这种非线性通常与团簇的能量、 团簇的大小、 离子的电荷态以及靶物质的结构有关。 通过研究二次离子发射有助于理解载能团簇离子与物质相互作用过程中的能量沉积与释放机制。 Using energetic cluster as projectile is a unique way to produce simultaneous impacts of several atoms and deposit extremely high energy density in a very small area. The cluster impingement on solids has exhibited some non linear effects not presented in collisions of individual atoms with those solids. The study of the secondary ion emissions can give insight into the energy deposition and relaxation steps of the cluster solid interaction. The dependence of the yields of secondary ion emission induced by clusters on the energy of clusters,cluster sizes,charge states and material structures of the targets was reviewed in this paper.     

Highly charged ions from laser-cluster interactions: local-field-enhanced impact ionization and frustrated electron-ion recombination

Our molecular dynamics analysis of Xe_{147-5083} clusters identifies two mechanisms that contribute to the yet unexplained observation of extremely highly charged ions in intense laser cluster experiments. First, electron impact ionization is enhanced by the local cluster electric field, increasing the highest charge states by up to 40%; a corresponding theoretical method is developed. Second, electron-ion recombination after the laser pulse is frustrated by acceleration electric fields typically used in ion detectors. This increases the highest charge states by up to 90%, as compared to the usual assumption of total recombination of all cluster-bound electrons. Both effects together augment the highest charge states by up to 120%, in reasonable agreement with experiments.     

Charge localization in collision-induced multiple ionization of van der Waals clusters with highly charged ions

Charge localization in multiple ionization and fragmentation of small argon clusters is reported. The processes are initiated by interaction of the neutral cluster with highly charged Xe(q+) (5< or =q< or =25). Products are detected by means of multicoincidence time-of-flight methods. A strong dependence of the fragmentation pattern on the Xe charge state q is observed. In particular, we find evidence for formation of multiply charged atomic Ar(r+) fragment ions up to r = 7. Such high charge states have neither been observed in fission of multiply charged van der Waals clusters nor in ion-induced fragmentation of fullerenes or metal clusters. This hints at fundamentally different excitation and fragmentation dynamics.     

Ar原子团簇与飞秒强激光相互作用产生的高能离子计算

利用改进的“等离子体球”模型模拟了Ar原子团簇与飞秒强激光相互作用的物理过程.改进后的模型弱化了原模型在共振吸收附近团簇内部屏蔽电场的异常增强行为，从而使其更为合理，计算得到的Ar离子平均动能与以往的实验结果符合.还定量地研究了Ar离子平均动能及其平均电荷态与团簇尺寸以及激光参数之间的变化关系. 关键词： 团簇 飞秒激光 高能离子 共振吸收     

Cluster model approach for electronic structure of Si and Ge(111) and GaAs(110) surfaces

For the purpose of exploring how realistic a cluster model can be for semiconductor surfaces, extended Huckel theory calculations are performed on clusters modeling Si and Ge(111) and GaAs(110) surfaces as prototypes. Boundary conditions of the clusters are devised to be reduced. The ideal, relaxed, and reconstructed Si and Ge(111) surfaces are dealt with. Hydrogen chemisorbed (111) clusters of Si and Ge are also investigated as prototypes of chemisorption systems. Some comparison of the results with finite slab calculations and experiments is presented. The cluster-size dependence of the calculated energy levels, local densities of states, and charge distributions is examined for Si and Ge(111) clusters. It is found that a 45-atom cluster which has seven layers along the [111] direction is large enough to identify basic surface states and study the hydrogen chemisorption on Si and Ge(111) surfaces. Also, it is presented that surface states on the clean Si and Ge(111) clusters exist independent of relaxation. Further, the calculation for the relaxed GaAs(110) cluster gives the empty and filled dangling-orbital surface states comparable to experimental data and results of finite slab calculations. The cluster approach is concluded to be a highly useful and economical one for semiconductor surface problems.     

