Fast quenching of metastable O2(a1Δg) and O2(b1Σg+) molecules by O(3P) atoms at high temperature

Oxygen molecules in the lowest metastable state, O2(a1Δg), play an important role in oxygen plasmas due to their high reactivity and significant concentrations. The accumulation of high densities of O2(a1Δg) occurs due to its low quenching rate. This paper demonstrates the existence, at high gas temperatures (700-1700 K), of fast quenching of O2(a1Δg) 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(a1Δg) 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(a1Δg) + O(3P) → O2(X3Σg−) + 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 Tg = 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(b1Σg+) 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(b1Σg+)]/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(a1Δg) and O2(b1Σg+) 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.