Dissociative attachment of electrons to excited molecules

The development and the first results from an experiment to carry out dissociative attachment to excited molecules are discussed. A brief summary of the relevance and status of such measurements are given. Apart from measuring the absolute cross sections from excited and state selected SO₂ molecule, we have been able to characterize the negative ion resonances using the excited state dissociative attachment. In addition, the state specificity of the electron attachment process has been used to derive information on the excited neutral state itself which has not been possible using optical spectroscopy. The applicability of this technique to other species are also discussed.     

Negative Ions in a Glow Discharge of Tetrafluoromethane

The Langmuir — probe measurements in a CF₄ — glow discharge reveal a large number of negative ions (10¹⁰ cm^?3) in the plasma between the planar C-electrodes. Under special conditions the density of them exceeds the electron density at several orders of magnitude and sustains a quasi — electron free plasma. The ratio depends on the residence time of the molecular gas in the discharge reactor. It is due to dissociative electron attachment to highly molecular gas components arising from the plasmachemical conversion of CF₄. F^? and CF₃^? are the most important negative ions.     

Generation of cold electrons in the downstream region of a microwave plasma source with near boundary resonances for production of negative ions

A microwave driven multicusp plasma based volume negative ion source equipped with a magnetic filter is developed. Instead of employing any electrodes or current carrying filaments, microwaves of frequency 2.45 GHz is used to generate plasma by resonance heating mechanisms namely the electron cyclotron resonance (ECR) and upper hybrid resonances (UHR), occurring near the boundary plasma layers. The principal process of negative ion production in hydrogen is dissociative attachment of low energy (0.5–1.0 eV) electrons to vibrationally excited neutral molecules generated from high energy (15–20 eV) electron impact. The source therefore necessitates two distinct spatial regions (a) production and (b) attachment chambers; which would contain electrons with optimum cross section for the aforementioned processes. A biased grid after the magnetic filter further helps to lower down the electron temperature to ≤1 eV which is favorable for the dissociative attachment process.     

The influences of gas and electron temperatures on electron attachment in gas electrical discharges

Various electron attachment processes are reviewed, emphasising the way in which the rates and products of some selected reactions vary with the attaching gas temperatureT _g, the temperature,T _e, and the energy of the attaching electrons. The examples illustrating the variety of reactions are the efficient dissociative attachment reaction to CCl₄, attachment to SF₆ which involves both dissociative and non-dissociative attachment, attachment to CHCl₃ which requires activation energy, and attachment to CCl₃Br which results in both Cl- and Br- product ions. A model has been presented which is able to quantitatively explain the difference influences ofT _g andT _e on the rates of some of these reactions. Also described are the unusually efficient attachment properties of the fullerene molecules C₆₀ and C₇₀ as revealed by our FALP experiments, noting that these molecules have potential importance as efficient suppressers of electrical breakdown through gases such as those used to insulate high voltage devices. We emphasise throughout this paper the importance of an understanding of the separate influences of gas and electron temperature on attachment reactions for the modelling of practical gas discharge media such as etchant plasmas. We dedicate this paper to Professor Jan Janča on the occasion of his sixtieth birthday in recognition of his major contributions to gas discharge physics.     

Zersetzung von CCl4 im HF-Niederdruckplasma für plasmachemisches Ätzen von Aluminium

After a brief summary of decomposition processes occuring in a plasma etching reactor investigations of CCl₄-decomposition by gaschromatography and time resolved emission spectroscopy are described. The decomposition takes place in a plasma tube with external r.f. electrodes. The qualitative identification of decomposition products of CCl₄ shows few main products only quite in contrast to plasma decomposition of hydrocarbons. These are CCl₄, Cl₂, C₂Cl₄, C₂Cl₆ and glow polymers. The dependence of the product distribution on power density and flow rate gives some hints for a rapid decomposition mechanism. The time resolved emission spectroscopy bases on a two channel method using a reference signal of an Ar line to eliminate the changing plasma conditions during the decomposition process. The limits of application of this method are discussed. Investigations of CCl-, Cl₂- and Cl-intensity-time-functions also reveal the existence of a the rapid decomposition mechanism for which a dissociative electron attachment is supported giving rate constants in the expected order of magnitude.     

