State-to-state rate constants for relaxation of highly vibrationally excited O2 and implications for its atmospheric fate

Abstract

Collisional relaxation of highly vibrationally excited O₂(X³Σ_g^–, v) has been investigated for vibrational states from v= 18 to 27, using O₂ and N₂ as quenching partners. These excited states of O₂ are now known to be produced in stratospheric ozone photolysis and this study gives information on their eventual atmospheric fate. State-to-state relaxation rate constants were obtained using the stimulated emission pumping method in the pump–dump–probe geometry. For self-relaxation, O₂(v= 19) was found to decay in single vibrational-quantum steps and the rate constants were found to agree well with theoretical predictions which rely on an ab initio potential-energy surface. For vibrational levels above v= 25, a dark channel is observed, where molecules prepared in a state v do not appear in vibrational states v– 1 or v– 2. The most likely explanation to this dark channel is the onset of the chemical reaction O₂(X ³Σ_g^–, v 26)+ O₂(X ³Σ_g^–)→ O₃+ O(³P)(1). The temperature-dependent data presented in this paper allow the derivation of activation energies for the dark channel when the reactants are prepared in vibrational states just below the known activation barrier of reaction (1). For these states, a fraction of the molecules ‘trickle down’ into lower vibrational states, allowing their kinetics to be probed. The derived activation energies are quantitatively consistent with the assignment of the dark channel to reaction (1). The first vibrational state which is above the activation barrier to reaction (1), v= 28, is not observed to ‘trickle down’, suggesting that it only follows the dark channel, i.e. reaction (1). The measurements diverge from the theory successful at v= 19 just at the energetic onset of reaction (1). Relaxation rate constants for N₂ quench gas decrease with O₂ vibrational quantum number and theory is not successful in reproducing these results. It appears that a more accurate O₂–N₂ potential-energy surface is needed.