The interaction of helium metastable atoms with a clean and co covered polycrystalline tungsten surface

An investigation has been made of secondary electron ejection resulting from the impact of helium metastable atoms on a clean and CO covered polycrystalline tungsten surface respectively. The ejected election energy distribution from the clean tungsten surface is found to differ from that resulting from the neutralization of slow incident ions. In the present case the distribution is narrow and peaked at approximately 2 eV. When the tungsten surface is exposed to CO it is found that the total yield of secondary electrons increases, but that the energy distribution remains essentially the same. The increase in the total yield following CO exposure is shown to be associated with bonding of the CO in the α-state. A tentative explanation for the de-excitation mechanism is proposed, based on Auger de-excitation of the helium metastable atom.     

Excitation of helium atoms in collisions with plasma electrons in an electric field

The rate constants are evaluated for excitation of helium atoms in metastable states by electron impact if ionized helium is located in an external electric field and is supported by it, such that a typical electron energy is small compared to the atomic excitation energy. Under these conditions, atomic excitation is determined both by the electron traveling in the space of electron energies toward the excitation threshold and by the subsequent atomic excitation, which is a self-consistent process because it leads to a sharp decrease in the energy distribution function of electrons, which in turn determines the excitation rate. The excitation rate constant is calculated for the regimes of low and high electron densities, and in the last case, it is small compared to the equilibrium rate constant where the Maxwell distribution function is realized including its tail. Quenching of metastable atomic states by electron impact results in excitation of higher excited states, rather than transition to the ground electron state for the electric field strengths under consideration. Therefore, at restricted electron number densities, the rate of emission of resonant photons of the wavelength 58 nm, which results from the transition from the 2¹ P state of the helium atom to the ground state, is close to the excitation rate of metastable atomic states. The efficiency of atomic excitation in ionized helium, i.e., the part of energy of an electric field injected in ionized helium that is spent on atomic excitation, is evaluated. The results exhibit the importance of electron kinetics for an ionized gas located in an electric field.     

Near-threshold electron impact excitation of ultraviolet-emitting levels of helium atoms

The excitation by electron impact of levels of helium atoms that decay by emission of ultraviolet light has been studied as a function of incident energy up to approximately the first ionization energy. The width (FWHM) of the energy spread of the incident beam was less than 30 meV. A crossed-beam interaction region was used and UV photons emitted approximately normally to the electron and gas beam directions were detected by a channel electron multiplier. Precautions were taken to avoid detection of scattered electrons and metastable atoms by the photon detector. The onset position of the lowest excited level was used in calibrating the incident energy scale. The energies of resonance structures in the cross-sections could therefore be obtained and are compared with the results of experiments detecting other products of the interaction.     

超强激光与固体气体复合靶作用产生高能氦离子

激光氦离子源产生的MeV能量的氦离子因有望用于聚变反应堆材料辐照损伤的模拟研究而得到关注.目前激光驱动氦离子源的主要方案是采用相对论激光与氦气射流作用加速高能氦离子,但这种方案在实验上难以产生具有前向性和准单能性、数MeV能量、高产额的氦离子束,而这些氦离子束特性是材料辐照损伤研究中十分关注的.不同于上述激光氦离子产生方法,我们提出了一种利用超强激光与固体-气体复合靶作用产生氦离子的新方法.利用这种方法,在实验上,采用功率密度5×10~(18)W/cm~2的皮秒脉宽的激光脉冲与铜-氦气复合靶作用,产生了前向发射的2.7 MeV的准单能氦离子束,能量超过0.5 MeV的氦离子产额约为10~(13)/sr.二维粒子模拟显示,氦离子在靶背鞘场加速和类无碰撞冲击波加速两种加速机理共同作用下得到加速.同时粒子模拟还显示氦离子截止能量与超热电子温度成正比.     

Research of Plasma Characteristics of the Radio-Frequency Discharge in O2 and Its Mixtures

Electron mean energy and the effects of gas mixture are studied theoretically and experimentally. The electron mean energy in O2 and its mixtures is obtained by solving Boltzmann＇s equation. The experiments of the Langmuir probe system and spectral analysis are carried out. It is shown that electron temperature goes down with the increasing pressure, narrowing pulse width and the addition of helium and argon. According to the intensity of oxygen atom at 777.19 nm, xenon is more effective in inhibition of O2 decomposition than helium and argon.     

