Use of particle-in-cell simulations to improve the actinometry technique for determination of absolute atomic oxygen density

Abstract

Actinometry is a non-invasive optical technique that can be used to quantitatively monitor atomic oxygen number densities [O] in gas discharges under certain operating conditions. However, careless application of the technique can lead to erroneous conclusions regarding the behaviour of atomic oxygen in plasma. One limitation on this technique is an accurate knowledge of the various rate constants required, which in turn is hampered by an insufficiently precise knowledge of the Electron Energy Distribution Function (EEDF) in the plasma. In this work Particle in Cell (PIC) simulations have been used to generate theoretical EEDFs. To validate a simulation the electron density n e produced by the PIC code is compared to experimental n e values measured using a hairpin probe. The PIC input parameters are adjusted to optimise agreement between the PIC and experimental n e results. This approach should in principle yield an EEDF that more accurately reflects the true EEDF in the plasma. The PIC EEDF is then used to generate rate constants for the actinometry model which should improve the accuracy of the quantitative [O] result for that particular set of plasma conditions. The actinometry [O] results are then compared to [O] results obtained using Two-photon Absorption Laser Induced Fluorescence (TALIF) to validate the approach.

Investigation of the radially resolved oxygen dissociation degree and local mean electron energy in oxygen plasmas in contact with different surface materials

“…These are consistent with the values found by previous studies which have used TALIF to measure atomic oxygen densities under similar conditions in similar reactors. 32,34,60 This consistency with TALIF measurements is in accordance with the results of our previous work which showed good agreement between atomic oxygen densities measured by ERA and TALIF in an atmospheric pressure plasma jet. 41 In Fig.…”

“…[12][13][14][15][16][17][18][19][20][21][22][23][24][25] This makes it particularly difficult to accurately control the densities of highly reactive species, such as atomic oxygen, in industrial processing applications. Previous investigations have sought insight into these processes by measuring the density of atomic oxygen and its surface recombination probability for various materials using twophoton absorption laser-induced fluorescence (TALIF), 12,[26][27][28][29][30] optical emission spectroscopy (OES), 21,22,[31][32][33][34] and VUV absorption spectroscopy. 23,35,36 TALIF and VUV absorption are well known for their accuracy, however, both are challenging to implement in industrial plasma reactors, particularly if spatially resolved measurements are required.…”

“…Various works have extended the basic actinometry approach for atomic oxygen (and atomic hydrogen 38 ) to incorporate the dissociative excitation process. [32][33][34] In order to do this, argon is typically used as a tracer gas with the emission from the Ar(2p 1 …”

“…Several works have demonstrated this coupling of experimental observations and simulations to be a reliable method of determining the atomic oxygen density in both high and low pressure plasma sources. [32][33][34] Here, we apply a further extension of the described techniques, known as Energy Resolved Actinometry (ERA) in order to measure atomic oxygen densities and local mean electron energies simultaneously and with high spatial resolution. This technique has been described in detail and benchmarked for the case of an atmospheric pressure plasma jet by Greb et al 41 Briefly, the ERA approach, which is inclusive of direct and dissociative excitation processes, modifies the commonly used actinometry technique in two significant ways (i) to additionally include atomic oxygen emission from the O(3p 5 P) excited state at k ¼ 777.4 nm and (ii) to utilise excitation ratios in place of emission intensity ratios.…”

Oxygen (3P) atom recombination on a Pyrex surface in an O2 plasma

“…This is preferable to using the O atom density since in this case the effect of gas temperature variations on the total gas density, N, are eliminated. The [O]/N variations were followed by time-resolved actinometry, using Ar (added as 5% of the gas flow) as the actinometer gas [1,3,4,6,[9][10][11]13,[35][36][37][38][39][40][41][42]. The effect of this small addition of Ar on the EEDF and other discharge parameters is negligible, since the EEDF is largely determined by electron collisions with the majority gas, O 2 [43].…”