Abstract
Non-equilibrium vibrational distributions and electron energy distributions of CO in nanosecond repetitively pulsed (NRP) discharges and afterglows have been determined from a coupled solution of the time dependent Boltzmann equation for the electron energy distribution function (eedf) of free electrons and the master equations for the vibrational distribution function (vdf) of CO and the electronic excited states of CO and O and C atoms. Emphasis is given to the role of dissociative electron attachment (DEA) from vibrationally excited states in affecting the eedf and vdf under extreme conditions, i.e., an optically thick plasma with quenching processes involving the electronic excited states, populated by a sequence of discharge pulses and corresponding afterglows. In particular, the quenching process of the a3Π electronic state of CO determines a pumping of vibrational quanta in the ground state, which in turn largely modifies the CO vdf promoting the activation of DEA process. DEA rate coefficients have been obtained by using a complete set of vibrational (v) dependent cross sections through the CO−(X2Π) channel and by using the experimental v = 0 cross section of Rapp and Briglia, which should include the contribution of other CO− resonant states. The importance of the last contribution has been also estimated by using a scaling law to extend the v = 0 cross section over all the vibrational ladder of CO. In particular, this mechanism becomes competitive with the other reactive channels for very short inter-pulse delay times, i.e., the tid = 1 μs, being less important for longer inter-pulse delay times, i.e., the tid = 25 μs.
Highlights
Non-equilibrium plasma kinetics is a topic of large interest for many applications in different fields such as plasma chemistry, plasma and laser physics, hypersonic and shock wave flows (Capitelli et al, 2016)
We report results for a nanosecond repetitively pulsed (NRP) discharge sustained by a sequence of modulated electric field pulses with a pulse duration tp = 20 ns and an inter-pulse delay time tid = 1 μs, while the results with tid = 25 μs are presented in section Long Inter-Pulse Delay Time
The introduction of dissociative electron attachment (DEA) from vibrationally excited states of CO plays an important role in NRP CO discharges with an interpulse delay times tid = 1 μs having a minor role with tid = 25 μs
Summary
Non-equilibrium plasma kinetics is a topic of large interest for many applications in different fields such as plasma chemistry, plasma and laser physics, hypersonic and shock wave flows (Capitelli et al, 2016). Particular attention is paid to the development of kinetic models which couple the Boltzmann equation for the electron energy distribution function (eedf) with the state-to-state vibrational kinetics for the calculation of the vibrational distribution function (vdf) of molecules and the collisional-radiative models for the electronic excited state densities This approach become essential when the chemistry at the basis of the relevant application is dependent on the high lying vibrational levels of the considered molecules (Capitelli et al, 2016). An example in this direction is represented by the formation of negative H− ions in magnetic multi-cusp H2 plasmas (Bretagne et al, 1985; Hassouni et al, 1998; Capitelli et al, 2006) and more in general in RF discharges to be used as negative ion beam source for neutral heating in tokamak devices. A second example is represented by expanding hypersonic and shock wave flows where the correct description of the dissociation process depends on the whole vibrational distributions of considered molecules (Capitelli et al, 2016)
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