R. Celiberto

The effect of N+N2 collisions on the non-equilibrium vibrational distributions of nitrogen under reentry conditions

Abstract Global (reactive+non-reactive) quasiclassical state-specific rate coefficients for the vibrational excitation and deexcitation of nitrogen molecules by collision with nitrogen atoms have been used to model a reacting flow. The one-dimensional model used describes the relaxation of N 2 vibrational distributions in the boundary layer surrounding a shuttle reentering the atmosphere. The results single out the strong non-equilibrium character of the vibrational distributions along the coordinate normal to the shuttle surface caused by the dissociation-recombination processes. The results also show the dependence of the vibrational distributions upon inclusion in the model of state-specific N+N 2 ( v ) rates.

Transport properties of high temperature air components: A review

The methods for the calculation of the transport properties of high temperature gases are reviewed from the points of view of both kinetic theory and transport cross sections. Particular emphasis is given to the poor convergence of the Chapman-Enskog method for calculating the thermal conductivity of free electrons and of the possible sources of errors when applying well known simplified formulae for calculating the translational thermal conductivity of heavy components and the viscosity of partially ionized gases. The transport cross sections (collision integrals) of high temperature air components are then discussed by comparing old and new calculations particularly emphasizing atom-atom, atom-molecule and atom-ion interactions. Special consideration is dedicated to the knowledge of transport cross sections of electronically excited states. The role of inelastic processes in affecting the transport cross sections is also briefly discussed. Finally the possibility to extend the results to non equilibrium situations is analysed.

Electron impact direct dissociative-ionization cross sections from vibrationally excited H2 molecules and translational energy distribution functions of protons

Abstract Dissociative-ionization cross sections for the processes e+H 2 (ν)→2e+H 2 + ( 2 Σ g + , 2 Σ u + )→2e+H + +H have been calculated by using the Gryzinski theory in combination with the Franck-Condon density for different vibrational levels of H 2 . The results show that the cross sections involving the completely repulsive state 2 Σ u + of H 2 + monotonically increase with increasing vibrational quantum number ν, while those involving the repulsive part of the bound 2 Σ g + state of H 2 + increase up to ν=4, presenting an opposite behaviour from ν=5 on. These results have been utilized to calculate the translational energy distribution function (TEDF) of protons. The presence of vibrationally excited H 2 molecules strongly affects TEDF by filling the gap between “cold” and “hot” protons.

Electron impact direct dissociation processes of vibrationally excited H2 molecules to excited atomic hydrogen H*(n=1–5).I. Cross sections

Abstract Direct dissociation cross sections for the process e + H2(ν)→e + H2*→ e + H + H*(n= 1–5) have been calculated by using the Gryzinski approximation in combination with the Franck-Condon density. A satisfactory agreement is found between the present ν=0 cross sections and corresponding theoretical and experimental results for the processes leading to unexcited H*(n= 1) and excited H*(n= 2–5) fragments. The role of vibrational excitation in affecting cross sections is such to decrease the threshold energy and to increase the maximum value of the cross section. This behaviour holds for all processes, with the exception of cross sections leading to H*(n= 2). In this last case the maximum value of the cross sections increases with increasing the vibrational quantum number ν for ν⩽5, having an opposite behaviour for ν>5.

Electron energy distribution function in He‐CO radio‐frequency plasmas: The role of vibrational and electronic superelastic collisions

The role of vibrational and electronic superelastic collisions in collision‐dominated He‐CO rf bulk plasmas has been studied by solving the time‐dependent Boltzmann equation in the presence of different concentrations of vibrational [CO(v=1)] and electronic [CO(a3Π),He(3S)] states. The results, which have been obtained for a field frequency ω of π 107 s−1 Torr−1, show that the presence of excited states is such to markedly enlarge at medium electron energies and to strongly enhance at high energies the electron energy distribution function with large consequences on the related macroscopic quantities of the electrons, particularly on the mean collision frequencies of high threshold inelastic collision processes.

Vibrational excitation of H2(X1Σ+g, v)/D2(X1Σ+g, v) through excitation of electronically excited singlet states and radiative cascade

Abstract A complete set of cross sections for the process e+H 2 (X 1 Σ + g , ν 1 )/D 2 (X 1 Σ + g , ν i )→e+H 2 (B 1 Σ + u , C 1 (Π u )/ D 2 (B 1 Σ + u , C 1 Π u )→e+H 2 (X 1 Σ + g , ν f )/D 2 (X 1 Σ + g , ν f )+ h ν has been obtained by using recent calculated excitation cross sections in combination with Einstein probabilities linking vibrational excited states of electronically excited states and the corresponding ones of ground state. An isotopic effect is found into the cross sections: in particular the H 2 cross sections are up to a factor 1.5 larger than the corresponding D 2 cross sections for ν i =O and ν f

Direct electron-impact collision cross sections involving vibrationally excited D2(v) molecules relevant to D− sources

Abstract Complete sets of direct electron-impact cross sections of vibrationally excited D2(v) molecules have been calculated by using a semiclassical approach. Calculated cross sections for dissociation, dissociative ionization, ionization and vibrational excitation according to the so-called E-V process show a decrease of threshold energy with increasing vibrational quantum number v of the target and a corresponding increase of the maximum cross sections. This last result is true for transitions to completely repulsive states (i.e. dissociation and dissociative ionization) while an intricate trend of maximum cross sections as a function of v is observed in the other cases.

Scaling of electron-impact electronic excitation cross sections of vibrationally excited diatomic molecules

Abstract A scaling law for the electron-impact electronic excitation cross sections of vibrationally excited diatomic molecules is derived and applied to the X 1 Σ g + →B 1 Σ u + transitions in hydrogen and deutrium.

Coupled solution of Boltzmann equation and non equilibrium vibrational kinetics in H2 volume discharges: An analysis of the input data

New sets of direct electron impact excitation cross sections involving vibrational excited states of H2 (including dissociation, ionization and E‐V processes) have been calculated by Gryzinski fomulation. These rates are then inserted in a numerical code based on the simultaneous solution of Boltzmann equation, of vibrational kinetics and H, H− production rates. Comparison of theoretical and experimental results shows satisfactory agreement for the electron temperature and electron density, while some discrepancy does exist on the negative ion concentration.

Non-Equilibrium Plasma Modeling

We present different examples of non-equilibrium plasma modeling interesting technological applications. In particular we discuss some aspects of plasma modeling in excimer lasers, negative ion sources, nitrogen afterglow and RF discharges emphasizing the theoretical common points linking different applications.