Energy dependence of the ionization probability of sputtered Cu and Ni

Despite its great sensitivity, the usefulness of secondary ion mass spectrometry (SIMS) for many applications has been limited by an inadequate understanding of the probability of sputtering an atom in an ionized state. To determine this ionization probability for clean Cu and Ni surfaces, I have measured the energy distribution of sputtered neutrals and ions by quadrupole mass filtering and retarding potential analysis using potential modulation differentiation. Analysis of sputtered neutrals was accomplished by electron impact ionization. Because the neutrals outnumber the ions by at least two orders of magnitude, the ratio of sputtered ions to neutrals is an accurate measure of the ionization probability. For energies below 20 eV the dependence of the ionization probability on energy goes as P(E) α Eⁿ, where n = 0.65 for clean Cu. The absorption of oxygen on the Cu surface increases the total ion yield while causing a reduction in the value of the exponent n. Similar results are found for nickel, where n = 0.54 for the clean surface.     

HL—1装置边缘分子过程对中性粒子输运的影响

本文采用新版的一维中性粒子输运程序,在平板几何下模拟了HL-1装置边缘8cm区域的分子过程及中性粒子与壁相互作用对中性粒子输运的影响,得到了电离速率的径向分布、稳态中性氢原子径向分布、壁与边缘等离子体区之间、芯部等离子体与边缘等离子体之间中性氢各种粒子通量、能量通量并与只含原子过程的类似计算进行了比较,结果表明边缘区的分子过程对巾性氢原子输运有显著的影响。有些结果得到实验的证实。     

Expansion Model of the Neutral Component in Vacuum Arc Spot Plasmas

Plasma generated by cathode spots in vacuum arcs expands towards the anode and the walls. In spite of a high degree of ionization (mostly), this plasma contains also a small but noticeable constituent of neutral atoms. Starting from the known analytical solution of a hydrodynamic model describing the plasma ions and electrons, the corresponding motion of neutrals is treated in a similar way. The results show: 1) The neutrals take part in the general plasma expansion; however, their final kinetic energy is lower (by about 50%) than the final energy of the ions. 2) The temperature of the neutrals also is lower than the ion temperature and decreases during expansion. 3) The acceleration of neutrals is caused partly by the pressure gradient, partly by interaction with the ions (friction); the forces diminish rapidly with increasing distance. Finally, the limitations of this approximate solution are discussed.     

Electron distribution function relaxation in monatomic gases

The authors discuss an analytic solution of the Boltzmann equation which describes the relaxation in time of the electron distribution function for electrons in a plasma derived from the monatomic gases He, Ne, Ar, and Xe. It is assumed that there are no perturbing forces on the electrons and that at t=0 they have a Maxwellian distribution function corresponding to an average energy of 2 eV. The electrons then lose energy through elastic collisions with neutrals and eventually energy-equilibrate with the neutrals, which are assumed to be cold. The evolution of the electron distribution function in time and velocity space is calculated for each gas. This model is approximately correct for the afterglow period of an electrical discharge in a monatomic gas. It is possible to calculate a time which is a measure of the decay time of the electron energy in an afterglow plasma     

等离子体屏蔽对He2+与H原子碰撞电离微分截面的影响

应用经典径迹Monte Carlo（CTMC）方法研究了He²⁺与H原子在等离子体环境下的碰撞电离过程,计算了在5—400 keV/u的能区随等离子体屏蔽作用变化的碰撞电离总截面和一阶微分截面.等离子体中带电粒子之间的相互作用采用Debye-Hückel模型来描述.由于等离子体屏蔽效应的存在,靶中束缚态电子能级及其经典微正则分布以及入射离子与靶电子的相互作用都发生了变化,而这些变化会直接影响碰撞电离过程.研究发现,碰撞电离总截面随等离子屏蔽的增加而增大,特别是在10 keV/u以下的低能区电离截面有量级的增加.对随能量变化的一阶微分截面,在低能碰撞过程中,屏蔽作用增加,微分截面呈量级增加,高能碰撞微分截面呈倍数增加.同时,屏蔽作用导致电离电子向高能方向移动,随着碰撞能量的增加两体碰撞机制的贡献越来越大,并在较高的出射电子能量出现了一个新的峰.对无屏蔽的自由原子碰撞过程,CTMC方法计算出的电离总截面在碰撞能量大于70 keV/u的较高能区在实验误差内与实验测量结果符合很好,而在较低的能区比实验值小30%—50%. 关键词： 重粒子碰撞电离 等离子体屏蔽效应 经典径迹Monte Carlo方法 Debye-Hückel模型     

