Time resolved diagnostics and kinetic modelling of a modulated hollow cathode discharge of NO₂

Abstract

The transients associated with the ignition and the extinction of the cold plasma produced in a low frequency, square-wave modulated, hollow cathode discharge of nitrogen dioxide are characterized by time resolved emission spectroscopy, mass spectrometry and electrical probes. The temporal evolution of the concentrations of neutral species created or destroyed in the NO 2 discharges are compared with the predictions of a simple kinetic model previously developed for discharges of other nitrogen oxides (N 2 O and NO). The physical conditions of pressure, gas flow rate, modulation frequency and electrical current in the NO 2 plasma were selected in order to highlight the time-dependent behaviour of some of the stable species formed in the discharge, especially the nitrogen oxide products, whose concentrations show transient maxima. The usefulness of the analysis of the transient results is emphasized as a means to evaluate the relevance of the different elementary processes and as a key to estimate the values of some of the rate constants critical to the modelling. This work is dedicated to the memory of Professor José Campos.

Time-resolved diagnostics and kinetic modelling of the ignition transient of a H2 + 10% N2 square wave modulated hollow cathode discharge

“…In previous publications, the authors have demonstrated how the employment of modulation techniques facilitates the distinction among processes, relevant in different time scales, for N 2 O [23,24], NO 2 [25] and air plasmas [26]. Time-resolved spectroscopy for the observation of molecular [23,24] or atomic species [25,26] in combination with kinetic modeling of the modulated discharges allowed to clarify the most important reactions [23,24] or the estimation of rate coefficients [26] in those plasmas. In this work, a similar procedure is followed to get further details in H 2 + 10% N 2 plasmas by using low frequency modulation, together with a recently developed kinetic model [8] that accounted quite satisfactorily for stationary state results.…”

Production of molecules on a surface under plasma exposure: example of NO on pyrex

“…Analysis of the possible NO ↔ NO 2 conversion channels showed that in O 2 plasma NO can be oxidized into NO 2 via reactions R1 and R2, but this oxidation is balanced by an efficient inverse process R3 (see Table 2) [3,5]. Heterogeneous oxidation of NO by O atoms may also take place, but this reaction is much slower than R1-R2 as it was found in [21,22]. Furthermore, reaction R2 may be ignored because in low pressure oxygen discharge conditions concentration of O 3 is too low [23] .…”

Spectroscopic diagnostics of laboratory air plasmas as a benchmark for spectral rotational (gas) temperature determination in TLEs

“…Diffusion of excited species to the cathode walls, where they might collide and experiment de‐excitation, is described by Fick's Law [ Levine , ; McDaniel , ]. We have assumed a model of rigid spheres and employed the effective diameters of N 2 and O 2 from Hirschfelder et al [] to estimate the typical diffusion times along the cathode radius, as it was done in Castillo et al [], Castillo et al [], and de los Arcos et al []. Under the present air pressures ranging from 0.1 to 2 mbar, these diffusion times result to be within the interval 0.1–1 ms, considerably longer than the radiative lifetimes of the excited N 2 and levels in this work, whereas the average time between successive collisions at these pressures (equivalent to some 68–50 altitudes) are of some 10 −6 –10 −7 s, respectively [ McDaniel , ].…”