Fast quenching of metastable O₂(a ¹Δ _g ) and O₂(b ¹Σ _g ⁺) molecules by O(³P) atoms at high temperature

Abstract Oxygen molecules in the lowest metastable state, O2(a 1Δ g ), play an important role in oxygen plasmas due to their high reactivity and significant concentrations. The accumulation of high densities of O2(a 1Δ g ) occurs due to its low quenching rate. This paper demonstrates the existence, at high gas temperatures (700–1700 K), of fast quenching of O2(a 1Δ g ) by O(3P) atoms, a process that has not been considered in previous models. Experiments were carried out at oxygen pressures of 10–100 Torr in an 81 MHz CCP discharge in a quartz tube with external electrodes. This setup provides high absorbed power density, leading to both high gas temperatures and significant O(3P) densities. We observe that the O2(a 1Δ g ) density is significantly limited at high gas temperatures by rapid quenching by atomic oxygen. The results were interpreted using a self-consistent 1D discharge model. The observations can only be explained by the inclusion of a rapid quenching reaction: O2(a 1Δ g ) + O(3P) → O2(X3Σ g −) + O(3P), with an activation energy in the range of 0.54–0.69 eV. The rate constant was determined over a wide range of discharge conditions (P = 20–100 Torr and T g = 800–1700 K), giving values between 3 × 10−11 exp(−8000/T) cm3 s−1 to 1.5 × 10−11 exp(−6300/T) cm3 s−1. A possible mechanism for this process is discussed. Measurements of the density of metastable O2(b 1Σ g +) molecules also indicated the existence of quenching by atomic oxygen, with a somewhat lower activation energy of ∼0.32 eV. The variations of the measured [O2(b 1Σ g +)]/N mole-fraction could be fitted by the model using a rate constant 2 × 10−11 × exp(−3700/T) cm3 s−1 for this process. These quenching processes of metastable O2(a 1Δ g ) and O2(b 1Σ g +) molecules by oxygen atoms are important for oxygen plasmas and could have a significant impact on the kinetics of oxygen-containing mixtures at higher gas temperatures, for example in plasma-assisted combustion or in high-pressure plasma processing reactors.