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[5.09] Experimental Measurement of Dissociative Recombination Relevant to Planetary Atmospheres

P.C. Cosby, J.R. Peterson, D.L. Huestis (SRI), A. Petrignani (FOM), W. van der Zande (Nijmegen U.), M. Larsson, R. Thomas, F. Hellberg (Stockholm U.)

Dissociative recombination (DR) is the primary mechanism for electron loss in ionized, low-pressure molecular gases and plasmas, such as planetary ionospheres. Through the use of heavy ion storage rings, there has been considerable progress in recent years in characterizing both the cross sections and the products produced by DR reactions. DR can in principle be strongly dependent on specific rotational, vibrational, and electronic states of the ion, but current work has allowed only investigating ground state species or uncharacterized internal state distributions. Above 150 km in the CO₂ atmosphere of Venus, the oxygen molecular ion is formed by the reactions: h\nu + CO₂ arrow O⁺ + O₂ and O⁺ + CO₂ arrow O₂⁺(v) + CO, where >60% of the O₂⁺(v) is produced in vibrationally excited levels. We report here the first experimental determination of cross sections and products produced in the DR of O₂⁺(v) in its ground state vibrational levels v = 0, 1, 2, 3, and 4. In particular, our measurements show substantial variations (factors of 5) in the DR cross sections among these low levels. Our measurements were made at CRYRING using a variable pressure electron-impact ion source with the ion populations characterized by dissociative charge transfer in cesium vapor. This work is partially supported by the NASA Planetary Atmospheres Program under grant NAG5-11173.

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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.