Double photoionization of helium atoms at 1 eV above threshold

An ab initio calculation of triple differential cross-section for double photoionization of helium atoms are presented for a very low energy of just 1 eV above threshold in the equal energy sharing kinematics. We have used hyperspherical partial wave theory together with a Numerov difference scheme in place of other schemes used in our earlier works. The results are in excellent qualitative agreement with the measurements of D?rner et al. [Phys. Rev. A 57, 1074 (1998)].     

Chemical ionization of hydrogen halide molecules in collision with metastable helium atoms

An anomaly of the temperature dependence of the cross section for chemical ionization of HBr molecules by metastable helium atoms He*(2³ S ₁) is discovered. It is shown that the interaction anisotropy induced by rotation of the HBr molecule has a significant influence on the formation of the anomaly. Zh. Tekh. Fiz. 68, 13–15 (February 1998)     

The role of collision cascades and helium atoms in cavity nucleation

Abstract

The simultaneous production of helium atoms and collision cascades during irradiation may enhance the nucleation of cavities. The influence of parameters such as helium generation rate and recoil energy on the possibility of cavity nucleation is investigated. The three most likely mechanisms by which cascade nucleation might occur are considered. Firstly, helium atoms that are present within the cascade may succeed in preventing the collapse of cascades into dislocation loops. Secondly, helium atoms in the cascades may be swept together during the cooling of the cascade to form cavity embryos. Thirdly, more distant helium atoms may be able to reach an uncollapsed vacancy aggregate in a cascade by diffusion before thermal annealing of the cascade.

Calculations indicate that it is unlikely that a sufficient number of helium atoms would be present in a cascade to prevent its collapse. The probability of several helium atoms being swept together within the volume of a cascade is also found to be rather small. Calculations suggest, however, that the cavity nucleation in cascades might become significant if helium atoms were to diffuse much faster than the substitutional diffusion rate during irradiation. The selfinterstitial replacement mechanism, for instance, would yield such a fast diffusion rate of helium.     

Role of metastable atoms and molecules of helium in excitation transfer to hydrogen atoms

The afterglow of a discharge in helium with a small admixture of hydrogen is studied spectroscopically (p=40 Torr, [e]≤10¹¹ cm^?3). The time-resolved measurements of intensities of the first four lines of the Balmer series are performed. The concentrations of metastable helium atoms and molecules are evaluated from the relative intensity of the absorption lines. The ratios of excitation transfer rates from atoms He(2 ³ S ₁) k ₁(n) and molecules of helium He₂(a 2sσ ³Σ _u ⁺ ) k ₂(n) to atomic hydrogen H*(n) are measured to be k ₁(n=3)/k ₂(n=3)=0.04±0.02 and k ₁(n=4)/k ₂(n=4)=0.01±0.02. The ratios of excitation rate constants k ₂(n) corresponding to different states H(n) are measured to be k ₁(n=4)/k ₂(n=3)=0.023±0.01; k ₁(n=5)/k ₂(n=3)≤0.013; and k ₁(n=6)/k ₂(n=3)≤0.007.     

Implantation and spectroscopy of metal atoms in solid helium

The first implantation of neutral Ba and Cs atoms into solid⁴He is reported. We discuss details of the experimental setup and techniques used to load the helium with atoms at concentrations of 10⁸ cm^–3. If photodissociation of Cs molecules and clusters is performed twice per hour this atomic concentration can be kept without a second implantation for almost a day. From the optical spectra of Ba in solid helium we infer no significant difference in the trapping site with respect to that in liquid helium.     

Thermal excitation of fourth sound in helium II

It has been experimentally verified, that fourth sound can be excited not only mechanically but also thermally. The pressure amplitudes of the generated fourth sound waves have been measured between 0.8 K and the lambda point.     

Charge transfer and charge transfer with excitation of a target ion in slow collisions of α particles with helium atoms. II

The cross sections of single- and double-electron capture in ³He²⁺-He collisions in the energy interval of incident α particles of 3–120 keV were obtained. The cross sections were calculated for the Coulomb trajectory of motion of nuclei in the context of the close-coupling equation method using the basis set of two-electron adiabatic molecular states. Introduction of additional requirements that must be satisfied by the numerical solutions of the system of coupled differential equations made it possible to obtain a good correspondence between the calculated values of total cross sections of single- and double-electron capture and the experimental data in the whole range of the considered collision energies.     

