Set Of Collision Cross Sections Research Articles

Electron collision cross section set for N2 and electron transport in N2, N2/He, and N2/Ar

Electron collision cross section set for N2 is proposed. The proposed set consists of vibrational elastic momentum transfer cross section and cross sections for 31 rotational excitation, 29 rotational deexcitation, 10 vibrational excitation, 17 electronic excitation, 2 neutral dissociation, and 5 ionization collision processes. Electron transport coefficients in N2, N2/He, and N2/Ar are calculated by Monte Carlo method with considering anisotropic inelastic electron scattering and partitioning of the residual energy after the ionization collision on the basis of experimental results. The reliability of the proposed electron collision cross section set is demonstrated by comparing the calculated and measured electron transport coefficients in a wide range of reduced electric fields, E/N (E: electric field strength, N: number density of gas molecules). Comparison of the values of the electron transport coefficients in N2 calculated from eight individual cross section sets partly provided on LXCat with the measured values is also carried out. Furthermore, the effect of anisotropic inelastic electron scattering on the electron transport coefficients is examined. The electron transport coefficients in N2 are found to be affected by considering anisotropic electron scattering with increasing E/N values, and the longitudinal third-order transport coefficient is found to be sensitive to considering anisotropic electron scattering even at middle E/N values.

Electron collision cross section set of Cl2 gas and electron transport analysis in Cl2 gas and Cl2/N2 mixtures

A detailed and reliable electron collision cross section set of Cl2 gas is constructed. The present cross section set consists of one elastic momentum transfer, three vibrational excitation, eight dissociative electronic excitation, two electronic excitation, one ion-pair formation, one dissociative electron attachment, and four ionization cross sections. The electron drift velocity, longitudinal diffusion coefficient, ionization coefficient, and electron attachment coefficient in both Cl2 gas and Cl2/N2 mixtures and the electron attachment rate coefficient in mixtures of N2 gas and a minute amount of Cl2 gas are calculated by Monte Carlo simulation using the present cross section set, and then the reliability of the present cross section set is demonstrated by comparing calculated and measured electron transport coefficients and rate coefficients. Furthermore, the rate coefficients for both elastic and inelastic electron collisions and the mean energy of electrons in Cl2 gas are calculated from the present cross section set and are provided.

Dissociative reactions induced by electron impact and electron transport in TEOS vapor

Reactions for dissociative ionization and neutral dissociation collisions between an electron and a tetraethoxysilane [Si(OC2H5)4, TEOS] molecule are reported, and neutral dissociation cross sections are calculated by applying the classical Rice–Ramsperger–Kassel theory. Detailed electron collision cross section set of TEOS vapor, including 18 neutral dissociation cross sections and 20 ionization cross sections, is constructed. Electron transport coefficients, such as the mean-arrival-time drift velocity, bulk drift velocity, longitudinal bulk diffusion coefficient, and ionization coefficient, and rate coefficients for both elastic and inelastic collisions in TEOS vapor are calculated from the present cross section set using a Monte Carlo method. The validity of the present cross section set is demonstrated by comparing the calculated electron transport coefficients with measured data. Furthermore, fragmentation of TEOS molecules by electron impact is simulated, and then the number of fragments produced in the simulation is compared with the measured mass spectra.

Cross-sections, transport coefficients and dissociation rate constants for molecular ion interacting with Kr

A hybrid method based on quantum mechanical formalism for electrons and a classical treatment for nuclei was used to study the dynamics of electronically ground state krypton dimer in collision with its parent gas. Two sets of collision cross-sections, namely non-reactive scattering and collision induced dissociation, were calculated from this method by using different sets of atom–atom interaction potentials (neutral–neutral, ion–neutral). These cross-sections were injected in Monte Carlo simulations for the determination of mobility, diffusion and dissociation rate constant. It is noteworthy that we also used an inverse method based on JWKB approximation to calculate a global momentum transfer collision cross-section without discerning the inelastic processes as dissociation. Thus, the transport coefficients obtained are also in a pretty good agreement with experimental data. All Monte Carlo simulations were performed at ambient pressure and temperature over a broad range of reduced electric field. Moreover, effects of rovibronic excitations were also studied thoroughly in order to improve correlation between theoretical mobility calculations and experimental data available in the literature.

