Modeling the formation of alkali clusters attached to helium nanodroplets and the abundance of high-spin states

The doping process of helium nanodroplets with alkali atoms has been modeled in order to study deviations from the Poissonian statistics of measured pick-up statistics which are important for assigning cluster or complex sizes in many experimental studies. Several, formally unexplained findings are reproduced and their origin has been analyzed: derivations from the expected functional form of the initial incline, the suppression of the formation of lithium clusters, the influence of the functional form and width of droplet size distributions. Furthermore, the controversially discussed formation of high-spin alkali clusters on helium droplets has been calculated within the model. The selection of high-spin states comes out to depend strongly on the experimental conditions, and is in general not pronounced for cluster sizes ≥ 3. The enhancement factor of 50 of high-spin states reported in earlier experiments is reproduced when choosing the conditions of these experiments.     

Formation of cations and anions upon electron interaction with (doped) helium droplets

Superfluid helium droplets have provided a new perspective for studying electron induced chemistry at extremely low temperatures. Helium droplets represent an ideal environment for the formation of novel and exotic agglomerates of atoms and molecules. Mass spectrometry can be used to detect the resulting ions formed upon electron ionization and electron attachment to doped droplets. In the case of electron ionization a helium atom of the droplet is ionized initially and after few resonant charge transfer reactions between helium atoms the charge finally localises on the dopant. An alternative process is Penning ionization of the dopant, where the scattered electron first electronically excites a helium atom on the surface of the droplet. The attachment of a low energy electron leads to formation of an electron bubble inside the droplet which decays by autodetachment or localization on a dopant, if present in the droplet. In the present minireview a general overview about the field of electron scattering with doped helium droplets is given and a presentation of important recent results related to these electron collision studies is given as well.     

Highly ordered Ga nanodroplets on a GaAs surface formed by a focused ion beam

The morphological evolution of a GaAs surface induced by a focused ion beam (FIB) has been investigated by in situ electron microscopy. Under off-normal bombardment without sample rotation, Ga droplets with sizes from 70 to 25 nm in diameter on the GaAs surface can self-assemble into a highly ordered hexagonal pattern instead of Ostwald ripening or coalescence. The mechanism relies on a balance between anisotropic loss of atoms on the surface of droplets due to sputtering and an anisotropic supply of atoms on the substrate surface due to preferential sputtering of As. The ratio of wavelength to the droplet diameter predicted by this model is in excellent agreement with experimental observations.     

纳米氦液滴-超低温化学研究装置

成功研制了新一代纳米氦液滴实验装置．氦液滴是由高压的高纯氦气通过超低温的脉冲阀绝热膨胀形成的,通过调节连接在超低温氦冷头上的脉冲阀的温度(10～30 K)及脉冲阀内氦气的背景压力(10～40 atm),氦液滴的大小在包含103到105个氦原子间连续可调,和传统的连续氦液滴束源比较起来,脉冲束源的强度提高了一个量级以上,提供了一个和商品化脉冲激光器结合使用,研究超低温条件下超流体氦中的化学反应动力学的机会．通过研究氦液滴中包裹的CH₃I分子的光解动力学来对仪器的性能进行测试,利用离子速度影像技术研究了超低温纳米氦液滴中包裹的CH₃I分子在252 nm下的光解动力学,光解产物甲基通过(2+1)共振增强多光子电离并运用离子影像进行检测,结果表明光解产物的平动能及角分布被氦原子环境有效的弛豫．也证实了运用脉冲氦液滴束源研究衰减光谱的可行性,通过对氦液滴中掺杂苯的衰减光谱的研究,还发现小于3%的衰减信号都可以被检测到,表明所研制的脉冲氦液滴束的稳定性及检测器的灵敏度都是很高的．     

氦离子低温预辐照对不锈钢中氦泡生长抑制作用的Monte Carlo模拟研究

实验中已发现氦离子低温预辐照不锈钢材料能有效抑制高温辐照阶段的氦泡生长,但这一结果一直缺少理论解释.基于持续注入粒子的三维格子气模型,采用Monte Carlo模拟方法研究了不同温度模式对氦泡生长行为的影响.研究发现,计算结果能很好地再现氦离子低温预注入抑制高温氦泡生长的实验现象,其原理为低温预注入时材料中形成了较高数密度的小氦泡,由于氦泡数量较多从而限制了氦泡平均尺寸的增大.     

