Static and dynamical distortions of helium clusters

We examine the effects of impurities (doping) and rotational excitation on the structural and energetic properties of helium clusters. Quantum Monte Carlo (QMC) techniques are used to study ground and rotationally excited states of pure and doped clusters. We use exponentially correlated wave functions and treat the molecular impurities as rigid. Whereas pure He_N show essentially monotonic decay of density from a central maximum value, addition of impurities induces local ordering of He to an extent dependent on the impurity-He binding. Rotational excitation of He_N gives rise to extremely large centrifugal distortions. The location of impurities also appears to change upon rotational excitation. The implications of these distortions on impurity spectra are discussed for SF₆He_N, and compared to recent experimental results.     

Localized diffusion of helium atoms around indium impurities in aluminium observed by means of perturbed angular correlations

The behaviour of He atoms implanted in ¹¹¹In doped Al has been investigated by means of perturbed angular correlation (PAC) measurements. The onset of He mobility was found to be at about 250K, probably due to vacancy-assisted migration. Mobile helium is effectively trapped at In impurities in small vacancy clusters that accomodate at most 11 He atoms. The PAC spectra taken at temperatures from about 510 K to 670 K exhibit a relaxation effect that is ascribed to hopping of He atoms from one vacancy to another, the saddle-point energy for this localized diffusion being 0.67(5) eV. A tentative model of the cluster is given.     

Electron impact ionization of thymine clusters embedded in superfluid helium droplets

Embedding molecules in helium clusters has become a powerful technique for the preparation of cold targets for spectroscopy experiments, as well as for the assembly of complex, fragile molecular species. We have recently developed a helium cluster source and a pick-up cell to produce neutral beams of doped helium droplets, to be used as targets in studies on electron collisions with molecules of biological relevance. In the present work we present the results of a series of experiments on electron-impact ionization of helium clusters doped with thymine and 1-methylthymine, where several interesting phenomena were observed, i.e., (i) electron impact ionization of molecular clusters inside the helium droplets leads predominantly to protonated clusters; (ii) the appearance energies are close to the ionization threshold of the helium atom but ionization efficiency curves in addition extend down by several eV; (iii) ionized molecular clusters can undergo metastable decay via the loss of one neutral monomer.     

金属钛中氦团簇融合的分子动力学模拟

运用分子动力学方法研究了金属钛中氦的扩散聚集行为.在300—800K的温度范围内,模拟了钛基底中氦团簇之间的融合过程.研究发现,温度的升高会加快氦团簇的融合.在300—800K,融合后的氦团簇在所模拟的时间尺度内三维结构保持不变.模拟结果还表明,常温下氦团簇之间的吸引力是导致氦团簇融合的重要因素. 关键词： 氦团簇 团簇融合 分子动力学模拟     

Atomic structure of the Zr–He,Zr–vac,and Zr–vac–He systems: First-principles calculation

The ab initio investigations have been performed for the atomic structure of the Zr–He, Zr–vac, and Zr–vac–He systems with concentrations of helium atoms and vacancies (vac) of ~6 at %. A heliuminduced instability of the zirconia lattice has been revealed in the Zr–He system, which disappears with the formation of vacancies. The most preferred positions of impurities in the metal lattice have been determined. The energy of helium dissolution and the excess volume introduced by helium have been calculated. It has been established that the presence of helium in the Zr lattice leads to a significant decrease in the energy of vacancy formation.     

Instability and reconstruction of a thin fluid film under inversion conditions

The development of mechanical instability of a neutral fluid film (liquid helium or hydrogen) under inversion conditions (it does not lie on a solid substrate but hangs from a ceiling) is discussed. Critical parameters of such an instability and the character of surface reconstruction under the action of van der Waals forces, bubble pressure, and gravitational forces are determined. The interrelation with the well-known Frenkel problem of a drop on a solid substrate is pointed out. An electrostatic mechanism is proposed for the stimulation of instability of a thin helium film. This mechanism is promising for the problem of superfluid helium leakage.     

Acoustic crystallization and heterogeneous nucleation.

By focusing a high-intensity acoustic wave in liquid helium, we have observed the nucleation of solid helium inside the wave above a certain threshold in amplitude. The nucleation is a stochastic phenomenon. Its probability increases continuously from 0 to 1 in a narrow pressure interval around P(m) + 4.7 bars ( P(m) = 25.3 bars is the melting pressure where liquid and solid helium are in equilibrium). This overpressure is larger by 2 to 3 orders of magnitude than what had been previously observed. Our result strongly supports the recent suggestion by Balibar, Mizusaki, and Sasaki that, in all previous experiments, solid helium nucleated on impurities.     