Electron Collision Cross Sections for the TRIES Molecule and Electron Transport Coefficients in TRIES-Ar and TRIES-O2 Mixtures

A reliable set of low-energy electron collision cross sections for the triethoxysilane (TRIES) molecule was derived based on the measured electron transport coefficients for a pure TRIES molecule by using an electron swarm method and a two-term approximation of the Boltzmann equation. The electron transport coefficients calculated using the derived set are in good agreement with experimental value over a wide range of E/N values (ratio of the electric field E to the neutral number density N). The present electron collision cross section set for the TRIES molecule, therefore, is the most reliable so far for plasma discharges and for materials processing using the TRIES molecule. Moreover, the electron transport coefficients for the TRIES-Ar and the TRIES-O2 mixtures were also calculated and analyzed over a wide range of E/N for the first time.

Measurement of electron swarm coefficients in chlorine and its mixtures with nitrogen

We have measured the electron drift velocity, density-normalized longitudinal diffusion and effective ionization coefficients for electrons in Cl2 and Cl2–N2 mixtures over the combined E/N range from 3 to 450 Td, using a pulsed Townsend apparatus especially designed to withstand the action of corrosive gases. Our measurements for the electron drift velocity and the effective ionization coefficient are compared with the only two previous measurements available, one of them indirect. In view of the present incompleteness and uncertainty in the electron collision cross section set for Cl2, Cl2–N2 mixtures with Cl2 contents from 1% to 80% have been measured in the aim of shedding more light into the completeness and reliability of the cross section set by testing the cross sections derived for pure Cl2 with those of the Cl2–N2 mixture for which the cross sections for N2 are well known.

Breakdown electric fields in dissociated hot gas mixtures of sulfur hexafluoride including teflon: Calculations with experimental validations and utilization in fluid dynamics arc simulations

Measurements of breakdown voltage Vb, gas temperature Tg, and density N and the associated critical electric field Ecr/N are performed in hot dissociated SF6 highly diluted in argon and in hot dissociated SF6 mixed with PTFE (Polytetrafluoroethylene or C2F4) also highly diluted in argon. Gases are heated using a microwave source and optical emission spectroscopy is used for measurements of Tg and N while Vb is measured from a specific inter-electrode arrangement placed inside of the cell of the hot gas conditioning. The experimental Ecr/N data in the numerous considered cases of gas temperatures and compositions have been used to evaluate and validate the sets of the collision cross sections of the 11 species involved in hot dissociated SF6 (i.e., SF6, SF5, SF4, S2F2, SF3, SF2, SF, S2, F2, F, and S), the 13 additional species involved either in hot C2F4 or CF4 (C2F6, C2F4, C2F2, CF4, CF3, CF2, CF, F2, F and carbon species as C, C2, C3, C4) and also the 2 further species (CS and CS2) present only in the considered mixtures SF6 + C2F4. The fitted sets of collision cross sections of all these 26 species are then used without argon dilution in hot SF6 and hot SF6 + C2F4 mixtures to calculate and to analyze the Ecr/N data obtained for a wide range of gas temperature (up to 4000 K) and gas pressure (8 bar and more) using a rigorous multi-term solution of the Boltzmann equation for electron energy distribution function and standard calculations of hot gas composition for the species proportions. Such Ecr/N data have been then successfully used to evaluate from a Computational Fluid Dynamics model the switching capacity at terminal fault from a coupled simulation of the electrostatic field and the hot gas flow after current zero.

Determination of the electron collision cross section set for the C2F6 molecule

The electron drift velocity and the density-normalized longitudinal diffusion coefficient in the pure C2F6 molecule over the E/N range of 0.1-100 Td have been measured using a double shutter drift tube, where E is the electric field and N is the neutral number density. Our electron drift velocity values are in good agreement with those reported in a previous work on the C2F6 molecule. The negative differential conductivity phenomenon was found in the electron drift velocity. The results of our new measurements on the pure C2F6 molecule, along with the available electron transport coefficients, in 0.524 and 5.47 % C2F6 - Ar mixtures were used to successfully obtain the most reliable set of electron-collision cross sections for the C2F6 molecule by using a multi-term Boltzmann analysis.

