CH spectroscopic observables in He–CH₄and N₂–CH₄atmospheric pressure dielectric barrier discharges

Abstract

In this paper we report on a spectroscopic study on CH observables including laser induced fluorescence (LIF) on CH and emission spectroscopy on the CH Gerö band. Direct observation by LIF has been found to be very difficult, both because of technical difficulties and an overall low CH density in the discharge. Analysis of CH(A) emission shows that it is due to CH4 dissociative excitation processes, by He(23S) in He–based mixtures, and by electron impact in N2–CH4. The analysis of spectra evidences the need for more precise knowledge on the collision quenching of electronically excited states and on the electron energy distribution function.

GC‐MS Investigation of Organosilicon and Fluorocarbon Fed Plasmas

“…In low pressure plasmas glow discharges, the actinometric approach, known as actinometric OES (AOES) [1,2], under particular conditions overcomes this limit allowing the estimation of the relative concentration of the emitting species in the ground state. Unfortunately, atmospheric pressure DBDs are collision-dominated non-equilibrium systems [5,6] that do not fulfil some crucial prerequisites for AOES application, for instance: i) the direct electron impact from the ground state can not be considered the main excitation process, in DBDs, in fact, emitting species can be generated also from dissociative excitation (e.g. in He-CH 4 glow DBDs CH(A) forms from dissociative excitation of CH 4 by metastable He) [5,6]; ii) the quenching of the emitting species is not mainly radiative, since at atmospheric pressure the occurrence of collisional quenching of excited states can not be neglected.…”

Optical diagnostics in dielectric barrier discharges at atmospheric pressure

“…2). The final findings show a very low CH density strictly confined into the time interval in which discharge electrons are active [15].…”

“…A filamentary discharge is instead a sum of short microdischarges, each lasting a few tens of ns. Local steady state is then impossible, but if the light emission is measured with (integrated in) a temporal gate much larger than the emitting state lifetime, it is easy to demonstrate by simple calculations that the relative C v populations (i.e., the vibrational distribution) deduced from the integrated emissions are the same as those obtained by the steady-state model, that can be then applied to this purpose also to the filamentary discharge case [15]. Few calculations, reported in Fig.…”

“…We have recently investigated this emission in N 2 -CH 4 and He-CH 4 mixtures in order to assess the possibility of extracting information on CH radical from its emissions [15]. Analysis of the intensity ratios between SPS and CH(A), and between FNS and CH(A), has instead shown that CH(A) excitation is not achieved by electron impact on CH but by dissociative excitation of CH 4 .…”

“…We have explored this realm, touching many of the issues described above by a multidiagnostic approach with time-resolved optical diagnostics, applied to common DBD devices. In this paper, we will briefly review our recent results on • the use of N 2 (C) emission (SPS) for electron temperature estimation by a collision-radiative model, with a complete characterization of the collision processes in the N 2 (C,v) manifold [12]; • optical-optical double resonance (OODR) LIF for N 2 (A) metastable detection [13]; • a combination of time-resolved emission and laser spectroscopy for the investigation of CN violet system emissions in a N 2 -CH 4 discharge [14]; and • the detection of CH in a He-CH 4 discharge, an archetype of all the difficulties one has to face when trying to detect a radical in this kind of discharge [15].…”