Laboratory Studies of Vibrational Excitation in O2( a1Δg, v) Involving O2, N2, and CO2.

Collisional removal of electronic energy from O2 in the low-lying a1Δg state is typically an extremely slow process for the v = 0 level. In this study, we report results on the deactivation of O2( a1Δg, 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( b1Σg+). 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.