Overdamped Alfvén waves due to ion-neutral collisions in the solar chromosphere

Alfvenic waves are ubiquitous in the solar atmosphere and their dissipation may play an important role in atmospheric heating. In the partially ionized solar chromosphere, collisions between ions and neutrals are an efficient dissipative mechanism for Alfven waves with frequencies near the ion-neutral collision frequency. The collision frequency is proportional to the ion-neutral collision cross section for momentum transfer. Here, we investigate Alfven wave damping as a function of height in a simplified chromospheric model and compare the results for two sets of collision cross sections, namely those of the classic hard-sphere model and those based on recent quantum-mechanical computations. We find important differences between the results for the two sets of cross sections. There is a critical interval of wavelengths for which impulsively excited Alfven waves are overdamped as a result of the strong ion-neutral dissipation. The critical wavelengths are in the range from 1 km to 50 km for the hard-sphere cross sections, and from 1 m to 1 km for the quantum-mechanical cross sections. Equivalently, for periodically driven Alfven waves there is an optimal frequency for which the damping is most effective. The optimal frequency varies from 1 Hz to 10^2 Hz for the hard-sphere cross sections, and from 10^2 Hz to 10^4 Hz for the quantum-mechanical cross sections. Future observations at sufficiently high spatial or temporal resolution may show the importance of high-frequency Alfven waves for chromospheric heating. For instance, the Atacama Large Millimeter/submillimeter Array (ALMA) may be able to detect the critical wavelengths and optimal frequencies and so to test the effective collision cross section in the chromospheric plasma.

Ar+/Ar, O2+/O2 and N2+/N2 Elastic Momentum Collision Cross Sections: Calculation and Validation Using the Semi-Classical Model

The aim of this paper is to obtain relevant sets of collision cross sections of the parent ions in low pressure discharges in argon, oxygen, and nitrogen, i.e., Ar+ in Ar, O2+ in O2 and N2+ in N2. These ion data are first discussed and then validated from comparisons between the calculated transport coefficients and those measured in the literature. The elastic momentum transfer collision cross sections are determined from a semi-classical approximation for the phase shift calculation based on a 12-6-4 inter-particle potential while ion transport coefficients are determined versus the reduced electric field from Monte Carlo simulations.

Determination of the electron collision cross-section set for the C4F6 molecule by using an electron swarm study

A set of electron collision cross-sections for the C4F6 molecule has been successfully determined for the first time by using an electron swarm method with a two-term approximation of the Boltzmann analysis. The set is evaluated by fitting the calculated electron drift velocities in pure C4F6 and in C4F6-Ar mixtures to the experimental data in the E/N range of 0.35 — 1200 Td (1 Td = 10−17 Vcm2), where E/N is the ratio of the electric field (E) to the neutral number density (N). The set is confirmed to be reliable due to our ability to reproduce satisfactorily the electron drift velocities in pure C4F6 and in 0.1, 0.5, 1, 5, and 10% C4F6-Ar mixtures with deviations of ±5%. The set may be the best available so far for use in the quantitative numerical modeling of a4 plasma discharge for various processing procedures with materials containing C4F6 molecules.

Electron attachment to oxygen in nitrogen buffer gas at atmospheric pressure

We have carried out experimental and theoretical studies of three body electron attachment (TBEA) to O2 in N2/O2 mixtures. We have applied three different experimental methods to determine the apparent rate constant k for TBEA to O2 for reduced electric fields E/ n from 0.5 Td up to 4.5 Td and O2 concentrations from 0.02% up to 3%. From the apparent rate constant k we have evaluated three body rate constant for electron attachment to O2 in pure O2 $$\left( {k_{O_2 } } \right)$$ and in pure N2 $$\left( {k_{N_2 } } \right)$$ . The comparison of present data with former studies shows that the former values of $$k_{N_2 }$$ overestimated the efficiency of this reaction, while in case of $$k_{O_2 }$$ we have found agreement with earlier studies. We have solved numerically the Boltzmann equation of the electrons and calculated the values of k, $$k_{N_2 }$$ and $$k_{O_2 }$$ using well established cross sections. Using the known collision cross section set for TBEA to O2, very good agreement between calculated and measured results for $$k_{O_2 }$$ was found, while in the case of k and $$k_{N_2 }$$ we had to introduce a scaling function, which describes the decrease of the efficiency of TBEA to O2 in presence of N2 and the dependence of the scaling function on E/n was determined.

