Quenching of O(2¹D₂) by Atmospheric Gases

Abstract

Laboratory investigations of electronically excited oxygen atoms in the O(2¹D₂) state are central to any detailed consideration of atmospheric chemistry, for this species is a photolysis product of the gases O₃(λ<310 nm), O₂(λ<180 nm) and CO₂(λ<167 nm) (ref. 1) following excitation by solar radiation. The O(2¹D₂) state, 1.967 eV above the O(2³P₂) ground state², is known to be highly reactive; this, combined with the low probability for the magnetic dipole-allowed spontaneous emission at λ=630 nm (2¹D₂→2³P₂, A_nm = 6.9×10⁻³ s⁻¹) (ref. 3) makes laboratory studies of this radiation extremely difficult⁴. Hence a number of indirect measurements on the kinetic behaviour of O(2¹D₂) have employed spectroscopic marker techniques which monitor a product of the reaction of this atomic state with an added molecule. Measurements of the electronic absorption of OH(A²Σ⁺←X²π) (ref. 5) and the emission from O₂(b¹Σ⁺_g→X³Σ⁻_g) (ref. 6) provide examples of such methods. Analyses of stable products have also been used to obtain kinetic data on this state⁷. These methods usually lead to the determination of relative rate constants. Several recent reviews on laboratory and aeronomical studies of O(2¹D₂) summarize the rate data derived from such investigations^8–12.