Superelastic Vibrational Collisions Research Articles

Self-consistent time dependent vibrational and free electron kinetics for CO2 dissociation and ionization in cold plasmas

A self-consistent time dependent model, based on the coupling between the Boltzmann equation for free electrons, the non equilibrium vibrational kinetics for the asymmetric mode of CO2 and simplified global models for the dissociation and ionization plasma chemistry, has been applied to conditions which can be met under pulsed microwave (MW), dielectric barrier discharge (DBD) and nanosecond pulsed discharges (NPD). Under MW discharge type conditions, the selected pulse duration generates large concentration of vibrational excited states, which affects the electron energy distribution function (eedf) through the superelastic vibrational collisions. Moreover, in discharge conditions, plateaux appear in the vibrational distribution function (vdf) through the vibrational–vibrational up pumping mechanism, persisting also in the post discharge. In post discharge conditions, also the eedf is characterized by plateaux due to the superelastic collisions between cold electrons and the CO2 electronic state at 10.5 eV. The plateau in vdf increases the dissociation of pure vibrational mechanism (PVM), which can become competitive with the dissociation mechanism induced by electron molecule collisions. The PVM rates increase with the decrease of gas temperature, generating a non-Arrhenius behaviour. The situation completely changes under DBD and NPD type conditions characterized by shorter pulse duration and higher applied E/N values. Under discharge conditions, both vdf and eedf plateaux disappear, reappering in the afterglow.

Influence of Electron Molecule Resonant Vibrational Collisions over the Symmetric Mode and Direct Excitation-Dissociation Cross Sections of CO2 on the Electron Energy Distribution Function and Dissociation Mechanisms in Cold Pure CO2 Plasmas.

A new set of electron-vibrational (e-V) processes linking the first 10 vibrational levels of the symmetric mode of CO2 is derived by using a decoupled vibrational model and inserted in the Boltzmann equation for the electron energy distribution function (eedf). The new eedf and dissociation rates are in satisfactory agreement with the corresponding ones obtained by using the e-V cross sections reported in the database of Hake and Phelps (H-P). Large differences are, on the contrary, found when the experimental dissociation cross sections of Cosby and Helm are inserted in the Boltzman equation. Comparison of the corresponding rates with those obtained by using the low-energy threshold energy, reported in the H-P database, shows differences up to orders of magnitude, which decrease with the increasing of the reduced electric field. In all cases, we show the importance of superelastic vibrational collisions in affecting eedf and dissociation rates either in the direct electron impact mechanism or in the pure vibrational mechanism.

Electron energy distribution functions and fractional power transfer in “cold” and excited CO2 discharge and post discharge conditions

A Boltzmann equation, in the presence of superelastic vibrational and electronic collisions and of electron-electron Coulomb collisions, has been solved in CO2 plasma in discharge and post discharge conditions. Superelastic vibrational collisions play an important role in affecting the electron energy distribution function (eedf) in a wide range of the reduced electric field E/N and of vibrational temperatures characterizing the vibrational modes of CO2. An important result is the dependence of fractional power losses and of the relevant rate coefficients on the vibrational temperatures of the system. Superelastic electronic collisions, on the other hand, are the main processes affecting eedf and related quantities in the post discharge conditions (i.e., E/N = 0). In particular at low vibrational temperatures, the superelastic electronic collisions form an important plateau in the eedf, largely influencing the rate coefficients and the fractional power transfer.

Breakdown conditions and thermal instability in air

Reduced electric fields (E/N) responsible for electrical breakdown in air have been calculated by solving a stationary Boltzmann equation including superelastic vibrational collisions. The results show a decrease ofE/N with increasing gas temperature. The possibility of air instability due to chemical processes producing electrons is then investigated by calculating the threshold of this instability as a function of a characteristic time for heat dissipation τ.

A Monte Carlo simulation of electron swarm parameters in He-CO mixtures

A computer simulation technique for the study of electron swarm parameters in gases is presented. The method enables calculation of the electron energy distribution function (EDF) as well as various average electronic properties in electrical discharges. The simulation is applied to He-CO mixtures and the results are in fair agreement with experimental values and with other computational works. A strong directional anisotropy in the EDF is observed at high relative concentrations of CO, especially at electron energies for which the inelastic collision cross section is relatively big and at high values of the reduced electric field (E/N). The calculations include the influence of elastic, inelastic and both superelastic vibrational collisions (SVC) and superelastic electronic collisions (SEC). The results show that the SEC create different plateaus in the EDF, affecting only the tail of the EDF. The role of SEC becomes more important as E/N is lowered, the effect being more evident in the absence of SVC.