脉冲氙灯放电过程分析与实验测定

脉冲氙灯是一种新型光源,利用量子理论以及气体放电理论分析了脉冲氙灯放电过程,设计了放电测试系统,实验测定了放电过程中的电压、电流和光脉冲信号。结果表明：在预电离阶段自由电子浓度较低,能量较低;气体放电阶段灯内形成电子崩,使得自由电子浓度的增大,放电电流迅速上升,电压下降;离子复合时释放出光子形成光脉冲,氙灯脉冲的光能量输出和光谱特性与复合电子能量、氙气复合能级相关,而当输入能量与氙灯的能耗不一致的时候,将导致充电电容反复充放电,而使电路产生振荡。对于分析脉冲氙灯放电过程,优化放电回路参数设计和脉冲氙灯的生产具有重要意义。     

氦氩射频容性放电发射光谱分析

光抽运亚稳态稀有气体激光器利用放电等离子体作为激光的增益介质.为掌握容性射频放电的放电参数对等离子体各项参数的影响的基本规律,利用等离子体发射光谱法研究了氦氩混合气体在不同装置、不同Ar组分、不同气压和不同射频注入功率下的等离子体参数.利用残留水蒸气产生的OH自由基A~2Σ~+→X~2Π的转动光谱分析获得气体温度;利用电子态光谱的玻尔兹曼做图法获得电子激发温度,利用Ar原子696.5 nm谱线的斯塔克展宽获得电子密度.结果表明:气体温度随气压增加略微上升,在一个大气压下改变组分和放电功率,气体温度变化不大;电子激发温度随总气压的下降而上升,且随着Ar组分的增加而略微下降;目前放电条件下的电子密度均在10~(15)cm~(-3)量级;长时间放电监测表明,残留的水蒸气会导致电子温度的下降,从而降低Ar亚稳态的产率.     

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.     

典型放电气体的击穿场强阈值研究

为了研究等离子体产生时的气体击穿特性,利用低气压条件下气体击穿场强阈值模型,分析了He、Ne、Ar、Kr、Xe和Hg蒸汽等6种典型放电气体的击穿阈值随入射波频率、电子温度、气体压强以及气体温度的变化规律。结果表明:气体击穿阈值随气体压强的增大而减小,随气体温度、电子温度和入射脉冲频率的增大而增大。气体压强和入射频率对击穿阈值的影响大于气体温度和电子温度,在所考虑的范围内,气体压强对击穿场强的影响约为100 V/m,入射脉冲频率对击穿场强的影响为50~300 V/m,气体温度和电子温度对击穿场强的影响为20~30 V/m。当考虑气体压强、气体温度以及电子温度等因素的影响时,各种气体的击穿场强阈值产生的变化规律相类似;但考虑入射频率的影响时,不同气体的击穿场强阈值差异很大。在所考虑的典型放电气体中,Xe具有最低的击穿场强阈值,He的击穿阈值最大。     

Electron ejection by helium metastable atoms incident on the clean and chalcogen covered Ni(100) surface

An intense, essentially photon free, helium metastable beam has been used to cause electron ejection from the clean, oxygen and sulphur covered Ni(100) surface, in a system equiped with an AES facility to monitor surface cleanness. The ejected electron energy distribution obtained for the clean Ni(100) surface is narrow and peaked at ～2 eV, unlike the distribution obtained from INS studies, and consequently indicates different de-excitation mechanisms for incident ions and excited atoms. The ejected electron distribution from the adsorbate covered surface is also narrow, but peaked at ～1 eV with structure which is essentially independent of the nature of the adsorbate. The yield of ejected electrons is found to increase linearly with coverage of both oxygen and sulphur, in contrast to the results obtained from INS. These data indicate that Auger neutralization does not occur at the surface; the possibility of Auger de-excitation is considered.     