The short-range reaction matrix in MQDT treatment of dissociative recombination and related processes

We discuss the Lippmann-Schwinger equation which governs the short-range reaction matrix (K-matrix) in the two-step multichannel quantum defect theory (MQDT) of dissociative recombination and related processes. We show that, if the energy dependence of the electronic coupling between the dissociative state and the ionization continua can be neglected, the convergence of the Born expansion of the Lippmann-Schwinger equation is achieved at second order. For the case of energy-dependent interaction, higher order effects are tested using a non-perturbative method for solving the Lippmann-Schwinger equation. Numerical examples are given for the dissociative recombination and vibrational de-excitation of the H₂ ⁺ molecular ion.Received: 18 April 2003, Published online: 5 August 2003PACS: 34.80.Gs Molecular excitation and ionization by electron impact - 34.80.Ht Dissociation and dissociative attachment by electron impact - 34.80.Lx Electron-ion recombination and electron attachment     

Glowing plasma formations in the atmosphere and their reproduction in the laboratory

The reasons for the appearance of glowing plasma formations in the real atmosphere are discussed. The results of laboratory modeling of these formations, in which the relative humidity of the air was varied, are reported. There exists a certain optimum concentration of water vapor, which leads to the formation of a cool, nonequilibrium plasma whose decay time is several seconds. Such atmospheric-electricity phenomena as St. Elmo's fire, ball lightning, and bead lightning are discussed as examples. The properties of the cool, nonequilibrium plasma are analyzed as a function of the content of H₂O molecules.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 87–104, March, 1992.     

Relaxing Phenomena in Negative Corona Discharge: New Aspects

Two theories, explaining the time dependence of the negative corona discharge current in air, are confronted with new experimental results. The influence of the ozone concentration on the discharge current was experimentally confirmed in dry air and in mixtures of nitrogen with oxygen. Assuming that only the dissociative electron attachment to ozone molecule is a process being responsible for a reduction of the electron component in the total mean discharge current, the mean value of the electron attachment rate constant k = (3 - 5.5) × 10^?9 cm^?3 s^?1 was derived from the measured dependence of the discharge current on the ozone concentration. The calculated value of the rate constant k corresponds to the dissociative attachment of electron to ozone molecule via process e + O₃ → products (O^? or O^?₂ negative ions).     

Hydrogen line ratios as electron temperature indicators in nonequilibrium plasmas

The ratio of intensities of hydrogen Balmer lines H_α-H_β is proposed as an indicator of electron temperature in a nonequilibrium plasma. Although different from the equilibrium case, the intensity ratio is a unique function of electron temperature provided that the optical depth is small for the visible lines and the plasma is far out of equilibrium. For such a plasma, the intensity ratios of the H_α-H_β lines are computed. The results are tabulated in the form of a conversion table between the measured excitation temperature and the true electron temperature. The ranges of applicability of the conversion table are also computed and are presented in separate tables. An example is shown in which particle densities are consistent with the Saha equilibrium condition at the apparent excitation temperature even though the plasma is in nonequilibrium at a different true electron temperature.     

A study of oxygen dissociative adsorption on close-packed metal surfaces using a hybrid analytical approach

The adiabatic and nonadiabatic formalisms developed for calculations of the X₂ molecule dissociative adsorption on metal surfaces are reviewed. The adiabatic formalism is extended here to account for the tunnel effect along the reaction coordinate. This approach is applied for calculating the activation energies, and rate constants associated with oxygen dissociative adsorption on close-packed metal surfaces of Pd, Pt, Rh, Ir, Ru, and Ni. Published information and data calculated in this work from a density functional treatment of a three-layer cluster model are used to determine the geometric and spectroscopic characteristics of atomic and molecular oxygen adsorbed on these surfaces. Results of the calculations are compared with published theoretical and experimental activation energies. The tunnel effect on the preexponential factor and on the activation energy of oxygen dissociative kinetic is discussed. A relationship between the kinetic and the corresponding thermodynamic characteristics of the reactions is considered.     

The physical parameters and unit processes in the plasma of noble/electronegative gas mixtures in high-energy systems

An experiment using probe and spectroscopic methods has been performed to study the physical processes and the kinetics of nonequilibrium processes in the steady-state and pulsed low-temperature plasmas of mixtures of noble gases with fluorine and some fluorine-containing halides. The results of the experiment are compared with predictions of a numerical simulation. It has been shown that as the percentage of the fluorine-containing gas is varied the average energy goes through a minimum, and the difference between the Townsend coefficients of ionization and attachment goes through an extremum as well. The numerical simulation has demonstrated that these variations in the physical parameters of plasmas are governed by kinetic factors. Ivanovo State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 18–24, May, 1999.     

Pressure dependence of electronic transitions of Fe,Co and Ni ions in layered dihalides

Abstract

The near infra-red absorption peaks due to transition metal ions in four halides of Fe, Co and Ni have been studied as a function of pressure. The behaviors of these ions' absorption peaks under pressure are found to be quite different. While the energy of the absorption peak in Col₂ increases with pressure similar to the behavior of transition metal ions in cubic crystal fields, the absorption peaks in the Fe halides are found to be almost independent of pressure. In Nil₂ two absorption peaks exhibit level-crossing at about 2GPa. The results have been interpreted in terms of a theory proposed by da Silva and Falicov [Phys. Rev. B 45,11511 (1992)] in which pressure changes the trigonal component of the crystal field at the transition metal ions.     