Electron Collision Cross Sections for the Cl2 Molecule from Electron Transport Coefficients

The measured electron transport coefficients (electron drift velocity, Townsend first ionization coefficient, electron attachment coefficient, and density-normalized effective ionization coefficient) in pure Cl 2 have been analyzed to derive the currently best available electron collision cross section set of the elastic and inelastic electron collision cross sections for the Cl 2 molecule using an electron swarm study and a two-term approximation of the Boltzmann equation for energy. The electron transport coefficients calculated using the derived cross sections are consistent with the experimental data over a wide range of E / N values (ratio of the electric field E to the neutral number density N ). The present electron collision cross section set for the Cl 2 molecule is the best available so far for quantitative numerical modeling plasma discharges for processing procedures with materials containing Cl 2 molecules.

Positrons in gas filled traps and their transport in molecular gases

In this paper we give a review of two recent developments in positron transport, calculation of transport coefficients for a relatively complete set of collision cross sections for water vapour and for application of they Monte Carlo technique to model gas filled subexcitation positron traps such as Penning Malmberg Surko (Surko) trap. Calculated transport coefficients, very much like those for argon and other molecular gases show several new kinetic phenomena. The most important is the negative differential conductivity (NDC) for the bulk drift velocity when the flux drift velocity shows no sign of NDC. These results in water vapour are similar to the results in argon or hydrogen. The same technique that has been used for positron (and previously electron) transport may be applied to model development of particles in a Surko trap. We have provided calculation of the ensemble of positrons in the trap from an initial beam like distribution to the fully thermalised distribution. This model, however, does not include plasma effects (interaction between charged particles) and may be applied for lower positron densities.

Electron transport coefficients in the mixtures of H2O with N2, O2, CO2 and dry air for the optimization of non-thermal atmospheric pressure plasmas

This paper presents the simultaneous measurement and calculation of the electron drift velocity in binary and ternary mixtures of N2, O2, CO2 with H2O. The main aim of this study has been the generation of a self-consistent set of validated collision cross sections that explain thoroughly the dependence of the electron drift velocity in the above pure gases and their mixtures. In doing this, changes to the collision cross section set for H2O had to be made, while all other cross section sets remained unchanged. It is worth mentioning that only a few experiments had been performed before dealing with water mixtures. The electron drift velocities in the binary and ternary mixtures under study show the effects of negative differential conductivity, and this has been explained thoroughly in terms of the collision cross sections and electron distribution functions through a multi-term Boltzmann code. It is important to note that two-term codes fail to predict the dependence of the drift velocity at low water concentrations and low E/N values. Calculated values of longitudinal and transverse diffusion coefficients, mean energies and distribution functions are also given over the E/N range 0.1 Td–2 kTd (1 Td = 10−17 V cm2).

Transport properties of in SF6–Ne, SF6–N2 and SF6–O2 mixtures

The mobility of has been calculated for the gas mixtures SF6–Ne, SF6–N2 and SF6–O2 using an optimized Monte Carlo code for the ion transport simulation in a drift tube. The elastic momentum transfer collision cross sections needed for the calculation were determined from a semi-classical JWKB approximation, while the inelastic ones (detachment, dissociative charge transfer and conversion to and F−) were taken from the literature for the case of the collision system. The resulting sets of collision cross sections were validated by comparing the calculated mobilities with those measured in the above mixtures with a time-resolved pulsed Townsend technique. The longitudinal and transverse density-normalized diffusion coefficients were calculated for these mixtures for the case where the share of SF6 in the mixture was 50%. Finally, the validity of Blanc's law was discussed at low and high electric fields, whereby we show that it fails at high fields, where inelastic processes are dominant.

