CN(B ²Σ⁺) formation and emission in a N₂–CH₄atmospheric pressure dielectric barrier discharge

Abstract

We have investigated CN(B 2 + → X 2 + ) violet system emission and laser induced fluorescence in an atmospheric pressure pulsed dielectric barrier discharge and found a high degree of vibrational non-equilibrium in both CN(B, v) and CN(X, v ). The vibrational distributions depend strongly on the gas feed composition and on the discharge/post-discharge regime. Analysis of the time resolved laser and emission spectroscopy measurements leads to the conclusion that two main chemi-luminescent mechanisms are active in the CN(B) excitation. One is the C + N + M → CN(A, B) + M recombination, that is dominant in the post-discharge; the other one, active in the discharge, is probably the reaction N + CH → CN(A, B) + H. Both reactions give intense emission when the discharge is operated in a pure N 2 gas feed in the presence of the surface deposit produced by a N 2 -CH 4 mixture discharge. C and CH must then be produced starting from some intermediate species coming from the surface. When CH 4 is added to the gas feed, the recombination reaction is strongly inhibited, while CH can be produced more easily from the methane contained in the gas phase. It is also likely that CN(X) is produced by the same reactions after radiative cascade from CN(A, B) states.

Advances in the optical diagnostics of kinetic processes in atmospheric pressure dielectric barrier discharges

“…Within this framework we approached fundamental studies in a N 2 -C 6 H 6 [21] and N 2 -CH 4 DBDs [22], and finding also the formation of a liquid deposit in nitrogen(argon)-methane discharges [23]. The CN(B-X) violet system is, as we have seen, the only prominent emission in atmospheric pressure discharge in nitrogen plus small amounts of hydrocarbons.…”

“…The production of CN(B) then showed peculiarities typical of chemiluminescent/recombination reactions involving the interplay between the gas-phase and the surface deposits, whose understanding requires a complex diagnostic approach. We addressed this study in a pulsed dielectric barrier discharge (a packet of sinusoidal applied voltage followed by a period of null voltage) by combining the results of the following, time resolved diagnostics [22]:…”

Optical diagnostics in dielectric barrier discharges at atmospheric pressure

“…We have encountered two such cases. The N 2 (C 3 Π u ,v) in a N 2 discharge, that shows large vibrational relaxation rates [12], and the CN(B 2 Σ + ,v) in a N 2 + CH 4 discharge, for which VET rates are low [14].…”

“…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].…”

LIF diagnostics in volume and surface dielectric barrier discharges at atmospheric pressure

“…Optical Emission Spectroscopy (OES), infact, is rarely applicable as a monitor of ground states species. In our recent studies on OES in dielectric barrier discharges, we have found a number of examples in which observable emissions are due to excitation of electronic states by energy transfers with metastable molecules (NO-γ System, nitrogen Herman Infrared System [1,2] dissociative excitation (CH 4300Å System [3]), recombination reactions (CN violet System [4]). In such a way that a relationship between emission features and the corresponding ground-state molecules cannot be established.…”