Laboratory Studies of Vibrational Excitation in O₂( a¹Δ_g, v) Involving O₂, N₂, and CO₂

Collisional removal of electronic energy from O₂ in the low-lying a¹Δ_g state is typically an extremely slow process for the v = 0 level. In this study, we report results on the deactivation of O₂( a¹Δ_g, v = 1-3) in collisions with O₂ and CO₂. Ozone photodissociation in the 200-310 nm Hartley band is the source of O₂( a, v), and resonance-enhanced multiphoton ionization is used to probe the vibrational-level populations. Deactivation of the a( v = 1-3) levels in collisions with O₂ at 300 K is fast, with rate coefficients of (5.6 ± 1.1) × 10^-11, (3.6 ± 0.4) × 10^-11, and (1.9 ± 0.4) × 10^-11 cm³ s^-1 (2σ) for v = 1, 2, and 3, respectively. The relaxation process appears to involve a near-resonant electronic energy transfer pathway analogous to that observed in vibrationally excited O₂( b¹Σ_g⁺). With CO₂ collider gas, the removal rate coefficient at 300 K is (1.8 ± 0.4) × 10^-14 and (4.4 ± 0.6) × 10^-14 cm³ s^-1 (2σ) for v = 1 and 2, respectively. Despite the small mole fraction of O₂ in the atmospheres of Mars and Venus, O₂ is at least as important as CO₂ in the final stages of collisional relaxation within the O₂ vibrational-level manifold.