The properties of helium atoms and as impurities in metals

When the atoms of simple metals condense from the gas phase to give a solid, the valence electron wavefunctions are strongly modified by their overlap. At first sight this strong overlap might be expected to produce a complex situation, difficult to treat quantitatively. Yet the simple metals are perhaps the best understood of all solids and the reason is that the electron wavefunctions are plane-wave-like to a first approximation and deviations can be treated by perturbation theory.     

Filament formation by impurities embedding into superfluid helium

The hydrogen molecules embedded in superfluid helium as a gas jet are shown to form long thin filaments. These filaments survived under a helium transition to anormal phase demonstrating their conjugated entity. The concentration of an impurity in the core of the vortex may be the mechanism of the impurity coalescence providing a cotton-like structure of the condensate obtained by introducing an impurity-containing gas helium jet to He-II. The text was submitted by the authors in English.     

圆锥磁绝缘传输线的横向空间电荷流

根据电磁场基本理论及电子运动守恒方程，导出圆锥传输线横向空间电荷流的数值模型和磁绝缘临界条件。通过数值计算，讨论了电压及圆锥几何结构参数等对横向空间电荷流和磁绝缘性能的影响。电压较高时,无磁场的空间电荷流较大，而磁绝缘性能更好。在传输线的三个几何参数中，几何因子对传输性能影响最大。     

亚稳态氦原子束在纳米结构制作中的应用

介绍两种用亚稳态氦原子束制作纳米结构的新方法。亚稳态原子束从原子源喷出后首先对其进行横向激光冷却,准直后的原子束穿过与之垂直的激光驻波场时发生淬火过程,原子的密度分布出现沟道化效应,给出基于光掩模制作纳米图形的基本原理、理论分析及模拟结果。介绍基于物理掩模制作纳米图形的原理和SAM抗蚀剂,利用沉积在基底上的亚稳态原子破坏基底上的SAM膜,结合刻蚀技术可制作出纳米量级的图形。     

Calculation of hydrogen and helium concentrations for CSNS target

The China Spallation Neutron Source(CSNS) is driven by protons whose energies are about 1.6 GeV.At such high energies, the spallation neutrons lead to the formation of large amounts of helium, hydrogen and new heavier species in the form of transmutation products. These hydrogen, helium and transmutation products have a critical effect on the mechanical properties on the one hand and exacerbate the displacement radiation damage on the other hand. In this paper, the background hydrogen/helium concentrations and the maximum hydrogen/helium concentrations near cracks in a tungsten target for CSNS have been calculated at temperatures of 100 and 300 by applying a theoretical model. For the CSNS tungsten target plate, we find the maximum hydrogen concentration near the tips of cracks ranges from 3.0×10~(-2)–2×10~(-1), which exceeds the hydrogen background concentration by 1.2–1.8 times; the maximum helium concentration near the tips of cracks ranges from 3.0×10~(-4)- 1.2×10~(-3), which exceeds the helium background concentration by 2- 4 times; the maximum hydrogen/helium concentration increases with the increase of the transfer length across the surfaces of the target and it decreases with the increase of temperature.     

Calculation of helium plasma satellites in turbulent plasmas

Neutral helium line profiles (2P-4D), under the influence of suprathermal electric field fluctuations, are calculated with the use of a relaxation line-broadening theory. First-order plasma satellites at the nearby forbidden component (2P-4F) are investigated for various turbulent plasma conditions. The spectral energy densities of the suprathermal electric field fluctuations are analytically modeled to infer quantitative relations between the plasma satellites and the enhanced electric field. It is found that the far satellite can grow stronger than the near satellite and both satellites will split into doublets as the turbulent electric field increases. The theory is generalized to higher order to describe the second harmonic plasma satellites near the allowed transition.     

Spectroscopic exploration of atomic scale superfluidity in doped helium nanoclusters

We present high resolution spectra of He(N)-OCS clusters with N up to 39 in the microwave and 72 in the infrared regions, observed with apparatus-limited line widths of about 15 kHz and 0.001 cm(-1), respectively. The derived rotational constant, B (proportional to the inverse moment of inertia), passes through a minimum at N=9, then rises due to onset of superfluid effects, and exhibits broad oscillations with maxima at N=24, 47 and minima at 36, 62. We interpret these unexpected oscillations as a manifestation of the aufbau of a nonclassical helium solvation shell structure. These results bridge an important part of the gap between individual molecules and bulk matter with atom by atom resolution, providing new insight into microscopic superfluidity and a critical challenge for theory.     

