A1等离子体与甲醇团簇的反应

利用激光烧蚀-分子束法(LA-MB)对激光烧蚀金属铝靶产生的Al等离子体与脉冲分子束超声膨胀产生的(CH3OH)n团簇在气相条件下的反应进行了研究.烧蚀激光相对于脉冲分子束之间的延时不同,观测到团簇离子序列及团簇尺寸的变化,反映出脉冲分子束状态对反应条件及激光烧蚀等离子体状态的影响.在激光烧蚀发生于脉冲分子束的前段,主要反应产物为Al (CH3OH)n,但团簇尺寸较小;在烧蚀发生于脉冲分子束的中段,主要产物为H (CH3OH)n,团簇尺寸增大,强度减弱;在烧蚀发生于脉冲分子束的后段,观测到尺寸更大的水合质子化团簇H (H2O)m(CH3OH)n.结合团簇离子速度分布的特征,对团簇的产生机理进行了讨论.     

Enhanced harmonic generation from expanding clusters

We report controlled enhancement of optical third harmonic generation (THG) from hydrodynamically expanding clusters of approximately 6x10(5) noble-gas atoms several hundred femtoseconds following ionization and heating by ultrashort pump pulses. This resonant enhancement is more pronounced for orthogonal than for parallel pump-probe polarizations, a consequence of faster cluster expansion along the pump polarization. Simulations show that the nonlinear susceptibility chi(3) of the individual clusters and the coherence length of the clustered plasma medium are optimized nearly simultaneously as the clusters expand, and both contribute to the observed THG enhancement. This dual enhancement mechanism may be scalable to relativistic probe intensity and to generation of high-order harmonics in the soft-x-ray regime.     

A self-consistent model for the production and growth of nanoparticles in low-temperature plasmas

A theoretical global model is presented for describing the kinetics of generation and growth of clusters and nanoparticles in low-pressure plasmas, where important processes for clusters and grains are collisions with monomers, electrons, and ions and particle coagulation and loss because of diffusion and gas flow drag. Simple equations are given for calculations of monomer density, particle-size distribution function, critical cluster size, the rate of particle production and particle density and mean size, and plasma characteristics (the densities and average energies of “cold” and “hot” electrons and the density of positively charged ions). The model is self-consistent; that is, the above-mentioned properties of clusters, nanoparticles, electrons, and ions are calculated jointly from coupled equations as functions of a small number of radio frequency (RF) discharge parameters (discharge geometry; absorbed electric power; voltage across the RF sheath; gas pressure; composition; and flow rate). Comparisons are made with the experimental data on SiH₄-Ar mixtures. Published in Russian in Khimicheskaya Fizika, 2008, Vol. 27, No. 4, pp. 79–93. The article was translated by the authors.     

Ion spectrum under excitation of a cluster beam by a laser pulse

A model describing the decay dynamics of large charged clusters formed during the interaction of a short laser pulse with a cluster beam is constructed. It is shown that the dynamics of the evolution of the cluster plasma and the energy spectrum of ions are affected by the presence of the decay products from neighboring clusters and the electron background due to ionization. The parameters of the plasma being formed are determined as functions of the cluster beam and laser pulse parameters.     

稠密气固两相流中颗粒团运动的DEM模型研究

颗粒聚集成团是稠密两相流动中的基本现象．本文直接以颗粒团为研究对象,建立了颗粒团运动的DEM软球模型,具体给出了稠密气固两相流中颗粒团大小的计算表达式,建立了非球形颗粒团运动、接触碰撞及破碎模型,并将此模型用于模拟一循环流化床内稠密气固两相流动,得到了流化床内颗粒团详细的运动碰撞经历及浓度、粒径分布,所得结果合理,与前人实验值相符．另外,计算表明,采用颗粒团运动的DEM软球模,能使计算量有效减少,计算时间明显缩短,说明本模型可有效地用于工程意义上的稠密气固两相流问题的模拟。     

Effective generation of characteristic <Emphasis Type="Italic">K</Emphasis>-rays from large laser-excited SF<Subscript>6</Subscript> clusters in the presence of an Ar carrier gas

The parameters of characteristic sulfur K _α-rays generated from SF₆ clusters that are surrounded by argon atoms and are irradiated by intense laser radiation have been analyzed. It has been found that the formation of large SF₆ clusters under the optimum experimental parameters is accompanied by the high-efficiency generation of the characteristic X rays, and the flux density of the characteristic X-ray photons is 100 photons/(mrad² pulse) at a laser-pulse energy of 5 mJ. It has been shown that the third-harmonic generation process can be used to characterize the spatial sizes of the gas-cluster jet and the region of the cluster plasma.     