Preventing the CO poisoning on Pt nanocatalyst using appropriate substrate: a first-principles study

Adsorption energies and stable configurations of CO on the Pt clusters are investigated using the first-principles density-functional theory method. It is found that the adsorption of CO on the top site of the Pt₄ cluster is more stable than that on the bridge site. The atomic charges are unevenly distributed within the charged Pt₄ cluster, and the structural positions of the Pt atoms determine their charge states and therefore their activity. A systematic study on the effects of electrons and holes doping in the Pt₄ clusters suggest an effective method to prevent the CO poisoning through regulating the total charge in Pt₄ clusters. The graphene-based substrate is an ideal catalyst support, which makes the Pt catalyst lose electron and weakens the CO adsorption. The results would be of great importance for designing high active nanoscale Pt catalysts used for fuel cells.     

Electron attachment to anionic clusters in ion traps

Ion traps are versatile tools for the investigation of gas-phase cluster ions, allowing, e.g., cluster-size selection and extended reaction times. Taking advantage of their particular storage capability of simultaneous trapping of electrons and clusters, Penning traps have been applied for the production of clusters with high negative charge states. Recently, linear radio-frequency quadrupole traps have been demonstrated to be another candidate to produce polyanionic clusters. Operation with rectangular, rather than harmonic, radio-frequency voltages provides field-free time slots for unhindered electron passage through the trap. Several aspects of electron-attachment techniques by means of Penning and radio-frequency traps are addressed and recent experimental results are presented.     

Direct observation of the non-supported metal nanoparticle electron density of states by X-ray photoelectron spectroscopy

Synchrotron-based X-ray photoelectron spectroscopy on copper and silver cluster beams created by a magnetron-based gas-aggregation source has allowed mapping the electron density of states (DOS) of free metallic nanoparticles. The cluster DOS profiles obtained in the experiments strongly resemble the infinite solid DOS shapes, but the extracted cluster work-functions are lower than those for the bulk metal. The latter observation is explained by the initial negative charge on most of the clusters, created by the source.     

Finite cluster studies of electron correlation effects

A review of the physical properties of small systems described by various model Hamiltonians is given. Their physical properties are found either exactly or by using Monte-Carlo methods, and are compared with properties of bulk systems. It is shown that (i) clusters differ in an essential way from the bulk, having electronic states strongly dependent on the geometry and cluster size, but, at the same time, (ii) important insights into the nature of electron correlation and low-energy excitations in solids may be gained by cluster studies. In particular, the cluster calculations allow one to identify the tendencies to various kinds of symmetry-broken states in the (extended) Hubbard model and give a simple qualitative understanding of the low temperature properties of the Kondo and the heavy-fermion systems. Furthermore, cluster calculations allow one to study such problems as the development of magnetic moment at chemisorbed atoms for strong correlations and the suppression of frustration by quantum fluctuations. Recently, cluster calculations gave important information about the possible role of charge transfer processes and magnetic interactions in the microscopic mechanism of pairing in high temperature superconducting oxides.     

密度泛函理论研究SinMn (n=2～14)团簇的结构和电子性质

采用密度泛函理论中的广义梯度近似对SinMn (n=2～14) 团簇的几何构型进行优化,并对能量、频率和电子性质进行了计算. 结果表明,当n≥10时,Mn原子完全陷入Si原子形成的笼内.二阶能量差分、分裂能和垂直电离势都表明Si5Mn和Si12Mn是稳定的团簇,且12是团簇的幻数.通过对电子性质的分析发现Si12Mn团簇具有较高的化学稳定性.布局数分析表明,在Si5Mn团簇中Mn原子的磁矩(3.923 μB)是最大的.较多的电荷转移以及Mn原子的4s, 3d态和Si原子的3s, 4p态的较强杂化是导致Mn原子磁矩减小的原因.当n≥7时,SinMn 的总磁矩是1 μB.     