The spectroscopic study of excitation and ionization in the collision of two metastable atoms

Collisions between two excited atoms leading to an increase in the excitation energy of the particles have been under investigation. All measurements were made in the afterglow of gas-discharge plasma. The cross sections of the following reactions have been determined: $\text{Hg}\text{(6}{}^3\text{P}{}_{012}\text{) +}\text{Hg}\text{(6}{}^3\text{P}{}_{012}\text{) →}\text{Hg}{}^7\text{+}\text{Hg}\text{(6}{}^1\text{S}{}_0\text{),}\text{He}{}_{\mathrm{m}}\text{(2}{}^{\mathrm{1,3}}\text{S}\text{) +}\text{Xe}{}_{\mathrm{m}}\text{(}{}^3\text{P}{}_{\mathrm{0,2}}\text{) → (}\text{Xe}{}^{\mathrm{+}}\text{)}{}^1\text{+}\text{He}{}_0\text{+}\text{e}$. The cross section of the first reaction for different transitions lies in the region (2?35) × 10^?15 cm² and the cross section of the second, in (0.2?2.4) × 10^?16 cm². Possible systematic errors and the role of cascade transitions are discussed. Cross sections of the Penning reaction He_m + Xe₀ → He₀ + Xe⁺ + e have also been measured. The result is σ (2³S) = (1.4 ± 0.2) × 10^?15 cm², σ (2¹S) = (1.2 ± 0.3) × 10^?15 cm².     

Ionization of helium atoms under the effect of the antineutrino magnetic moment

Differential cross sections for inelastic antineutrino interaction with a helium atom are calculated. It is shown that, in the energy-transfer range extending up to 1 keV, the cross sections in question are considerably enhanced in the electromagnetic-interaction channel in relation to the cross sections for elastic scattering on a free electron. Absolute cross-section values are of interest in searches for the antineutrino magnetic moment, provided that its value in Bohr magneton units falls within the range (10^?13?10^?12)_µB.     

He原子与HI分子碰撞截面的密耦计算

基于发展的分子间相互作用势, 采用密耦方法计算了入射能量从1到140?meV范围内He原子与HI分子碰撞的微分截面、分波截面和积分截面.通过与He-HX（X=F,Cl,Br）体系分波截面的比较, 印证了He-HI体系相互作用势以及密耦计算结果的可靠性.结果表明：小角散射的概率大于大角散射的概率;碰撞能量越高,散射概率就越小, 尾部效应也越弱.总积分截面主要来自弹性碰撞的贡献;非弹性积分截面以00→01和00→02跃迁的贡献为主,其中00→02跃迁的贡献最大. 关键词： 碰撞截面 密耦计算 HI-He体系     

Collision induced transitions between Zeeman substates of laser excited sodium atoms in very high magnetic fields

This is a report on a new method of measuring cross sections for the collision induced population transfer between single Na — 3p ² P fine structure Zeeman states. The experiments are done for the five inert gases at the magnetic field strengths of 6, 17, 24, and 51 kOe. From the optically excited² P _3/2,±3/2 and² P _1/2,±1/2 states, respectively, the transfer to the other Zeeman states is studied. The method allows the determination of cross sections for which the influence of the magnetic field is cancelled and which render possible the calculation of cross sections for the transfer and the relaxation of all² P density tensor components of the degreek=1, 2, and 3. The comparison with previous theoretical and experimental results gives satisfactory agreement. As an application the six Grawert parameters are deduced for the inert gases.     

Ionization of long-lived highly excited atoms by collision with molecules. II

A theoretical treatment of the collisional ionization of a highly excited atom with a molecule is improved by use of the exact form factor of the hydrogen atom in highly excited states. The present treatment is still based on the previously proposed mechanism in which the Rydberg electron is ionized by gain of energy from rotational de-excitation of the molecule in a rotationally excited state. Closer comparison of experiment with theory is made for symmetric- as well as asymmetric-top molecules.     

Experimental investigation of electron impact excitation of zinc atoms to the 41P1 state

Polarization analysis version of the electron-photon coincidence technique was applied to completely determine quantum state of zinc atoms excited to the 4¹P₁ state by electron impact. We report first experimental values of Stokes parameters and Electron Impact Coherence Parameters (EICPs) for incident electron energy 80 eV.     

Fragmentation of positronium in collision with He atoms

The absolute cross section for the fragmentation of positronium in collision with He atoms has been measured. The results are compared with available theories. The longitudinal energy distributions of positrons resulting from fragmentation have also been determined and are found to display a peak situated just below half the residual energy. This is suggestive of the occurrence of "electron loss to the continuum" in which the two residual charged particles lie in a low relative-velocity Coulomb-continuum state.     

平行电磁场中He原子回归谱中的排斥势效应

利用包括实散射的闭合轨道理论和分区自洽迭代的计算方法,研究了在标度能量~ε=-0.03和标度电场~f=0.01的情况下He(M=1)原子在平行电磁场中的闭合轨道和光吸收的回归谱.与相同条件下的He(M=0)原子的回归谱相比较,由于z轴处的排斥势,减小了原子实短程衍射效应和电子的关联,使谱变得简单,这与实验的结果一致.