气粒并流上行流动中颗粒团特性的实验研究

用流动显示法和图像分析法研究了气粒并流上行流动中颗粒团的瞬态行为,通过灰度梯度识别颗粒团,由颗粒团成像的象素确定其尺寸,采用相关分析法计算颗粒团的速度。随着颗粒流量的提高和表现气速的降低,气粒并流上行流动中开始出现颗粒团,由于颗粒团与气相的相互作用,流态具有不稳定和不规则的现象.在一定操作条件下,在壁面附近出现了一些长时间停留的大尺寸颗粒团.     

Self-action of laser radiation in cluster plasma

We have analytically and numerically studied the self-action dynamics of laser radiation in a plasma with ionized gas clusters. Based on the simplified model of a cluster in the form of a superposition of two charged (electron and ion) bunches, we analyze the nonlinearity mechanisms. We refine the electrodynamic cluster model by the molecular dynamics method. The polarization behavior of the plasma bunch in the main part of the laser pulse is shown to be the same as that in the simplified model. We investigate the self-action dynamics of laser radiation under conditions when the nonlinearity of the stratified medium is determined by the anharmonicity of the electron motion in the cluster, while the group velocity dispersion is determined by both the background plasma and the ionized clusters. Since the characteristic field for the electron nonlinearity depends strongly on the cluster size, the peculiarities of the self-action dynamics result from plasma bunch expansion. The spatiotemporal evolution of the wave field is shown to be accompanied by pulse self-compression near the trailing edge.     

Influence of flow on the structure and dynamics of clusters in complex plasma systems

The influence of flow with different strengths, positions, and widths on the structure and dynamics of clusters is studied by two-dimensional (2D) Langevin molecular dynamics simulations. The particles are confined by a quadratic confining potential. The horizontal position of the system centre, average inter-particle distance, and coupling parameter are calculated to characterize the effect of changing the strength, position, and width of the flow on the cluster structure. The trajectories, the velocity autocorrelation function, and the mean square displacement are obtained to further uncover the dynamic properties of the 2D dusty plasma system.     

Processes occurring in cluster plasmas and cluster beams

A method of generating a cluster beam of a refractory metal is analyzed. Clusters are formed in a high-pressure arc discharge plasma in which the refractory metal is an additive to the buffer gas. The conditions for cluster formation to occur in the intermediate region of the discharge are found, and the cluster instability, as a result of which all of the refractory metal collects in the form of clusters in this zone of the discharge, is studied. The processes studied are the basis for of a method of generating intense cluster beams. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 10, 741–746 (25 November 1998)     

Some properties of ion and cluster plasma

The aggregate of problems connected with the physics of ion and cluster plasma is qualitatively considered. Such a plasma can exist when a dense gas is ionized by a hard ionizer. The conditions for the formation of an ion plasma and the difference between its characteristics and those of an ordinary electron plasma are discussed; a solvated-ion model and the distribution of the clusters with respect to the number of solvated molecules are considered. The recombination rate of the positively and negatively charged clusters is roughly estimated. The parameters of a ball-lightning plasma are estimated on the basis of the cluster model.Translated from Trudy Fizicheskogo Instituta im. P. M. Lebedeva, Vol. 120, pp. 50–64, 1980.     

Hard X-ray emission by clusters in an intense femtosecond laser field during collective recombination

We consider the new mechanism of X-ray generation by clusters under irradiation by femtosecond laser pulses, the so-called collective photorecombination. We develop the theory of the photo-recombination of electrons that pass from atomic clusters at the outer ionization to the ground level of a homogeneously charged cluster. Such a cluster is considered to be a quantum potential well. The dipole approximation is inapplicable for this process. We conclude that X-ray photons in collective photorecombination on a charged cluster as a whole have an energy that is much larger than that for photorecombination on separate atomic ions inside the cluster. For a typical cluster of 2.25 × 10⁶ electrons, with a radius R = 300 Å, and a number density of plasma electrons n _e = 2 × 10²² cm^?3, we find that at a 5% outer ionization of this cluster, the energy of hard X-ray photons is 7.2 keV.     