Electron delocalization in magnesium clusters grown in supercold helium droplets

The formation of bare clusters from highly reactive metals can be achieved very effectively by the pickup of atoms into superfluid helium droplets. We report on the experimental observation of electronic shells in small magnesium clusters produced by this method. Mass spectra taken under various ionization conditions show steps and outstanding peaks, as well as pronounced minima. The abundance distribution suggests a transition to full electron delocalization which is complete at about 20 atoms. A so-far-not-reported electron reorganization is observed, leading to a novel shell structure.     

Imaging the translational dynamics of CF3 in liquid helium droplets

The translational dynamics of CF3 in liquid helium have been investigated by photodissociating CF3I dissolved in helium droplets consisting of several thousands of atoms. The velocity distribution of CF3 fragments that have escaped from the droplets has been determined using ion imaging techniques and is found to be considerably shifted to lower speeds with respect to the photodissociation of gas phase CF3I. The fragments furthermore show a speed dependent angular distribution that is isotropic for the slowest and approaches the gas phase distribution for the faster fragments. These distributions point to a nonthermal escape process in which, at least for the speeds relevant for the present experiment, the kinetic energy and momentum transfer from the fragments to the solvent appears to be governed by binary collisions with the individual helium atoms.     

Magnetic dichroism of potassium atoms on the surface of helium nanodroplets

The population ratio of Zeeman sublevels of atoms on the surface of superfluid helium droplets (T=0.37 K) has been measured. Laser induced fluorescence spectra of K atoms are measured in the presence of a moderately strong magnetic field (2.9 kG). The relative difference between the two states of circular polarization of the exciting laser is used to determine the electron spin polarization of the ensemble. Equal fluorescence levels indicate that the two spin sublevels of the ground-state K atom are equipopulated, within 1%. Thermalization to 0.37 K would give a population ratio of 0.35. We deduce that the rate of spin relaxation induced by the droplet must be <520/s. For the K2 triplet dimer we find instead full thermalization of the spin.     

Pinning of quantized vortices in helium drops by dopant atoms and molecules

Using a density functional method, we investigate the properties of liquid 4He droplets doped with atoms (Ne and Xe) and molecules ( SF6 and hydrogen cyanide). We consider the case of droplets having a quantized vortex pinned to the dopant. A liquid-drop formula is proposed that accurately describes the total energy of the complex and allows one to extrapolate the density functional results to large N. For a given impurity, we find that the formation of a dopant+vortex+(4)He(N) complex is energetically favored below a critical size N(cr). Our results support the possibility to observe quantized vortices in helium droplets by means of spectroscopic techniques.     

铝中氦原子行为的密度泛函研究

用密度泛函理论计算了大量He原子存在时He在金属铝中不同位置的能量,并在理论上预测了铝中的氦原子行为。结果表明,铝晶胞内He原子择优占位区是空位,而在整个晶体范围,最有利于容纳He原子的区域是晶界,其次是空位和位错。在fcc-铝的两种间隙位中,He原子优先充填四面体间隙位。间隙He原子的迁移能很小,易于通过迁移在晶内聚集,或被空位、晶界、位错等缺陷束缚。     

铝中氦原子行为的密度泛函研究

用密度泛函理论计算了大量He原子存在时He在金属铝中不同位置的能量,并在理论上预测了铝中的氦原子行为.结果表明,铝晶胞内He原子择优占位区是空位,而在整个晶体范围,最有利于容纳He原子的区域是晶界,其次是空位和位错.在fcc-铝的两种间隙位中,He原子优先充填四面体间隙位.间隙He原子的迁移能很小,易于通过迁移在晶内聚集,或被空位、晶界、位错等缺陷束缚.     

Formation of K*He exciplexes on the surface of helium nanodroplets studied in real time

Superfluid helium nanodroplets are doped with potassium atoms to form complexes with the alkali atom residing on the surface of the droplets. Dispersed laser-induced fluorescence spectra of such systems already revealed the formation of M(*)He ( M = Na,K) exciplexes upon electronic excitation [Reho et al., Faraday Discuss. 108, 161 (1997)]. By means of femtosecond pump-probe spectroscopy, this formation process now is followed in real time. We find K(*)He(n = 1) to be formed within 180 fs. Furthermore, the existence of exciplexes with n>1 is quantified suggesting that the first ring around the potassium atom contains four helium atoms.     

