2D imaging of absolute methyl concentrations in nanosecond pulsed plasma by photo-fragmentation laser-induced fluorescence

Abstract

The methyl radical plays a central role in plasma-assisted hydrocarbon chemistry but is challenging to detect due to its high reactivity and strongly pre-dissociative electronically excited states. We report the development of a photo-fragmentation laser-induced fluorescence (PF-LIF) diagnostic for quantitative 2D imaging of methyl profiles in a plasma. This technique provides temporally and spatially resolved measurements of local methyl distributions, including in near-surface regions that are important for plasma-surface interactions such as plasma-assisted catalysis. The technique relies on photo-dissociation of methyl by the fifth harmonic of a Nd:YAG laser at 212.8 nm to produce CH fragments. These photofragments are then detected with LIF imaging by exciting a transition in the B-X(0,0) band of CH with a second laser at 390 nm. Fluorescence from the overlapping A-X(0,0), A-X(1,1), and B-X(0,1) bands of CH is detected near 430 nm with the A-state populated by collisional B-A electronic energy transfer. This non-resonant detection scheme enables interrogation close to a surface. The PF-LIF diagnostic is calibrated by producing a known amount of methyl through photo-dissociation of acetone vapor in a calibration gas mixture. We demonstrate PF-LIF imaging of methyl production in methane-containing nanosecond pulsed plasmas impinging on grounded metallic or dielectric surfaces. Absolute calibration of the diagnostic is demonstrated in a diffuse, plane-to-plane discharge. Measured profiles show a relatively uniform distribution of up to 30 ppm of methyl. Relative methyl measurements in a filamentary plane-to-plane discharge and a plasma jet reveal highly localized intense production of methyl. The utility of the PF-LIF technique is further demonstrated by combining methyl measurements with formaldehyde LIF imaging to capture spatiotemporal correlations between methyl and formaldehyde, which is an important intermediate species in plasma-assisted oxidative coupling of methane.

Generation and decay of N2(A3Σu +) molecules in reacting CO2 and CH4 plasmas

“…The synthetic spectrum is generated using PGOPHER software [32], with the pressure broadening coefficient measured in reference [8]. The rotational-translational temperature is inferred from the intensity ratio of R 11 (6) and P 33 (14) lines, labeled in figure 15, with the estimated uncertainty of ±20 K. The uncertainty is controlled by the N 2 (A, v = 0) population in the plasma and by the acoustic waves generated in the discharge section, which result in the laser beam steering. The temperature rise during the 3 kHz, 50-pulse discharge burst at the conditions of figures 7-10, due to the quenching and relaxation of the excited states generated in the plasma, is several tens of degrees, as summarized in table 1.…”

“…This limits the confidence in the predictions of CO and CH 3 radical yield in the plasma. Measurements of time-resolved, absolute CO [5] and CH 3 [6] number densities at well characterized plasma conditions may improve considerably the understanding of the underlying kinetics. Complementary measurements of the absolute number density of the metastable excited electronic state of nitrogen, N 2 (A 3 Σ u + ), in these plasmas can help characterize the yield of the molecular dissociation processes.…”

Power concentration determined by thermodynamic properties in complex gas mixtures: the case of plasma-based dry reforming of methane

“…The detection of CH 4 dissociation products, such as CH 3 and CH radicals, requires more sophisticated laser techniques (e.g. laser-induced fluorescence (LIF) [64], coherent anti-Stokes Raman scattering [65][66][67] and photo-fragmentation LIF [68]), which are currently not suitable for our setup. Therefore, the quantification of the species formed in the plasma upon the addition of CH 4 and the full characterization of the gas mixture are delegated to future studies.…”

Quantifying the thermal effect and methyl radical production in nanosecond repetitively pulsed glow discharges applied to a methane-air flame

“…Qualitative 2D imaging of CH 3 has been demonstrated on laminar and turbulent methaneair flames [33][34][35]. Van den Bekerom et al have recently published quantitative CH 3 PF-LIF measurements in a plasma at low pressure [36]. The absolute calibration was done by producing a known CH 3 profile by photo-dissociation of acetone vapor.…”