Fe–Si core/Si-shell clusters prepared by double glow discharge sources

Journal of Nanoparticle Research - Using an improved plasma gas condensation cluster deposition system, Fe and Si clusters are prepared by a single glow discharge source and Fe/Si hybrid clusters...     

Cluster generation by laser ablation

We produced niobium clusters over a wide size range (Nb_n: 1≤n≤1000) by a laser ablation method in a low-pressure condensation source. Mass spectra of the neutral clusters, positive and negative ion clusters were measured by a time-of-flight mass spectrometer with a high mass resolution and a wide mass range. Similarity and difference in their mass spectra were examined to understand the cluster generation process in different charged states.     

Overrun phenomenon and neutron yield in Coulomb explosion of deuterated alkane clusters driven by intense laser field

By using a simplified Coulomb explosion model, the laser-driven Coulomb explosion processes of three deuterated alkane clusters, i.e., deuterated methane(CD_4)_N, ethane(C_2D_6)_N and propane(C_3D_8)_N clusters are simulated numerically.The overrun phenomenon that the deuterons overtake the carbon ions inside the expanding clusters, as well as the dependence of the energetic deuterons and fusion neutron yield on cluster size, is discussed in detail. Researches show that the average kinetic energy of deuterons and neutron yield generated in the Coulomb explosion of(C_2D_6)_N cluster are higher than those of(CD_4)_N cluster with the same size, in qualitative agreement with the reported conclusions from the experiments of(C2 H6)_N and(CH4)_N clusters. It is indicated that(C_2D_6)_N clusters are superior to(CD_4)_N clusters as a target for the laser-induced nuclear fusion reaction to achieve a higher neutron yield. In addition, by comparing the relevant data of(C_3D_8)_N cluster with those of(C_2D_6)_N cluster with the same size, it is theoretically concluded that(C_3D_8)_N clusters with a larger competitive parameter might be a potential candidate for improving neutron generation. This will provide a theoretical basis for target selection in developing experimental schemes on laser-driven nuclear fusion in the future.     

Enhancement of high-harmonic generation by laser-induced cluster vibration

Sharp periodic enhancement of high-harmonic generation synchronized to prepulse-induced cluster vibration is observed. Tenfold enhancement of high-harmonic-generation efficiency is achieved by optimal selection of the prepulse-pump delay. This strong correlation between high-harmonic generation and cluster vibration also provides a new tool for studying the vibrational dynamics of nanometer atomic clusters.     

Recombination processes in atomic clusters irradiated by superintense femtosecond laser pulses

Various mechanisms of recombination of electrons with multiply charged atomic ions in atomic clusters irradiated by superintense femtosecond laser pulses are discussed. All of the recombination mechanisms are shown to take a time considerably longer than the laser pulse duration and, hence, they can develop only in a homogeneous, fairly rarefied cluster plasma after pulse termination. All autoionization states of multiply charged ions in a dense cluster plasma have been found to be destroyed by the Holtsmark electric field.     

Si6X(X=Cu,Ag,和Au)团簇多激子产生的从头算研究

利用对称性组态相互作用理论（SAC-CI）和LANL2DZ基组研究Si₆X（X=Cu、Ag和Au）团簇的多激子产生（MEG）.在团簇的MEG计算之前,基于密度泛函理论和全电子基组DZP-DKH,对团簇的几何结构进行优化并且经过频率分析确认团簇的能量稳定性.结果表明：多激子的主导成分是双激子,这些团簇的MEG都出现在可见光范围内,团簇的MEG强的部分在紫外线范围内,为利用MEG制备光俘获材料提供信息.     

Morphological studies of nanostructures from directed cluster beam deposition

Tin clusters with a most probable size of 2.2 nm are formed from a magnetron plasma aggregation cluster source and deposited on the amorphous carbon and silicon surface. The morphologies of the cluster-based nanostructures formed by directed cluster beam deposition with different cluster energies and incident angles are analyzed based on the transmission electron microscope (TEM) and atomic force microscope (AFM) observation.     

An escape of fast electrons from a plasma cluster in subcritical electric field

A behaviour of electrons with high initial thermal velocities in plasma clusters is discussed in the paper. These electrons from the tail of the Maxwell velocity distribution can be accelerated into considerable energies by means of an external electric field. Both numerical and analytical estimates of the energy of the escaping electrons and an estimate of the total energy transferred by the electrons from the cluster are treated in the paper. The calculations are performed for a situation usual in coaxial and rail accelerators of the plasma clusters.