Temperature Dependence of the Collisional Removal of O 2 (A³Σ u + , υ=9) with O 2 and N 2

The temperature dependence of the collisional removal of O 2 molecules in the υ=9 level of the A³Σ u + electronic state has been studied for the colliders O 2 and N 2 , over the temperature range 150 to 300 K. In a cooled flow cell, the output of a pulsed dye laser excites the O 2 to the υ=9 level of the A³Σ u + state, and the output of a time‐delayed second laser monitors the temporal evolution of this level via a resonance‐enhanced ionization. We find the thermally averaged removal cross section for O 2 collisions is constant (∼10 Ų) between room temperature and 200 K, then increases rapidly with decreasing temperature, doubling by 150 K. In contrast, the N 2 cross section at 225 K is ∼8% smaller and gradually increases to a value at 150 K that is ∼60% larger than me room temperature value. The difference between the temperature dependence of the O 2 and N 2 collision cross section implies that the removal by oxygen becomes more important at the lower temperatures found in the mesosphere, but removal by N 2 still dominates.