Charged clusters in liquid helium

The appropriateness of the experimental study of charged clusters in liquid helium has been supported. The interaction potential of negative ions (electron bubbles) with inert clusters formed by Ne, Ar, Kr, and Xe atoms or H₂ and N₂ molecules has been found. Small clusters levitate at a distance of 13–16 Å above the negative ion. The scalings laws for the properties of charged inert clusters have been discovered and grounded; the number of quantum levels and states of such clusters has been determined. The mobility measurement of charged clusters may provide a new technique of probing the properties of impurity nanoparticles in helium.     

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通过参量转化，将杂质速率方程转化为具有三角对称系数矩阵的矢量方程，并进行了数值求解，得到了托卡马克等离子体中常见轻杂质特征参量 随等离子体温度的变化曲线。计算结果显示，对常见的轻杂质如氦、碳、氧、氖及氩等在1eV～10keV温度范围内特征参量基本分布在1015～1019m-3•s数量级范围内；平均特征参量数值与核电荷数Z没有明确的函数关系；在曲线最后两个峰值处，相应杂质的平衡时间基本一致；当等离子体中杂质主要以类氦、类氖离子形态(具有闭合壳层形态离子)存在时，对应温度下特征参量曲线达到极小值。     

对理想条件下Rn及其子体垂直运移实验数据的再分析

当1个Rn与8个以上的He原子结合成团簇后,该团簇可以克服重力向上运移;当与1个Rn结合的He原子少于8个时,该团簇将向下运移.因此向上运移的RnHe团簇的平均原子数要明显大于向下运移的RnHe团簇的平均原子数.对Rn及其子体的长距离运移实验数据进行分析表明：在不考虑扩散的情况下,平均每个Rn需要与7.5个He原子结合,则铀矿石源所能提供的He原子数量可能不够;如果考虑扩散的情况,平均每个Rn只需与4.26—5.57个He原子结合,则铀矿石源可以提供足够数量的He.这一推导结果说明,在采用团簇运移机制解释Rn及其子体垂直运移实验结果时,不能忽略扩散运移的存在. 关键词： Rn 垂直运移 RnHe团簇     

Superfluidity in a doped helium droplet

Path-integral Monte Carlo calculations of the superfluid density throughout 4He droplets doped with linear impurities are presented. After deriving a local estimator for the superfluid density distribution, we find a decreased superfluid response in the cylindrically symmetric region of the first solvation layer. The helium in this region has a superfluid transition temperature similar to that of a two-dimensional helium system and may be responsible for previously unexplained experimental Q-branch measurements.     

High-resolution mass spectrometric study of pure helium droplets,and droplets doped with krypton

Mass spectra of doped and undoped helium droplets are presented. The high resolution of the time-of-flight spectrometer (m/Δm ≅ 5000) makes it possible to fully resolve small helium cluster ions from impurities and to unambiguously identify abundance anomalies in the size distribution of He _n ⁺. The yield of He₄ ⁺ shows the well-known enhancement relative to other small cluster ions when the expansion changes from sub- to supercritical, provided the electron energy exceeds a value of 40 ± 1 eV, the threshold for formation of electronically excited ions. Upon doping with krypton, pure Kr _n ⁺ cluster ions containing up to 41 Kr atoms are observed. The spectra exhibit abundance anomalies at 13, 16, 19, 22 & 23, 26 and 29, in agreement with spectra obtained by ionization of bare krypton clusters that are formed in neat supersonic beams. Mixed clusters He _m Kr⁺ indicate closure of a solvation shell at m = 12.     

Excited-state relaxation of Ag8 clusters embedded in helium droplets

Neutral silver clusters Ag(N) are grown in ultracold helium nanodroplets. By exploiting a strong absorption resonance recently found for Ag8, first photoelectron spectra of this neutral species are recorded. Variation of the laser photon energy reveals that direct vertical two-photon ionization is hindered by rapid relaxation into the lower edge of a long-living excited state manifold. The analysis of the dynamics gives a precise value of (6.89+/-0.09) eV for the vertical ionization potential of Ag8. The influence of the helium matrix on photoemission is discussed.     