Electron energy spectrum in the field-ionized plasma

In this article, the effect of ionization on the energy spectrum of electrons within the interaction of a laser pulse with hydrogen atoms is investigated using particle-in-cell simulation codes. The results show that the behaviour of electrons' energy distribution function in the field-ionized plasma, which occurred due to the field ionization, compared with that in the pre-plasma strongly depends on the pulse shape. For short rise-time pulses (here 30 fs), due to the rapid enhancement of laser electric field, ionization occurs quickly, and as a result, there is not much difference in the electron energy in both the media. However, for pulses with rise time of 40 fs, in the pre-plasma state, the electron population reaches higher energies compared with the field-ionized plasma state. The main reason for this difference is the nonlinear wave breaking that happens earlier due to density inhomogeneity in the field-ionized plasma. On the other hand, at longer rise-time pulses (here 60 and 70 fs), electrons achieve higher energies in the field-ionized plasma than those in the case of pre-plasma. In this case, because of density fluctuations in the field-ionized plasma, the Raman backscattered radiations are seeded by a strong initial noise at the earlier times and the Mendonca condition for chaos threshold is met sooner. Therefore, the electrons gain more energy through the stochastic mechanism that is in agreement with chaotic nature of the motion.     

Contribution of near threshold states to recombination in plasmas

Resonance states in atoms or ions at low energies can control the rates of important plasma processes (e.g., dielectronic recombination). We examine the role of states at negative energies just below the ionization threshold of the recombined system and find that they can contribute as much, or more, to recombination as positive energy states. In plasmas, negative energy states can be populated by three body recombination, photorecombination, or continuum lowering. Properly including these negative energy states in a theoretical treatment of plasma processes can change the thermally averaged rate coefficients and, in some cases, removes much of the sensitivity to the energy of a state.     

Onset of breakdown and formation of cathode spots

When an increasing diode voltage is applied, enhanced field emission of electrons begins from a growing number of small spots or whiskers on the cathode surface. This stimulates desorption of weakly bound adsorbates from the surface of a whisker. As the diode voltage increases, the 100-V equipotential surface moving toward the cathode is met by the desorbed neutrals moving away from the cathode, resulting in sharp risetime for the onset of ionization of desorbed neutrals by field-emitted electrons. Positive ions produced in the ionization region a few microns from the electron emitting spot are accelerated back to it. This bombardment leads to surface heating of the spot. The onset of breakdown by this mechanism requires much less current than the Joule heating mechanism. The localized buildup of plasma above the electron emitting spot leads to pressure and electric field distributions that ignite unipolar arcs. The high current density of the unipolar arc and the associated surface heating by ions result in the explosive formation of cathode spot plasma     

Effects of inverse bremsstrahlung in laser-induced plasmas from a graphite surface

The kinetic energy of electrons emitted due to laser interaction with a graphite surface was studied with a time-of-flight spectrometer. In addition the yields of carbon atomic and molecular ions were measured as a function of laser pulse energy. Pulse energy thresholds for ion emission are observed to correlate with the observed maximum electron energies. Furthermore, the data suggest that ionic carbon clusters can be dissociated by energetic electrons or photons created in the plasma. We believe that initially photoemitted electrons are accelerated by inverse bremsstrahlung to the energies required for electron impact ionization and dissociation     

Multi-reference configuration-interaction calculations on multiply charged ions of carbon monosulfide

The potential energy curves for neutrals and multiply charged ions of carbon monosulfide are computed with highly correlated multi-reference configuration interaction wavefunctions.The correlations of inner-shell electrons with the scalar relativistic effects are included in the present computations.The spectroscopic constants,dissociation energies,ionization energies for ground and low-lying excited states together with corresponding electronic configurations of ions are obtained,and a good agreement between the present work and existing experiments is found.No theoretical evidence is found for the adiabatically stable CSq+(q>2) ions according to the present ab initio calculations.The calculated values for 1st-6th ionization energies are 11.25,32.66,64.82,106.25,159.75,and 224.64 eV,respectively.The kinetic energy release data of fragments are provided by the present work for further experimental comparisons.     

