A new mechanism for OH vibrational relaxation leading to enhanced CO₂ emissions in the nocturnal mesosphere

Abstract

On the basis of experimental and theoretical studies, this paper proposes a new mechanism that contributes to nocturnal 4.3 µm CO₂ emissions. It suggests that collisions of ground state O atoms with highly vibrationally excited OH(v), produced by the reaction of H with O₃, remove a substantial fraction of the OH(v) vibrational energy by a fast, spin-allowed, multiquantum vibration-to-electronic energy transfer (ET) process that generates O(¹D): OH(v ≥ 5) + O(³P) → OH(0 ≤ v' ≤ v - 5) + O(¹D). The electronically excited O(¹D) atom is subsequently deactivated by collisions with N₂ in a fast spin-forbidden ET process that leaves the N₂ molecule with an average of 2.2 vibrational quanta. Finally, the vibrational excitation of N₂ is transferred by a fast, near-resonant vibration-to-vibration ET process to the asymmetric stretch (v₃) mode of CO₂, which promptly radiates near 4.3 µm.