Selecting molecules embedded in nanodroplets (clusters) of superfluid helium

A method of selecting molecules embedded in nanodroplets (clusters) of superfluid helium is proposed, which is based on the selective vibrational excitation of embedded molecules by intense IR laser radiation. This action leads to a significant decrease in size of the excited clusters, after which these clusters are separated with respect to size via scattering of the cluster beam on a crossing atomic beam. The method is described in detail and the possibility of selecting SF₆ molecules in liquid helium nanodroplets using the excitation by CO₂ laser radiation and the angular separation via scattering on a xenon atomic beam is demonstrated. The results show that, by using this technique, it is possible to separate molecules with respect to isotope (element) composition. Advantages and drawbacks of the method are analyzed.     

Adaptive clustering of a quantum solvent: the LiH+ cation in bosonic helium from stochastic calculations

Calculations of the quantum structures describing the initial solvation shells of bosonic helium atoms around a polar, ionic system like LiH⁺ are reported, together with the corresponding quantum energies. The calculations were carried out using the Diffusion Monte Carlo (DMC) approach and parametric trial functions. Its final radial and angular distributions for clusters of varying size are analysed and discussed. The solvation of this ionic dopant is shown to occur in a way which is strongly affected by the orientational induction forces between the latter molecule and the solvent atoms, indicating the onset of “snowball" structures at the location of the dopant and the clear distinction between “heliophilic" and “heliophobic" regions of microsolvation.     

Influence of hydrogen and helium implantation on the properties of structural materials

The influence of hydrogen and helium impurities on physical and mechanical properties of EP-450 ferrite steel has been studied. The techniques of elastic recoil detection analysis, Rutherford backscattering, acoustic emission, acoustic energy dissipation rate measurements, magnetic and eddy-current methods were used. Microhardness of steel samples containing hydrogen and helium has been studied by nanoindentation. Microscopic investigation of the steel surface after hydrogenation and after exposing to monoenergetic 40-keV helium ions has been performed.     

Mass spectrometric investigation of anions formed upon free electron attachment to nucleobase molecules and clusters embedded in superfluid helium droplets

Here we report the first mass spectrometric study of negative ions formed via free electron attachment (EA) to nucleobases (NBs) embedded in helium clusters. Pure and mixed clusters of adenine and thymine have been formed by pickup of isolated NB molecules by cold helium droplets. In contrast to EA of isolated molecules in the gas phase we observe a long-lived parent anion NB- and, in addition, parent cluster ions NB-n up to size n=6. Moreover, we show that a low energy electron penetrating into a doped helium droplet causes efficient damage of the embedded nucleobases via resonant, site selective, dissociative electron attachment.     

Helium implanted CuHf as studied by TDPAC and positron lifetime measurements

¹⁸¹Ta time differential perturbed angular correlation (TDPAC) and positron lifetime measurements were carried out on homogeneously α-implanted CuHf samples. TDPAC measurements indicate the trapping of vacancy clusters and helium associated defect complexes by Hf atoms. The presence of helium-vacancy complexes and helium stabilised voids has been identified by positron lifetime measurements. Further the nucleation and growth stages of helium bubbles have been identified. TDPAC and positron lifetime measurements indicate that Hf atoms act as heterogeneous nucleating centers for helium bubbles. Hf atoms are found to suppress the bubble growth in CuHf as indicated by the results of positron lifetime measurements.     

Clustering of helium in vacancies bound to a 12〈111〉{110} edge dislocation in α-iron

Atomistic calculations show that helium trapped in a vacancy bound to an edge dislocation in α-iron desorbs into the dislocation core with lower activation energy (<1.0 eV) than required for the release from this core into the bulk of the crystal. This latter desorption step requires an additional 2.3 eV. The calculations indicate the possibility to estimate from a helium desorption spectrum the length of an edge dislocation between two vacancy clusters (or jogs).     

Helium wetting: Theme and variations on inhomogeneous helium

The surprising discovery, in 1991, that liquid helium does not wet a cesium surface at low temperature has triggered an important activity both theoretical and experimental: helium has become a model system for the study of wetting transitions. After summarizing the main theme of helium wetting, I will focus on more recent studies, such as the structure and excitations of helium interfaces, experiments on the capillary rise, the “surfactant effect” of helium-3 impurities on liquid helium-4 and the “quantum prewetting transition” of pure helium-3. Unexpected consequences on the phase separation of³He?⁴He mixtures in restricted geometry will be drawn.