On the Theory of Neutral and Charged Cluster Ion Sputtering of Metal

A calculation method for large neutral and/or charged cluster (with number of atoms N > 5) elastic sputtering of a metal during ion bombardment is proposed. The result is presented as a simple assymptotic formula for the probability of cluster ejection and cluster charge state. A conclusion is made on the exponential nature of the dependence of the total cluster yield on the number of atoms the cluster consists of.     

Charge composition of a cluster plasma upon irradiation of large atomic clusters by the field of a superatomic femtosecond laser pulse

A theory is developed for calculating the charge composition of a cluster plasma produced upon irradiation of large atomic clusters by the field of a superatomic femtosecond laser pulse. The theory is based on the overbarrier process of a successive multiple internal ionization of atomic ions inside a cluster accompanied by the external field ionization. Collision ionization is also taken into account in the calculations. The theory is illustrated by the example of a cluster consisting of 10⁶ xenon atoms irradiated by a 50-fs laser pulse with a peak intensity of 2×10¹⁸ W/cm². In this case, the Xe²⁶⁺ ions dominate. The amounts of atomic xenon ions with multiplicity up to 31 are calculated.     

Energies,charges, and sizes of clusters under ion sputtering of a metal

A theory of ion sputtering of a metal in the form of neutral and singly charged clusters with a number of atoms of N ⩾ 5 has been developed. This theory is based on simple physical assumptions and agrees well with experiment. The results are presented in the form of expressions convenient for practical use. The energy spectra of clusters, charge distributions, ionization coefficients, and total yields of neutral and singly charged clusters at different target temperatures are calculated in terms of the proposed theory as an example.     

(OsnN)0,±(n=1-6)团簇电子结构与光谱性质的理论研究

采用密度泛函理论中的杂化密度泛函B3LYP方法在赝势基组LANL2DZ水平上对Os_nN^0,±(n=1-6)团簇的各种可能构型进行了几何结构优化,得出了它们的基态构型,并对基态结构的磁学性质、自然键轨道(NBO)、光谱和芳香性进行了理论研究. 研究结果表明:OsN^-和Os₅N^-团簇发生了"磁矩猝灭"的现象,在Os₂N和Os₄N< 关键词： nN^0,±(n=1-6)团簇')" href="#">Os_nN^0,±(n=1-6)团簇 电子结构 光谱性质 密度泛函理论     

Structural properties and reactivity of bimetallic silver-gold clusters

Bimetallic silver-gold clusters are well suited to study changes in metallic versus ionic properties involving charge transfer as a function of the size and the composition. We present structures, ionization potentials (IP) and vertical detachment energies (VDE) for neutral and charged bimetallic Au_nAg_m ( 2(n + m)5) clusters obtained from density functional level of theory. In the stable structures of these clusters Au atoms assume positions which favor charge transfer from Ag atoms. In clusters with equal numbers of hetero atoms (n = m = 1- 4) heteronuclear bonding is preferred to homonuclear bonding, giving rise to large values of ionization potentials. For larger clusters (n=m=5, 10) stable structures do not favor neither hetero bonding nor segregation into the single components, although they exhibit more metallic than ionic features. This remains valid also for Au₈Ag₁₂ cluster characterized by strong charge transfer to gold subunit. The influence of doping of pure gold clusters with silver atoms on VDE and IP values is discussed in context of their reactivity towards O₂ and CO molecules. As a starting point we consider reactivity towards CO and O₂ molecules on the example of AgAu^- dimer. The results show that the catalytic cycle can be fullfilled.     

Distribution and evolution of electrons in a cluster plasma created by a laser pulse

We analyze the properties and the character of the evolution of an electron subsystem of a large cluster (with a number of atoms n～10⁴?10⁶) interacting with a short laser pulse of high intensity (10¹⁷?10¹⁹ W/cm²). As a result of ionization in a strong laser field, cluster atoms are converted into multicharged ions, part of the electrons being formed leaves the cluster, and the other electrons move in a self-consistent field of the charged cluster and the laser wave. It is shown that electron-electron collisions are inessential both during the cluster irradiation by the laser pulse and in the course of cluster expansion; the electron distribution in the cluster therefore does not transform into the Maxwell distribution even during cluster expansion. During cluster expansion, the Coulomb field of a cluster charge acts on cluster ions more strongly than the pressure resulting from electron-ion collisions. In addition, bound electrons remain inside the cluster in the course of its expansion, and cluster expansion therefore does not lead to additional cluster ionization.     

