Relative Yield of O2(b1σ+_g, v = 0 and 1) in O(1D) + O2 Collisions

In the thermosphere, energy transfer from excited O-atoms leads to production of O2 molecules in the first two vibrational levels of the O2(b1σ+_g) state: O(1D) + O2 → O(3P) + O2(b1σ+_g, v = 0, 1). Subsequent radiative decay of O2(b1σ+_g, v = 0 and 1) to the ground state, O2(X3σ-_g), results in the Atmospheric Band emission. The relative yield for production of O2(b1σ+_g, v = 0 and 1) in the above process, k1/k0, is an important parameter in modeling of the observed Atmospheric Band emission intensities. We report laboratory measurements of k1/k0, in which the output of a pulsed fluorine laser at 157 nm is used to photodissociate molecular oxygen in a O2/N2 mixture. Photodissociation of O2 produces a ground-state O(3P) atom and an excited O(1D) atom. O(1D) rapidly transfers energy to the remaining O2 to produce O2(b1σ+_g, v = 0, 1). The temporal evolution of the O2(b1σ+_g, v = 0 and 1) populations is monitored by observing emissions in the O2(b--X) 0-0 and 1-1 bands at 762 and 771 nm, respectively. The value of k1/k0 is extracted from the time-dependent O2(b1σ+_g, v = 0 and 1) fluorescence signals, based on a detailed understanding of the kinetics involved. Two published studies reported that energy transfer to O2(b1σ+_g, v = 0) is favored, with a k1/k0 ratio in the range 0.3--1 [1, 2]. In contrast, our more direct measurements clearly indicate that production of O2(b1σ+_g, v = 1) dominates that of O2(b1σ+_g, v = 0), with a value of k1/k0 in the range 3--4. Comparisons with high-altitude spectra of the O2(b--X) 0-0 and 1-1 bands obtained during the Arizona Airglow Experiment (GLO) support our experimental finding and suggest that a major revision of the input for k1/k0 in atmospheric models is warranted. This work was supported by the NSF Aeronomy Program under grant ATM-0209229. The fluorine laser was purchased under grant ATM-0216583 from the NSF Major Research Instrumentation Program. 1. M. J. E. Gauthier and D. R. Snelling, Can. J. Chem. 52, 4007 (1974). 2. L. C. Lee and T. G. Slanger, J. Chem. Phys. 69, 4053 (1978).