Prospects for precision measurements of atomic helium using direct frequency comb spectroscopy

We analyze several possibilities for precisely measuring electronic transitions in atomic helium by the direct use of phase-stabilized femtosecond frequency combs. Because the comb is self-calibrating and can be shifted into the ultraviolet spectral region via harmonic generation, it offers the prospect of greatly improved accuracy for UV and far-UV transitions. To take advantage of this accuracy an ultracold helium sample is needed. For measurements of the triplet spectrum a magneto-optical trap (MOT) can be used to cool and trap metastable 2³S state atoms. We analyze schemes for measuring the two-photon 2³S →4³S interval, and for resonant two-photon excitation to high Rydberg states, 2³S →3³P →n³S, D. We also analyze experiments on the singlet-state spectrum. To accomplish this we propose schemes for producing and trapping ultracold helium in the 1¹S or 2¹S state via intercombination transitions. A particularly intriguing scenario is the possibility of measuring the 1¹S →2¹S transition with extremely high accuracy by use of two-photon excitation in a magic wavelength trap that operates identically for both states. We predict a “triple magic wavelength” at 412 nm that could facilitate numerous experiments on trapped helium atoms, because here the polarizabilities of the 1¹S, 2¹S and 2³S states are all similar, small, and positive.     

Observations on lifetimes and excitation processes for the upper states in atomic helium

Lifetime observations have been made on 49 energy levels of helium. Good overall agreement with theory is obtained, with significant exceptions, the most striking of which are the 3¹D and 3³D states, where 50% discrepancies with theory and some of the older measurements are found. The new measurements are, however, in harmony with recent results using the Hanle effect.The existence of the upper-state transfer process is clearly delineated by the results for n > 6.     

Similarity law for kinetic coefficients of hot electrons in atomic liquids with molecular impurities

A straightforward similarity law is proposed in the framework of the Lekner-Cohen theory which makes it possible to estimate the effect of the impurities on the kinetic coefficients of electrons in liquids with molecular additions.     

利用激光冷却原子束测量氦原子精密光谱

⁴He原子2³S₁→2³P_0,1,2跃迁的精细结构分裂,目前在理论和实验上都能够达到10^-8水平的精度,并可被应用于测定精细结构常数α, 和对量子电动力学进行检验.该方面实验研究的关键, 是需要提高测量信噪比,并消除各种可能的系统偏差, 将这一精细结构分裂测量到亚kHz水平.在设计的这套实验方案中, 首次结合激光冷却原子技术,通过激光横向冷却来提高亚稳态氦原子束的束流强度,并对三态亚稳态氦原子进行偏折, 将其从原子束中分离,从而大幅降低测量背景,并利用频率锁定激光器的边带扫描的方式来进行光谱测量,以使得扫描测量中保持足够的频率精度. 在目前基本搭建成的实验装置上,实验方法的可行性已经获得验证,分析表明有望实现亚千赫兹水平的测量准确度.     

Calculation of the static healing length in helium II

The healing length for stagnant liquid helium II at an infinite plane boundary is calculated from the Hills-Roberts equations. A new thermodynamic function needed to complete the calculation is derived from neutron scattering and thermodynamic data. Results for the healing length are obtained from absolute zero to about a tenth of a degree from the lambda point, and at all pressures. Comparison with experimental evidence is presented.     

Pressure shift of atomic resonance lines in liquid and solid helium

We show that for atoms trapped in a liquid He matrix the excitation and emission spectra can be quantitatively described by the bubble model which treats the matrix as an incompressible continuous medium. Comparison with experimental results shows that the model also holds for a solid matrix. The model is only valid in the case where the attractive He–He interaction dominates the attractive part of the foreign atom-helium interaction. Its failure in the case of implanted Na atoms is discussed.     

Semiempirical atomic mass formula

An atomic mass formula based on the liquid-drop model with shell and deformation energies determined from the Nilsson model and BCS pairing energy has been adjusted simultaneously to ground-state binding energies and to fission barrier heights. A short table of calculated values is included, and additional calculations are available.