Effect of the target density on the cross section of charge exchange between fast ions and atoms

When fast X^q+ X^{q^ + } ions collide with atomic or molecular targets, the total charge exchange cross section decreases with increasing target density. This is because the excitation levels of resulting X^{(q - 1)+} X^{(q - 1)^ + } ions are suppressed because of ionization by target atoms. The effect of target density on the total charge exchange cross section may amount to one order of magnitude or more depending on the charge and energy of an incident ion, as well as on the density and inner shell configuration of target atoms. Numerical calculations are performed for partial (in the principal quantum number n) cross sections σ(n) and total cross sections σ_tot=Σ_nσ(n) of charge exchange in the case of collisions of fast multiply charged ions having an energy E in the range 100 keV/u-10 MeV/u with gas or solid targets.     

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     

Surface heating of deuterium clusters by the field of a superintense ultrashort laser pulse for implementing the nuclear reaction d+d → 3He+n

A new mechanism for heating the electron component of plasmas formed upon the application of a superintense ultrashort laser pulse to atomic clusters is proposed. Clusters considered here consist of deuterium atoms. Upon the emission of a large number of electrons, an irradiated cluster, which acquires a positive charge, explodes (Coulomb explosion). Deuterons that are ejected as the result of this possess high kinetic energies, so that collisions between them can result in ³He formation accompanied by neutron emission. The new mechanism of the heating of the electron plasma from clusters is based on the conjecture that, when an ionization electron is reflected from the inner surface of the cluster ion in the presence of a laser field, it predominantly absorbs (rather than emits) laser photons.     

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.     

Ion-induced sputtering of a metal in the form of large clusters

A model is proposed for the ion-induced sputtering of a metal in the form of large clusters with a number of atoms N⩾5. The model is based on simple physical assumptions and is consistent with experiment. As an example, calculations are made of the relative cluster yield as a function of the number of atoms in the cluster as a result of the bombardment of various metals by singly charged 5 keV argon ions. A comparison is made with experimental data. Zh. Tekh. Fiz. 69, 64–68 (March 1999)     

Multiple ionization of argon in coincidence with projectile ions in 60–120 MeV Si q+-Ar collisions

A projectile ion-recoil ion coincidence technique has been employed to study the multiple ionization and the charge transfer processes in collisions of 60–120 MeV Si ^q+ (q = 4−14) ions with neutral argon atoms. The relative contribution of different ionization channels, namely; direct ionization, electron capture and electron loss leading to the production of slow moving multiply charged argon recoil ions have been investigated. The data reported on the present collision system result from a direct measurement in the considered impact energy for the first time. The total ionization cross-sections for the recoil ions are shown to scale as q ^1.7/E _p ^0.5 , where E _p is the energy in MeV of the projectile and q its charge state. The recoil fractions for the cases of total- and direct ionizations are found to decrease with increasing recoil charge state j. The total ionization fractions of the recoils are seen to depend on q and to show the presence of a ‘shell-effect’ of the target. Further, the fractions are found to vary as 1/j ² upto j = 8+. The average recoil charge state 〈j〉 increases slowly with q and with the number of lost or captured electrons from or into the projectile respectively. The projectile charge changing cross-sections σ _qq′ are found to decrease with increasing q for loss ionization and to increase with q for direct-and capture ionization processes respectively. The physics behind various scaling rules that are found to follow our data for different ionization processes is reviewed and discussed.     

Geometries and electronic properties of NbnV(0, ±1) (n = 1−6) clusters studied by density-functional theory

Using density functional theory (DFT) with valence basis set LANL2TZ to study the relative stabilities and electronic properties of the most stable structures of Nb _n V^(0,?±1) (n = 1?6) clusters. The ground state structures of Nb _n V ^(0,?±1) keep the similar geometric structure as the host Nb _n clusters. The doping of vanadium atom enhances the chemical activities of Nb _n clusters. The Nb₃V and Nb₆V are more stable than other clusters. The average binding energy of charged systems (Nb _n V⁺ and Nb _n V^? clusters) are generally larger than neutral Nb _n V clusters natural population analysis shows that there are charge transfers from niobium to vanadium atoms in the small Nb_1?4V, however, for larger clusters (Nb₅V and Nb₆V), the charge transfers are from vanadium to niobium atoms. The vertical and adiabatic ionization potentials (VIP and AIP) are estimated and the vertical one is more close to experimental results.     