7.87 eV postionization of peptides containing tryptophan or derivatized with fluorescein

Chemical tags such as anthracene can be attached to a molecular analyte and serve as chromophores for 7.87 eV laser postionization by lowering the overall ionization potential of the tagged molecular complex. Fluorescein and tryptophan are demonstrated as two new tags for 7.87 eV laser postionization of various amino acids and peptides. Other molecular species that are efficient fluorescence probes should also serve as tags for 7.87 eV postionization since they display highest occupied molecular orbitals with extended π-conjugation that lead to ionization potentials below this photon energy and an ability to stabilize the net positive charge of the radical cations. This technique is demonstrated here for laser desorbed species, but is also applicable to keV ion sputtered neutrals. Overall, 7.87 eV laser postionization of derivatized species promises to expand the capabilities of mass spectrometric surface analysis.     

Features of the film-growth conditions by cross-beam pulsed-laser deposition

Spatial and energetic characteristics of the plasma plume by cross-beam pulsed-laser deposition (CBPLD) were investigated. Effective droplets filtering together with high efficiency of material usage are observed by this approach. Time-of-flight (TOF) technique with electrostatic ion collectors (Langmuir probes operating in the ion-collecting mode) were applied to obtain kinetic energy distribution functions of ionized particles and to compare the ionization degrees of the plasma by the CBPLD and by the conventional PLD. The average and maximum kinetic energies of the ions by the CBPLD are found to be 2–3 times lower as compared to the conventional PLD. At the same time, the fraction of ionized species and highly exited neutrals (Rydberg atoms) in the CBPLD plasma is 1.5–2 times larger in comparison to the conventional approach. Re-sputtering of the material of the growing film by fast ions is a considerable effect in both the PLD methods by the chosen experimental conditions. The angular width of the directional pattern of the plasma plume by CBPLD is comparable to that typical for the conventional PLD. Received: 21 December 1998 / Accepted: 28 March 1999 / Published online: 7 July 1999     

Kinetic analysis of the ionization ratio effects on a three‐species plasma

In partially ionized plasmas, the energy transferred to electrically charged species by the electromagnetic field can be partly channelized to the population of neutrals, due to interspecies collisional processes. Depending on the relative density of neutrals, these effects may govern the collective plasma dynamics by drastically modifying particle dynamics and energy‐transport processes with respect to the fully ionized plasma‐approximation models. In this work, the influence of the ionization ratio r_i on a partially ionized plasma is analysed by means of a three‐species one‐dimensional kinetic model to compute transient and steady state velocity‐dependent distribution functions. The conservative collision operators accounting for charge–charge and charge–neutral interactions allow studying several plasma scenarios with the same entire number of particles per unit of volume but for an increasing r_i parameter, in the presence of a modulated signal‐like electric field. For a sequence of plasma scenarios of fixed r_i, ranging from typical weakly ionized to highly ionized plasma values r_i ～ 10^?7–10^?4, the mass species flows are examined. These flows behave linearly with respect to r_i up to a value r_i ? 10^?5 from which the quasi‐linear dependence is critically altered. The convection–diffusion equations are solved with the semianalytical Propagator Integral Method, which behaves well to deal with conservative operators, density, and field discontinuities, allowing for the use of collision terms of disparate time and spatial characteristic scales. The results can be relevant to a wide class of plasma systems and to analyse the ionization ratio effects on transport coefficients.     

Interferometry of flashboard and cable-gun plasma opening switcheson Hawk

Interferometry of plasma opening switch (POS) plasmas on the Hawk generator has shown many important features of the plasma evolution during conduction and opening. Opening occurs when a low-density region forms at a radial location determined by plasma redistribution during the conduction phase, consistent with J×B forces and the measured plasma distributions produced by the sources alone. High neutral densities have been detected in the POS region during conduction. Low-density plasma appears between the POS and the load at the time current appears in the load, and high-density plasmas appear there later in time. There are two important differences between the density evolution of POS's utilizing flashboard and cable-gun plasma sources. 1) There is a substantial (two-three times) increase in the electron inventory during conduction using cable guns that is not detected using flashboards. This is attributed, primarily, to ionization of ions and neutrals for the cable-gun case. 2) The conduction scaling with plasma density implies that the cable-gun POS has an effective ion mass/charge ratio about double that for the flashboard POS     

The Impact of Direct Ionization by Beam Electrons on the Electron Kinetics of the Beam Discharge Plasma in Molecular Hydrogen