Dynamics simulation on interaction of intense laser pulses with atomic clusters

Under classical particle dynamics, the interaction process between intense femtosecond laser pulses and icosahedral noble-gas atomic clusters was studied. Our calculated results show that ionization proceeds mainly through tunnel ionization in the combined field from ions, electrons and laser, rather than the electron-impact ionization. With increasing cluster size, the average and maximum kinetic energy of the product ion increases. According to our calculation, the expansion process of the clusters after laser irradiation is dominated by Coulomb explosion and the expansion scale increases with increasing cluster size. The dependence of average kinetic energy and average charge state of the product ions on laser wavelength is also presented and discussed. The dependence of average kinetic energy on the number of atoms inside the cluster was studied and compared with the experimental data. Our results agree with the experimental results reasonably well.     

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.     

Chaotic cluster itinerancy and hierarchical cluster trees in electrochemical experiments

Experiments on an array of 64 globally coupled chaotic electrochemical oscillators were carried out. The array is heterogeneous due to small variations in the properties of the electrodes and there is also a small amount of noise. Over some ranges of the coupling parameter, dynamical clustering was observed. The precision-dependent cluster configuration is analyzed using hierarchical cluster trees. The cluster configurations varied with time: spontaneous changes of number of clusters and their configurations were detected. Simple transitions occurred with the switch of a single element or groups of elements. During more complicated transitions subclusters were exchanged among clusters but original cluster configurations were revisited. At weaker coupling the system itinerated among lower-dimensional quasistationary chaotic two-cluster states and higher-dimensional states with many clusters. In this region the transitions showed characteristics of on-off intermittency.     

超短强激光脉冲激励甲烷团簇的内电离机理

采用三维粒子动力学模拟方法研究了甲烷团簇在超短强激光脉冲激励下的爆炸动力学行为,重点讨论了几种典型的内电离机理对团簇爆炸过程中离子的价态和动能的影响.研究表明,在激光脉冲强度比较小的情况下,团簇中的原子主要是在光场作用下通过隧道电离的方式发生电离.当激光场进一步增强时,势垒压低电离是电离的主要方式.在相同的较高激光强度下,团簇更容易通过势垒压低电离达到高的电离价态.团簇发生电离后,其内部库仑电场的点火电离效应和内部滞留自由电子的碰撞电离效应也将增强团簇的再次电离过程. 关键词： 超短强激光脉冲 甲烷团簇 内电离     

Neutralization of mass-selected cluster ions by charge transfer reactions

Mass-separated beams of clusters with a single, well-defined number of atoms or molecules are indispensable for studies of cluster properties. This paper presents a technique with the potential to generate such a beam which contains neutral clusters of one size only. For this purpose mass-selected cluster ions have been neutralized by charge transfer reactions in metal and sulphur vapors. Relative charge exchange cross sections were measured for different cluster sizes. They reveal pronounced variations of the electronic structure of these particles. Thus, the study of charge transfer processes is introduced as a method to probe the positions of electronic energy levels in neutral clusters.     

过渡金属杂质与ZnSe配位环境的相互作用

把Cluster分为内部原子和边界原子,考虑电荷转移效应对Cluster环境势的影响,利用离散变分自洽电荷密度方法分析非冻结原子轨道的占有数,具体计算了过渡金属离子在ZnSe中的电子结构,Cluster的净电荷和电荷转移。得到了过渡金属离子与配位原子相互作用特性的一些新的规律性认识。 关键词：     

Investigation of the charge distribution in small cluster ions Ar<Stack><Subscript>13</Subscript><Superscript>+</Superscript></Stack> and Ar<Stack><Subscript>19</Subscript><Superscript>+</Superscript></Stack>

The results of ab initio studies of the atomic and charge structure of small clusters and cluster ions formed by 13 and 19 argon atoms are reported. It was found that the icosahedral atomic structure is energetically the most favorable for such clusters. The calculations demonstrate that when a single electron is removed from a cluster, the excess positive charge is distributed primarily over the surface of the formed cluster ion.