Direct observation of reaction intermediates in catalytic synthesis of ammonia

Reaction intermediates in ammonia synthesis on metal surfaces are observed directly in the pulsed-laser stimulated field desorption mass spectrum to be N, NH, and NH₂. The dominant mechanism of ammonia formation on metal surfaces at low temperature and gas pressure is therefore by dissociative mechanism.     

Physicochemical Processes in Heavy-Metal Azides and the Dislocation Structure

The present paper gives new experimental results that allow slow decomposition of metal azides of different types (including radiolysis, photolysis, electrolysis, and thermolysis) to be analyzed in the context of the common reaction pathway. Filamentary crystals of silver and lead azides are investigated. It is demonstrated experimentally that the reaction goes on the vacancy cluster formed by an edge dislocation in the near-surface region of the crystal. The reaction produces nonequilibrium electrons and holes considered as reactants. In the bulk of the crystal, the reaction in the anionic sublattice forms an intermediate product, hypothetically, N₆. Methods of controlling the defect crystal structure of metal azides are suggested. They allow the reactivity of metal azides to be preset.     

Gas-discharge plasma initiated in air by a radiation pulse

Formation of a nonequilibriuin gas-discharge plasma in air under the action of radiation at a wavelength of 10.6 with a duration of 10^–6 sec is considered. It is shown that the air discharge occurs under nonequilibrium conditions by formation of an autoionization complex with subsequent oscillatory excitation of molecules. The molecular distribution function over oscillatory degrees of freedom is found in the diffusion approximation. The electron temperature and concentration in the breakdown plasma are calculated with consideration of multistep ionization of molecules and dissociative recombination of charged particles. Results are compared with experiment.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 9–13, February, 1985.     

Nonadiabatic rate processes on metal surface: Limitation by spin conversion

Rate processes occurring on a metal surface may sometimes be limited by spin conversion. We present a generic model describing this case. The results obtained are compared with the conventional two-state Landau-Zener model and with a multistate model implying one-electron transfer between the reactant and the metal. In this context, the specifics of the dissociative adsorption of O₂ on Ag(111) are briefly discussed. The text was submitted by the author in English.     

The vaporization of a metallic target by a microsecond pulsed TE-CO2 laser radiation

The analysis of some new experimental data has proved that the evaporation of a stainless steel target in vacuum under the action of powerful TE-CO₂ microsecond pulsed laser radiation is initiated on the initial spike or not at all. The minimum “tail” duration to get evaporation was deduced as a function of the temporal pulse shape characteristics and the metal thermophysical properties. The vaporisation process was accompanied by nonequilibrium heating of vapour clouds leading to the target surface shielding. Two self-regulated evolutions were recognised which were attributed to the intense vaporisation and the target evaporation by the UV plasma emitted radiation, respectively.     

紫外光诱导光电子发射负离子源的阈值光电子成像仪设计 (cited: 2)

设计了一套紧凑的光电子成像装置,它包括解离式光电子贴附负离子源、垂直安装的高分辨阈值光电子速度成像装置和线性飞行时间质谱仪．紫外光辐射金属表面诱导低能光电子发射,再通过低能电子贴附超声分子束产生高强度和冷的负离子源．结合这种负离子源和飞行时间质谱-光电子成像仪装置,仪器的质量分辨能达到200左右,能量分辨优于3%(即对1 eV动能的电子,分辨达到30 meV)．此外,使用该实验装置获得了CH₃S^-和S₂^-在611.46 nm下的低能阈值光电子成像结果．同时得到了CH₃S和S₂的更精确的电子亲和势分别为1.8626±0.0020和1.6744±0.0035 eV．初步的结果证明了该装置对研究阈值光电子成像精确测量光电子亲和势非常有效     

The dynamics of dissociative adsorption on metal surfaces: “First principles” theory – Experiment confrontations

This review tries to asses our current state of understanding of the dynamics of dissociative adsorption on single crystal metal surfaces by quantitatively comparing “first principles” theory, i.e. that based on density function theory, with detailed dynamical experimental results. Examples are chosen for H₂ and N₂ dissociation on transition metal surfaces.     

An alternative approach on the calculation of cohesive energy density and isothermal compressibility of alkali metal halides

In the present paper, new and useful theoretical methods for the estimation of cohesive energy density (Ced) and isothermal compressibility (k_T) of alkali metal halides are described. The mentioned theoretical methods include the use of Kaya molecular hardness equation published by us in recent years. Cohesive energy density and isothermal compressibility of alkali metal halides were calculated in the framework of mentioned theoretical methods and the results obtained were compared with both experimental data and the results of previous theoretical methods proposed to calculate the aforementioned quantities, namely cohesive energy density and isothermal compressibility. It is important to note that the results obtained in the study are in good agreement with the available experimental data and with the results of previous theoretical methods. Additionally, we also investigated the correlation with lattice energy of cohesive energy density and isothermal compressibility for alkali halides.