M. Cacciatore

Non-equilibrium dissociation and ionization of nitrogen in electrical discharges: The role of electronic collisions from vibrationally excited molecules☆

Abstract Dissociation and ionization rates by electron impact from vibrationally excited nitrogen molecules have been calculated by using a set of cross sections obtained with the Gryzinski method and a self-consistent electron energy distribution function. The results show that in both processes, the presence of vibrationally excited molecules enhances the direct electronic mechanisms (DEM). In the case of dissociation, DEM is competitive with the pure vibrational mechanism (PVM), while in the ionization process DEM gives ionization rates several orders of magnitude smaller than PVM. A satisfactory agreement is finally found between theoretical and experimental dissociation and ionization rates.

Electron energy distribution functions under N2 discharge and post-discharge conditions: A self-consistent approach

Abstract 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.

From dynamics to modeling of plasma complex systems: negative ion (H-) sources

Abstract We present an approach for calculating the H 2 vibrational distribution, electron energy distribution function and negative ion concentration (H - ) based on the self-consistent solution of the vibrational master equation and of the Boltzmann equation. Most of the input data of the present model have been obtained by our group in these last years by using classical, semiclassical and quantum approaches. In particular we discuss the most important process involving vibrationally excited H 2 molecules colliding with themselves as well as with atomic hydrogen, electrons and metallic surfaces. Part of these data have been obtained by using dynamic calculations. The main result of our approach, which mixes different techniques, is the ability of reproducing the main experimental features of negative ion (H - ) sources.

Vibrational Kinetics, Dissociation, and Ionization of Diatomic Molecules Under Nonequilibrium Conditions

This chapter deals with the study of nonequilibrium vibrational distributions created by the so-called vibration-vibration (V-V) up-pumping mechanism [2.1] (or Treanor’s mechanism) and with the effects of such a mechanism on chemical reactions of diatomic molecules. Treanor’s mechanism is responsible for the overpopulation of the vibrational distribution of a diatomic molecule under physical conditions characterized by large vibrational temperatures [θ1 = E10/k ln(N0/N1)] and small transiational ones (Tg). (E10 is the energy difference between level 1 and level 0, N1 and N2 being the corresponding population densities; k is Boltzmann’s constant.) Indeed, in this case V-V energy exchanges are more effective than vibration-translation (V-T) ones thus creating a quasi-stationary vibrational distribution made up of a Treanor’s distribution for the low-lying vibrational levels, a plateau, and a tail. The extension of this plateau depends on the effectiveness of V-V rates in overcoming the V-T ones [2.2–5].

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.

Non-equilibrium vibrational kinetics of CO pumped by vibrationally excited nitrogen molecules: General theoretical considerations

Abstract The non-equilibrium vibrational kinetics of CO pumped by vibrationally excited N2 has been calculated by solving the vibrational master equations for both N2 and CO molecules, linked by V-V (N2 CO) energy exchange processes. The results have been obtained for different values of the parameters governing the kinetics (in particular gas temperature, initial vibrational content of N2, different mixing ratios N2/CO). Emphasis is also given to dissipative channels present in CO, due to bimolecular reactions involving highly vibrationally excited CO molecules. The results shows the essential features of the temporal evolution of CO and N2 vibrational distribution as well as the strong coupling existing between them.

A database for V–V state-to-state rate constants in N2–N2 and N2–CO collisions in a wide temperature range: dynamical calculations and analytical approximations

Extensive semiclassical calculations have been performed regarding the generation of accurate collision data for single- and multi-quantum vibrational state-selected V–V exchanges in N2(v)–N2(u) and N2(v)–CO(u) collisions over a wide range of vibrational quantum numbers (v, u) and gas temperatures. This highly accurate database is also used to test the applicability of simplified analytical approximations frequently used in vibrational kinetic modeling to calculate large sets of state-to-state V–V rate constants in diatom–diatom collisions. The semiclassical calculations clearly show that the long-range attractive branch of the interaction potential cannot be neglected, even for the N2–N2 system. Therefore, new analytical approximations for V–V exchanges for the two collision systems are proposed which agree very well with the semiclassical rates. The ab initio data, together with that obtained from the newly proposed analytical formula, constitute an accurate database that can be used with confidence in the vibrational kinetic modeling of N2 and CO-based gases and plasmas over a wide temperature range, from thermal to hyper-thermal gas temperatures.

Vibrational kinetics in HeCO reacting discharges

Abstract Combined measurements of vibrational distributions ( N υ ) of CO and CO 2 yields (β) in HeCO discharges have been performed at different residence times in radiofrequency discharges. The experimental results on N υ have been obtained by IR emission spectroscopy and on β by gas-chromatographic and mass-spectrometric techniques. A theoretical model including the most important relaxation channels of the vibrational energy has been set up and coupled to the plasma chemistry describing the rate of formation of species such as CO 2 , C, and O. Theoretical and experimental results are in good agreement, emphasizing the role of a vibrational mechanism in dissociating CO in HeCO mixtures.

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.