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, Anm = 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 investigations8–12.