Rate Constants Of Elementary Processes Research Articles

Atmospheric pressure plasma jets generated by the DBD in argon-air, helium-air, and helium-water vapour mixtures

Plasma jets are increasingly used in plasma medicine, in microelectronics and for surface treatment due to their unique properties, such as low gas temperature and high concentration of active species. This paper presents the results of a study of plasma jets generated by the atmospheric pressure dielectric barrier discharge while passing through it of argon-air, helium-air and helium-water vapor mixtures. Comparison of the plasma jets characteristics shows that the gas flow rate at which the maximum power of UV radiation is achieved increases in the series of He-H2O, He-air, Ar-air. Replacing Ar-air working mixture with He-air mixture leads to an increase in the plasma jet radiation power in ∼ 3 times. The maximum overall radiation power achieved in the He-air plasma jet was 0.07 W. The emission spectra of the plasma jets were identified in the spectral region of 270–900 nm, it was found that the molecular bands of the second positive and first negative systems of nitrogen predominate in the UV region of spectrum and lines of He I or Ar I prevail in the visible region. The distributions of the radiation intensity of excited plasma species along the jet length were measured. The analysis of difference in the emission spectra of the plasma jet in pure Ar and He and in their mixtures with air, as well as in a He-H2O mixture, has been performed taking into account values of the mean electron energy and rate constants of elementary processes in the plasma calculated using the Bolsig + code. The molecular and ionic reactions responsible for emission of radiation in the UV range of spectrum are considered.

A calculation of semi-empirical one-electron wave functions for multi-electron atoms used for elementary process simulation in nonlocal plasma

Subject of Research. The paper deals with development outcomes for creation method of one-electron wave functions of complex atoms, relatively simple, symmetrical for all atom electrons and free from hard computations. The accuracy and resource intensity of the approach are focused on systematic calculations of cross sections and rate constants of elementary processes of inelastic collisions of atoms or molecules with electrons (ionization, excitation, excitation transfer, and others). Method. The method is based on a set of two iterative processes. At the first iteration step the Schrödinger equation was solved numerically for the radial parts of the electron wave functions in the potential of the atomic core self-consistent field. At the second iteration step the new approximationfor the atomic core field is created that uses found solutions for all one-electron wave functions. The solution optimization for described multiparameter problem is achieved by the use of genetic algorithm. The suitability of the developed method was verified by comparing the calculation results with numerous data on the energies of atoms in the ground and excited states. Main Results. We have created the run-time version of the program for creation of sets of one-electron wave functions and calculation of the cross sections and constants of collisional transition rates in the first Born approximation. The priori available information about binding energies of the electrons for any many-particle system for creation of semi-empirical refined solutions for the one-electron wave functions can be considered at any step of this procedure. Practical Relevance. The proposed solution enables a simple and rapid preparation of input data for the numerical simulation of nonlocal gas discharge plasma. The approach is focused on the calculation of discharges in complex gas mixtures requiring inclusion in the model of a large number of elementary collisional and radiation processes involving heavy particles in different quantum states.

Impurity radiation from a tokamak plasma

In tokamak operating modes, energy balance is often governed by impurity radiation. This is the case near the divertor plates, during impurity pellet injection, during controlled discharge disruptions, etc. The calculation of impurity radiation is a fairly involved task (it is sometimes the most difficult part of the general problem) because the radiation power is determined by the distribution of ions over the excited states and by the rate constants of elementary processes of radiation and absorption. The objective of this paper is to summarize in one place all the approximate formulas that would help investigators to describe radiation from the most often encountered impurities in a fairly simple way in their calculations accounting for plasma radiation, without reference to special literature. Simple approximating formulas describing ionization, recombination, and charge-exchange processes, as well as radiative losses from ions with a given charge, are presented for five impurity species: beryllium, carbon, oxygen, neon, and argon. Estimating formulas that allow one to take into account plasma opacity for resonant photons in line impurity radiation are also presented.

