Kinetics of highly vibrationally excited O₂(X) molecules in inductively-coupled oxygen plasmas

Abstract

The high degree of vibrational excitation of O 2 ground state molecules recently observed in inductively coupled plasma discharges is investigated experimentally in more detail and interpreted using a detailed self-consistent 0D global kinetic model for oxygen plasmas. Additional experimental results are presented and used to validate the model. The vibrational kinetics considers vibrational levels up to v=41 and accounts for electron impact excitation and de-excitation (e-V), vibration-to-translation relaxation (V-T) in collisions with O 2 molecules and O atoms, vibration-to-vibration energy exchanges (V-V), excitation of electronically excited states, dissociative electron attachment, and electron impact dissociation. Measurements were performed at pressures of 10-80 mTorr (1.33 and 10.67 Pa) and radio frequency (13.56 MHz) powers up to 500 W. The simulation results are compared with the absolute densities in each O 2 vibrational level obtained by high sensitivity absorption spectroscopy measurements of the Schumann-Runge bands for O 2 (X, v=4-18), O( 3 P) atom density measurements by two-photon absorption laser induced fluorescence (TALIF) calibrated against Xe, and laser photodetachment measurements of the O − negative ions. The highly excited O 2 (X, v) distribution exhibits a shape similar to a Treanor-Gordiets distribution, but its origin lies in electron impact e-V collisions and not in V-V up-pumping, in contrast to what happens in all other molecular gases known to date. The relaxation of vibrational quanta is mainly due to V-T energy-transfer collisions with O atoms and to electron impact dissociation of vibrationally excited molecules, e+O 2 (X, v)→O( 3 P)+O( 3 P).

Measurement of the two-photon excitation cross-section of the 6p′[3/2]2 and 6p′[1/2]0 levels of Xe I at the wavelengths 224.3 and 222.6 nm

“…TALIF via the 6p [3/2] 2 level of Xe was also used directly to investigate neutral gas depletion in a helicon reactor (Aanesland et al 2007). As told by Annušová et al (2018), however, "continuing work at the Laboratory de Physique des Plasmas on DC discharges suggests that using the value of the ratio determined by Niemi et al (2005) may overestimate the O atom density by up to a factor of two". Dealing with the calibration of O, H and N TALIF with Xe and Kr, Stancu (2020) also noted some inconsistencies, especially with cross-section ratios, and recommended to revisit the twophoton absorption cross-sections of interest with more precise techniques, especially by direct absorption measurements (Blondel et al 2020).…”

Advances in non-equilibrium $$\hbox {CO}_2$$ plasma kinetics: a theoretical and experimental review

“…In addition, theoretical cross sections are proposed in [229] for the full matrix of e-V transitions up to v = 42, while statedependent dissociative attachment and state-dependent dissociation cross sections are given in [230]. vdependent cross sections of excitation of O 2 (a,b) from O 2 (X) were considered in [231]. It is worth noting that the validation of the electron impact dissociation cross section and its v-dependence is still an open problem, with some indications that the dissociation cross sections based on Phelps [220] can be overestimated [232,233].…”

“…( 24), (26) and (27). The self-consistent approach has a long history in literature and was used to describe several plasma mixtures, such as H 2 , H, N 2 , O 2 , N 2 -O 2 in discharge and post-discharge, shock wave and expanding flow conditions [231,272,[312][313][314][315][316][317][318][319]. Recently, the self-consistent model was applied to CO 2 plasmas by Pietanza et al [266][267][268][269][270][271] to better understand the role of vibrational excitation in CO 2 conversion by cold nonequilibrium plasmas, giving particular attention to the electron kinetics and to the EEDF's calculation.…”

Zero-dimensional and pseudo-one-dimensional models of atmospheric-pressure plasma jets in binary and ternary mixtures of oxygen and nitrogen with helium background

“…A detailed resolution of the vibrational distribution function (VDF) has gained much attention in low-pressure plasma [30][31][32][33][34][35][36][37] (e.g., N 2 (v < 46) [30], O 2 (v < 42) [31,33]). Its influence on the electron energy distribution function (EEDF) [30,32,[36][37][38] and the plasma properties [31,33] is presented. An important role of the nitrogen VDF in the production of nitric oxide and nitrogen atom is reported [35,39], and the VDF is intensively investigated [37].…”