Electron energy distribution functions under N 2 discharge and post-discharge conditions: A self-consistent approach

Electron energy distribution functions (EDF) under N 2 discharge and post-discharge conditions have been calculated by self-consistently solving the Boltzmann equation, the vibrational master equation (including dissociation) and the kinetics of the most important electronic states of N 2. The results show that the relaxation of EDF in the post-discharge is strongly linked to the residence time of N 2 in the discharge, which determines the initial conditions. In particular at low residence times the role of metastable states in affecting EDF through superelastic electronic collisions (SEC) overcomes the corresponding one from superelastic vibrational collisions (SVC). On the contrary, SVC dominate at long residence times, when a well-developed vibrational distribution has been built up by the discharge. At intermediate residence times both SVC and SEC affect the relaxation of EDF, which in general presents a non-monotonic behaviour.

Electron energy distribution functions in He–CO vibrationally excited post discharges

Electron energy distribution functions (edf) and related parameters have been calculated in the post-discharge regime of vibrationally excited He–CO mixtures by solving the Boltzmann equation including superelastic vibrational collisions. The calculations point out to the importance of small concentration of CO molecules in affecting both edf ’s and related quantities in the post-discharge reigme. In particular, the mixture closely follows the behavior of pure CO for molar fractions of CO (XCO) approximately equal to 20%. Helium affects edf ’s by increasing elastic losses in the post discharge, for XCO<20%, leading to average electron temperatures, Te, lower than the corresponding vibrational temperatures, ϑ1. The results obtained by using Treanor’s and Boltzmann distributions emphasize the role of excited vibrational levels (v>2) in affecting edf and the average electron energy.

Coupled solutions of the collosional Boltzmann equations for electrons and the heavy particle master equation in nitrogen

Self-consistent electron energy distribution functions (edf's) in non-equilibrium nitrogen have been calculated by solving the Boltzmann equation coupled to a system of vibrational master equations including the dissociation process. The results show a temporal evolution of the electron energy distribution due to the corresponding evolution of the vibrational distribution and of the concentration of atoms arising from the dissociation process. The superelastic vibrational collisions are the main cause of this evolution. The calculated edf's are then used to obtain quantities such as drift velocity, average and characteristic energy and rate coefficients for ionization, vibrational and electronic excitation processes. All these quantities exhibit a similar temporal evolution following the temporal evolution of the edf. In particular the rate coefficients for ionization and electronic excitation, which depend on the high energy portion, present variations of several orders of magnitude.

Influence of superelastic vibrational collisions on the relaxation of the electron energy distribution function in N2 post discharge regimes

Influence of superelastic vibrational collisions on the relaxation of the electron energy distribution function in N2 post discharge regimes

Selfconsistent Electron Energy Distribution Functions in Non Equilibrium oxygen: Effects on the Dissociation Rate

AbstractElectron energy distribution functions (edf) in non equilibrium oxygen have been calculated by solving the Boltzmann equation coupled to a system of vibrational master equations. The results show the importance of both superelastic vibrational collisions and of the presence of oxygen atoms in affecting edf.The coupling between the Boltzmann equation and the system of vibrational master equations brings to a temporal evolution of edf, which progressively changes from a cold molecular gas situation (all molecules in the ground vibrational level) to a vibrationally excited molecular gas and finally to a gas composed by oxygen molecules and oxygen atoms.All electronic rate coefficients follow a temporal evolution. due to the corresponding evolution of edf.Finally the present results are used for discussing the dissociation rate of molecular oxygen in electrical discharges.

Electron Energy Distribution Functions of Hydrogen: The Effect of Superelastic Vibrational Collisions and of the Dissociation Process

Abstract Electron energy distribution functions (EDF) of molecular H2 have been calculated by numerically solving the Boltzmann equation including all the inelastic processes with the addition of superelastic vibrational collisions and of the hydrogen atoms coming from the dissociation process. The population densities of the vibrational levels have been obtained both by assuming a Boltz-mann population at a vibrational temperature different from the translational one and by solving a system of vibrational master equations coupled to the Boltzmann equation. The results, which have been compared with those corresponding to a vibrationally cold molecular gas, show that the inclusion of superelastic collisions and of the parent atoms affects the EDF tails without strongly modifying the EDF bulk. As a consequence the quantities affected by the EDF bulk, such as average and characteristic energies, drift velocity, 0-1 vibrational excitation rate are not too much affected by the inclusion of superelastic vibrational collisions and of parent atoms, while a strong influence is observed on the dissociation and ionization rate coefficients which depend on the EDF tail. Calculated dissociation rates, obtained by EDF's which take into account both the presence of vibrationally excited molecules and hydrogen atoms, are in satisfactory agreement with experimental results.