密度泛函理论研究ScnO(n=1—9)团簇的结构、稳定性与电子性质

采用基于密度泛函理论的BP86/CEP-121G （O原子采用6-311G^**基组）方法,对Sc_nO （n=1—9）团簇的几何结构、能量与稳定性、电子结构性质及其随团簇尺寸的变化趋势进行了研究.随着团簇原子个数的增加,O原子从位于Sc_n团簇结构的边缘转变为占据团簇的内部位置.O原子的掺入增加了Sc_n团簇的稳定性,使其能隙升高,并改变了其稳定性及电子结构性质随团簇尺寸变化的规律;含有偶数个Sc原子的氧化物团簇比其周围邻近的含有奇数个Sc原子的氧化物团簇具有相对较高的稳定性.Sc_nO团簇电离势的理论计算值与实验值符合得较好,而其电子亲和势呈现振荡交替上升的变化趋势;用最大化学硬度规律等方法表征了Sc_nO氧化物团簇的稳定性和电子结构性质. 关键词： nO团簇')" href="#">Sc_nO团簇 几何结构 电子性质 密度泛函理论     

Structures and electronic properties of Mo2nNn（n=1-5）:a density functional study

The lowest-energy structures and the electronic properties of Mo2nNn(n=1-5) clusters have been studied by using the density functional theory(DFT) simulating package DMol 3 in the generalized gradient approximation(GGA).The resulting equilibrium geometries show that the lowest-energy structures are dominated by central cores which correspond to the ground states of Mo n(n = 2,4,6,8,10) clusters and nitrogen atoms which surround these cores.The average binding energy,the adiabatic electron affinity(AEA),the vertical electron affinity(VEA),the adiabatic ionization potential(AIP) and the vertical ionization potential(VIP) of Mo2nNn(n=1-5) clusters have been estimated.The HOMO-LUMO gaps reveal that the clusters have strong chemical activities.An analysis of Mulliken charge distribution shows that charge-transfer moves from Mo atoms to N atoms and increases with cluster size.     

Is NaI soluble in water clusters?

clusters (solvents being , or ) have been studied by resonance enhanced two photons ionization, leading to the detection of clusters. When water is the solvent, large clusters up to n>50 can be observed, whereas for and no clusters larger than 10 could be evidenced. Because the first step in the ionization process is the excitation from the ground solvated () ion pair state to a covalent excited state, the differences in the cluster size distribution for different solvent may be interpreted as a difference in cluster structures leading to a difference in the charge separation in the ground state. Received: 30 September 1997 / Revised in final form: 30 October 1997 / Accepted: 30 October 1997     

Ionization and fragmentation of clusters evaporated from the surface of a metal by accelerated ions

A mechanism is proposed for the ionization of clusters evaporated from the surface of a metal subjected to fast-ion bombardment. According to this mechanism, part of the internal energy of the cluster nuclei is transferred to its electronic subsystem, with subsequent ionization. Within the framework of this approach, the dependence of the degree of ionization on n (the number of atoms in the cluster) is calculated for the evaporated clusters. The results obtained are in agreement with experimental data. Zh. Tekh. Fiz. 67, 1–5 (December 1997)     

OsnN0,±(n=1—6)团簇几何结构与稳定性的理论研究

采用密度泛函理论(DFT)中的B3LYP方法,在Lanl2dz基组水平上对(Os_nN)^0,±(n=1—6)团簇的各种可能构型进行了几何参数全优化,得到了它们的基态构型;并对基态构型的平均结合能(E_b) 、二阶能量差分(Δ₂E_n) 、解离能(E_d)和能隙(E_g)进行了理论研 关键词： nN^{0')" href="#">Os_nN0 (n=1—6)团簇')" href="#">±(n=1—6)团簇 几何结构 稳定性 密度泛函理论}     

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