Helium nano-bubble bursting near the nickel surface

We have investigated the expansion and bursting of a helium nano-bubble near the surface of a nickel matrix using a molecular dynamics simulation. The helium atoms erupt from the bubble in an instantaneous and volcano-like process,which leads to surface deformation consisting of cavity formation on the surface, along with modification and atomic rearrangement at the periphery of the cavity. During the kinetic releasing process, the channel may undergo the "open" and"close" states more than once due to the variation of the stress inside the nano-bubble. The ratio between the number of helium atoms and one of vacancies can directly reflect the releasing rate under different temperatures and crystallographic orientation conditions, respectively. Moreover, a special relationship between the stress and He-to-vacancy ratio is also determined. This model is tested to compare with the experimental result from Hastelloy N alloys implanted by helium ions and satisfactory agreement is obtained.     

A quantum mechanical study of the He,H-, He-He and He-H systems

This paper applies the methods for treating diatomic molecules described in papers I and II of this series, to the interation of two normal helium atoms and the interaction of a normal helium atom and a hydrogen atom. The results are compared to the experimental results of Amdur and referred to other theoretical results where they exist. The results on the He-H interaction are in fair agreement with Amdur's experimental work. The repulsive potential of two normal helium atoms disagrees with Amdur's experimental results at the distances between 1 and 2 angstroms. The helium atom and negative hydrogen ion are also discussed.     

Buoyancy corrections for the potential of an impurity in a 4He nanodroplet

This paper presents correction terms for the effective potential for the translation and rotation of an impurity atom or molecule solvated in a helium nanodroplet that were previously published (Lehmann, K. K., 1999, Molec. Phys., 97, 645). The correction arises from changes in the total He-He potential energy of the displaced liquid as a function of the solute position within the droplet. For the alkali atoms, this buoyancy type correction removes the large barrier to ejection of the atom from the droplets, which is predicted if this term is neglected.     

Adsorption of helium on isolated C60 and C70 anions

Adsorption of helium on free, negatively charged fullerenes is studied in this work. Helium nanodroplets have been doped with fullerenes and ionised by electron attachment. For suitable experimental conditions, C^?₆₀ and C^?₇₀ anions are found to be complexed with a large number of helium atoms. Prominent anomalies in the ion abundances indicate the high stability of the commensurate 1×1 phase in which all hollow adsorption sites are occupied by one atom each. The adsorption energy for an additional helium atom is about 40% less than for atoms in the commensurate layer, similar to our previous findings for fullerene cations and in agreement with theoretical dissociation energies. Similarly, an anomaly in the adsorption energy occurs when 60 helium atoms are attached to C^?₆₀ or 65 to C^?₇₀. For C₆₀, the anomaly coincides with the one observed for cationic complexes but for C₇₀ it does not. Implications of these features are discussed in light of several theoretical studies of neutral and positively charged helium–fullerene complexes.     

Collisional-radiative volume recombination and ionization coefficients for He+-He2+-e plasmas

Coefficients have been calculated for collisional-radiative recombination of doubly ionized helium atoms with electrons and collisional-radiative ionization of singly into doubly ionized helium atoms due to electronic collisions and radiative absorption. Up to one hundred coupled equations have been solved which corresponds to a level system in which levels up to principal quantum number one hundred have been taken into account. The numerical results are presented in form of curves for a large number of parameters corresponding to very different experimental situations. Comparison of our values with those experimentally determined by Mosburg and by Gippiuset al. is made.     

The direct ionization processes in the collisions of partially stripped carbon and oxygen ions with helium atoms at low-to-intermediate energies

The values of direct double- to-single ionization ratio R of helium atoms induced by C^q＋, O^q＋ （q = 1 -4） ions at incident energies from 0.2 to 8.5MeV are measured. Based on the existing model （Shao J X, Chen X M and Ding B W 2007 Phys. Rev. A 75 012701） the effective charge of the projectile is introduced to theoretically estimate the value of R for the partially stripped ions impacting on helium atoms. The results calculated from our ＂effective charge＂ model are in good agreement with the experimental data, and the dependence of the effective charge on the ionization energy of the projectile is also discussed qualitatively.     

相对论离子与氦二聚体碰撞电离的理论研究

基于对称程函近似,从理论角度研究了相对论离子与氦二聚体碰撞双电离的直接碎裂通道:由入射离子与两个原子的作用造成的双电离。从不同的近似角度出发,建立了两个理论模型,第一个模型将氦二聚体的两个原子看做一个整体,而另外一个模型完全忽略了两个氦原子之间的相互作用。模型的数值计算结果表明,总截面与氦二聚体的空间取向有很强的依赖关系,当氦二聚体与入射离子束流平行时,总截面达到最大值,在垂直时为最小值。进一步研究发现,模型一的结果在氦二聚体核间距较小时更为合理,核间距较大时,模型二更为精确。