Dynamic behavior of hydrogen isotopes in tungsten–carbon composite materials

The thermal re-emission of hydrogen isotopes from WC layers on graphite, implanted with 5 keV H + 2 and D + 2 ion beams up to saturation at room temperature, has been studied by means of the elastic recoil detection (ERD) technique. It is found on isochronal annealing for 10 min that the re-emission takes place in three stages: the first stage occurs for retained fraction of 1.0–0.6, the second stage for retained fractions of 0.6–0.2 and the third stage below the retained fraction of 0.2, which is ascribed to re-emission from carbon segregated in the grain boundaries of the WC layers. It is also found on isothermal annealing that the concentration of retained hydrogen decreases rapidly in the beginning and hereafter very gradually with increasing the annealing time. The isothermal re-emission curves have been analyzed by taking into account the two trapping sites and the mass balance equations with two kinds of thermal detrapping ( Σ d 1 , Σ d 2 ), retrapping (Σ t) and local molecular recombination ( K 1) between free hydrogen atoms. The isotope difference of the rate constants for elementary processes determined by best fitting of the solution to the experimental re-emission profiles is discussed with relevance to the nature of two trapping sites.

Template photopolymerization of methacrylic acid in water—IV. Determination of rate constants of elementary processes

Initiation rate for the polymerization of methacrylic acid in water, in the absence and in the presence of polyvinylpyrrolidone as a template, have been determined by the inhibition method. A water-soluble inhibitor (2,2,6,6-tetramethyl-4-hydroxy-piperidine-l-oxyl) was used. The rate constants for template propagation ( k p) and termination ( k t) were calculated and compared with the corresponding constants for non-template polymerization. It was found that k p and k t in the template polymerization are lower by several orders of magnitude that those in the polymerization without template. The mechanisms of elementary processes of the template polymerization are discussed.

Method for determining rate constants of processes in atoms and molecules

A variant method is proposed for determination of rate constants of elementary processes occurring within a plasma, including irradiation of the plasma by resonant radiation. In the proposed variant the radiant power is varied during experiment, but remains less than the value which causes saturation of fluorescence. It is shown that this method permits simultaneous determination of several constants. The technique is demonstrated for a C2 molecule. The effective section for thermal excitation of the level d3πg from states (X1∑g+ + a3πu) is determined, equal to 3·10−18 cm2±80%.

Monte Carlo evaluation of TST rate constants of elementary processes

The Monte Carlo simulation of Xenon atoms adsorbed at a Platinum (111) surface is carried out in order to compute the rate constant of the thermal desorption process at several temperatures.

Chemical relaxation study of the heterogeneous silicon-hydrogen system under plasma conditions

The chemical relaxation technique consists of measuring the response of a chemical system to a small disturbance of an equilibrium or a nonequilibrium steady state. Since, for a small perturbation, the response of any complex and stable system is linear, rate constants of elementary processes under actual plasma conditions can be evaluated directly from the relaxation data. Applications of this technique to the kinetic study of the formation and decomposition of silane are presented and compared with previous data obtained by a flow method. In addition, relaxation data are presented which show that the main reaction channel of silance decomposition in glow discharges is the electron-impact-induced fragmentation into SiH2 radicals (or ions) and H2 molecules.

Kinetics of photophysical and photochemical processes involving selective excitation of ortho-I 2 by laser radiation

A method is described for studying elementary processes in an ortho-para iodine molecular mixture during selective excitation of ortho-I 2 and direct fluorescent control of the concentration of para-I 2 and ortho-I 2 molecules. Selective predissociation of ortho-I 2 molecules was carried out with argon laser radiation. It is shown that the conversion of para-I 2 into ortho-I 2 proceeds under formation of the complex I 3. The rate constants of elementary processes have been evaluated: the probability of iodine atom recombination on the cell walls, δ=(1.04±0.07)×10 4s −1; the desorption probability, K *=(2.30±0.25)×10 −4s −1; the conversion rate constant, β=(1.88=0.09)×10 16 cm 6 mol −2 s −1; the quantum yield of predissociation from the excited state B 3Π o+u( u=43) of I 2 molecules, γ=1.66×10 −5. The selective photochemical reaction of electronically excited iodine molecules with 2-hexene has been investigated. When the mixture (iodine-40 mtorr, 2-hexene - 2 torr) was irradiated for 10-12 min by the 5145 Å line of an argon laser with a power of 2.2 W, photochemical separation of ortho- and para-modifications of iodine was realized with the enrichment coefficient being equal to 4.

Use of stable radicals for the determination of rate constants of elementary processes

A direct method is proposed for determining the concentration of active centers by introducing the stable radical 2, 2, 6, 6-tetramethyl-4-hydroxypiperidine-1-oxyl at the commencement, and during the course of, the polymerization process of tetrafluoroethylene. On the basis of the data obtained, the rate constants of the elementary processes are calculated.