Die Reaktion54Xe(n, α)52Te mit Neutronen der Energie 12,5 bis 18,0 MeV

The reaction₅₄Xe(n, α)₅₂Te was investigated in the neutron energy range 12.5 to 18.0 MeV. A high pressure gas scintillator filled either with pure xenon of natural isotopic abundance or with a mixture of xenon and helium was used as a target and as a detector simultaneously. The helium served as a monitor, (n, α)-spectra were measured and analyzed by evaporation theory. Large components of direct processes were found. The level density parameter of⁵²Te was determined as (17.0±1.7) MeV^?1. The cross section excitation function is given as well as the branching ratios for evaporation and direct processes as a function of neutron energy.     

ELECTRON MOMENTUM SPECTRA OF EXCITED He(2~1S) AND He(2~3S)

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Collisional — Radiative Model of Helium Plasma. I. Description of the Model

We present a collisional-radiative model of helium plasma with explicit inclusion of metastable state influences and give results of numerical calculations of population coefficients and effective reaction rates (generalization of effective ionization and recombination coefficients for the case when metastable states are not explicitly included) for a large region of electron densities and temperatures for optically thin and thick helium plasma. The model is described in this paper.     

Effects of discharge current and voltage on the high density of metastable helium atoms

Both hollow-cathode and Penning-type discharges were adopted to excite helium atoms to a metastable state. Experimental data indicate that Penning discharge is more suitable for generating high fractions of metastables in a low-density helium beam for laser-induced fluorescence technique in measuring electric fields at the edge of a plasma. The metastable density increases with increasing helium gas pressure in the range of 1.33×10^{-2}－66.7Pa. The highest metastable density of 3.8×10^{16}m^{-3} is observed at a static gas pressure of 66.7Pa. An approximately linear relationship between the density of metastable helium atoms and the plasma discharge current is observed. Magnetic field plays a very important role in producing a high density of metastable atoms in Penning discharge.     

A simulation toolkit for electroluminescence assessment in rare event experiments

A good understanding of electroluminescence is a prerequisite when optimising double-phase noble gas detectors for Dark Matter searches and high-pressure xenon TPCs for neutrinoless double beta decay detection.A simulation toolkit for calculating the emission of light through electron impact on neon, argon, krypton and xenon has been developed using the Magboltz and Garfield programs. Calculated excitation and electroluminescence efficiencies, electroluminescence yield and associated statistical fluctuations are presented as a function of electric field. Good agreement with experiment and with Monte Carlo simulations has been obtained.     

Electron Temperatures and Electron Densities in Microwave Helium Discharges with Pressures Higher than 0.1 MPa

We adopted laser Thomson scattering for measuring the electron density and the electron temperature of microwave plasmas produced in helium at the pressures higher than the atmospheric pressure. The electron density decreased while we observed the increase in the electron temperature with the pressure. These are reasonable results by considering the decrease in the reduced electric field, the dominant loss of electrons via three‐body recombination with helium as the third body, and the production of electrons with medium energy via heavy particle collisions at the high gas pressure. The temporal variation of the electron temperature had the rise and the fall time constants of approximately 10 ns. The rapid heating and cooling of the electron temperature are due to the fast energy transfer from electrons to helium because of the high collision frequency in the high‐pressure discharge. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electron-impact multiple ionization of xenon (σn+, n=2–9)

Cross sections for production of multiple charged xenon ions (Xeⁿ⁺, n=2–9) by electron collision are presented in the 100–2500 eV impact energy range. We determinate the apparent ionization thresholds and the integrated oscillator strengths for the reactions. The present values are compared with available data from experimental groups using diverse techniques.     

Electron beam effects in auger analysis of physisorbed xenon

Adsorbed xenon on evaporated films of nickel and platinum has been analysed by Auger electron spectroscopy. The primary electron beam is shown to cause some surface heating resulting in a displacement of the isotherms. This temperature effect is the same for both metals and is due to the limiting thermal conductivity of the glass substrate. A further effect, the electron induced desorption (EID) of xenon is evidenced by a distortion of the isotherms at low equilibrium gas pressures. This effect is more clearly observed on platinum because the coverage is higher than on nickel. The EID cross-section for xenon on both metals is found to be 1 × 10^?17cm². The attenuation of the metal Auger peaks by the xenon overlayer is found to be less for platinum than for nickel. This difference is attributed to a lower packing density of xenon on the platinum surface.     

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.