In a recent paper the stationary beam plasma discharge in partially dissociated hydrogen was investigated where the electron component was described by the Boltzmann equation for a mixture of atomic and molecular hydrogen and the main heavy charged and neutral particles by balance equations. It was assumed that, via the quasilinear beam plasma interaction, the electron beam produces only the turbulent electric field whilst an additional production of plasma electrons due to direct ionization by the beam and thus a direct influence on the balances of charge carriers were neglected. Now the additional production of plasma electrons due to direct ionization by the beam is studied on the basis of a generalized Boltzmann equation but for the simpler model of a purely molecular hydrogen plasma. For experimentally obtainable values of the turbulence energy density, beam energy, beam ionization degree and electron life time the calculation of the electron energy distribution function and of the direct beam contribution to the electron particle balance shows a marked influence of the direct beam ionization with increasing degree of beam ionization.     

Internal energy content of n-butylbenzene, bromobenzene, iodobenzene and aniline molecular ions generated by two-photon ionization at 266 nm. A photodissociation study

A technique to investigate photodissociation kinetics on a nanosecond time scale has been devised for molecular ions generated by multiphoton ionization (MPI) using mass-analyzed ion kinetic energy spectrometry. The branching ratio or rate constant has been determined for the photodissociation of the n-butylbenzene, bromobenzene, iodobenzene, and aniline molecular ions generated by MPI at 266 nm. The ion internal energies have been estimated by comparing the measured kinetic data with the previous energy dependence data. The analysis has shown that only those molecular ions generated by two-photon ionization contribute to the photodissociation signals. Around half of the available energy has been found to remain as molecular ion internal energy in the two-photon ionization process. Copyright 1999 John Wiley & Sons, Ltd.     

Rates and channels of water ionization by a high-current high-voltage pulsed discharge

The experimentally observed growth of the plasma density in a high-current high-voltage pulsed discharge in a liquid medium is compared with the results of calculations based on the effective cross sections for electron-impact ionization and other elementary processes. It is found that, in the initial stage of the discharge, the plasma density grows linearly with time, whereas at densities above 3 × 10¹⁰ cm^?3, the growth becomes exponential due to the collective acceleration of plasma electrons. The gas-vapor fraction of the water medium is ionized by two groups of electrons: low-energy electrons, with energies about several tens of electronvolts, and high-energy ones, with energies in the kiloelectronvolt range. The energy spent on water ionization is estimated and is found to be several times higher than the energy required to ionize a rarefied gas.     

利用静电场中光电离效率谱精确确定1,3-二乙氧基苯分子的电离能

电离能是原子和分子的重要的特性参数,在光物理和光化学过程中起着重要作用,精确电离能对相关研究具有重要意义.电离能是调试零动能光谱信号的重要参考数据,在判断异构物数量和分子构型方面也起着关键作用.1,3-二乙氧基苯是一种重要的苯的衍生物,实验证实在超声分子束中包含两种旋转异构物I(down-up)和III(down-do...     

Energy level offset analysis of lead atom in laser plasma

The optical emission spectra of the plasma generated by a 1064 nm laser irradiation of lead target in air were recorded and analyzed. Temporal evolvement trait of spectral lines was investigated. The Stark width and line shift were measured at different delay time and laser energies. The electron densities were determined using Stark-broadening parameters of spectral lines. The atomic energy level offset in plasma surroundings was explored by analyzing the line shift. The experimental data of Stark widths and line shifts were analyzed using the regularity of the Stark parameters’ dependence on effective ionization potential. However an inverse experimental result was found compared with the theoretical calculation. In addition, the change of the Stark widths and line shifts with the delay time and laser energies was discussed.     

Electron-impact ionization and dissociative ionization of sulfur in the gas phase

We describe the methods and the results of investigation of the yield of positive ions formed as a result of electron-impact ionization of sulfur. The ionization energy for the basic molecule and the energies corresponding to the emergence of fragment ions are obtained from the ionization efficiency curves. The dynamics of formation of molecular sulfur ions in the temperature range 320–700 K is investigated. The energy dependences of efficiency S _n of the ion formation for n = 1–6 are analyzed, and their appearance energies are determined. The total cross section of sulfur ionization by a monochromatic electron beam is also investigated. Using the linear approximation method, we marked out features on the ionization function curve, which correspond to the ionization and excitation energies for multiply charged ions. The total cross section of the formation of negative sulfur ions is measured in the energy range 0–9 eV.