Swarm data of O−, NO−, He+, N2O+, and ions in N2O, He and N2O–He mixtures for PECVD applications

The aim is first to calculate and validate, from an ion swarm unfolding technique, the sets of collision cross sections of several ions (O−, NO−, He+, N2O+, , and ) colliding with N2O and He gases and their mixtures. The aim is then to calculate the corresponding ion swarm parameters in these pure gases and their mixtures in a large E/N range. These ion interactions and swarm data are very useful more particularly for the modelling of the electrical and energetic behaviour of RF discharge in N2O–He mixtures with a small admixture of SiH4 used in PECVD applications for thin film depositions. The sets of collision cross sections for these different ion/gas systems involve both elastic and inelastic processes in the case of an ion energy range varying from thermal energy up to about 100 eV. The elastic ion–gas interaction has been described by using a rigid core potential model which is well adapted for both polar and non-polar systems and also symmetric and asymmetric systems. Momentum transfer cross sections are then determined using a semi-classical JWKB approach while inelastic cross sections are taken from the literature and completed from empirical laws. Then by using these different cross section sets, it is possible to obtain the ion swarm data for different gas mixtures involving N2O and He whatever their relative proportions. These ion swarm data are obtained from an optimized Monte Carlo method well adapted for the ion transport in pure gas as well as gas mixtures. The specific behaviour of these swarm data (ion mobility or drift velocity, diffusion coefficient and reaction rates) is analysed in the case of each ion in different N2O–He mixtures over a quite large E/N range (1–1000 Td) showing more particularly the non-validity of the classical linear approximations such as Blanc law.

Development of the electron impact cross section set for CHF3. Monte Carlo simulation of the swarm experiments in pure CHF3 and CHF3/Ar mixtures

Trifluoromethane (CHF3) is widely used in semiconductor processing. However, electron collision cross section set in CHF3 is not finally completed and verified yet. Cross section sets developed before were tested only in pure CHF3 and calculations with these sets do not describe swarm parameters in CHF3 diluted by Ar. In this work a numerical code based on the Monte Carlo method has been developed and used to calculate swarm parameters in pure CHF3 and Ar/CHF3 mixtures. Calculations in CHF3 diluted in Ar allowed us to validate the absolute value of vibration cross sections. The values and the shapes of momentum transfer, attachment and dissociation cross sections in CHF3 were renormalized on the base of the thorough analysis of all available experimental data and quantum-mechanical calculations. As a result physically well-grounded cross section set for CHF3 was developed and verified on various swarm experiments in CHF3 and CHF3/Ar mixtures.

Measured and calculatedSF6−collision and swarm ion transport data inSF6-Ar andSF6-Xe mixtures

The measurement of the mobility of SF-6 in the mixtures SF6 -Ar and SF6 -Xe is reported over the density-reduced electric field strength E/N 1-180 Td (1 Townsend = 10(-17) V cm(2)), from a time-resolved pulsed Townsend technique. Simultaneously, the mobility of SF-6 in the same binary mixtures has been calculated from a set of collision cross sections for SF-6 -Ar, SF-6 -Xe, and SF-6 - SF6 using a Monte Carlo simulation procedure for ion transport. The good agreement between measured and calculated mobilities in these gas mixtures has led us to conclude that the validation of our cross section sets is confirmed. The elastic collision cross section, a predominant process for ion energies lower than about 10 eV, was determined from a semiclassical JWKB approximation using a rigid core potential model for the ion-neutral systems under consideration. This elastic cross section was then added to several other inelastic collision cross sections found in the literature for ion conversion, electron detachment of SF-6 and charge transfer. Moreover, the calculations of the mobility and the ratios of the transverse and longitudinal diffusion coefficients to the mobility were extended into a much wider E/N range from 1 to 4000 Td. Additionally, we have also calculated the energy distribution functions and the reaction coefficients for ion conversion and electron detachment. Finally, we have shown that the range of validity for the calculation of the mobility in gas mixtures from Blanc's law is only valid for the low E/N region, where the interaction is dominated by elastic collisions and the ion distribution function remains essentially Maxwellian.

Swarm derived electron collision cross section set for the perfluorocyclobutane molecule

The cross section set of perfluorocyclobutane (c-C4F8) is determined by the electron swarm method. The modification is applied to the vibrational excitation cross section and the elastic momentum transfer cross section. Our previously measured electron swarm parameters (drift velocities, longitudinal diffusion coefficients and effective ionization coefficients) in 0.468% and 4.910% c-C4F8/Ar mixtures are used for the determination of the vibrational excitation cross section. The measured drift velocity and longitudinal diffusion coefficient in pure c-C4F8 are used to determine the correct elastic momentum transfer cross section. The calculated electron transport coefficients were derived using the Monte Carlo simulation method.