Temperature Dependence of the Collisional Deactivation Processes in Excited O2: A Probe to the Relaxation Pathways and Energetics

Emission from electronically excited O₂ is an important component of the nightglow of the Earth and Venus. Since these emissions occur at altitudes where the temperature is significantly below room temperature, understanding the temperature dependence of collisional removal is crucial to modeling the emission. Recent results have shown an unexpectedly large variation in the removal rates for nearby vibrational levels in the a ¹Δ_g, b ¹Σ _g^+, and c ¹Σ _u^- states. As an example, the b ¹Σ _g^+ (ν = 13) removal rate constant for collisions with O₂ at 150 K is 4 to 60 times larger than that of the neighboring ν = 14 and 12 levels, respectively. Such a large discrepancy can be caused by the presence of a resonant energy transfer pathway. A difference in behavior is also seen in how the magnitude of the rate constant varies with temperature. Knowledge of this behavior helps us to achieve a better understanding of the relaxation pathways and the relevant kinetic parameters in the collisional deactivation of the highly excited levels. The removal rates for the b ¹Σ _g^+ state ν = 14 and 15 levels by collisions with O₂ were measured to be 2--4 x 10^-12 cm³s^-1 and found to be almost independent of temperature over the entire atmospheric range (150--300 K), which points to a deactivation process with little or no energetic barrier. However, the rates for ν = 11 and 12 at 150 K are an order of magnitude slower than those for ν = 13--15. We find that the ν = 12 rate increases by a factor of 5 going from 150 to 240 K, indicative of a relaxation process with an activation barrier of about 350 +/- 120 cm^-1. For the c ¹Σ _u^- state at 155 K, the collisional removal rate constants in O₂ are (2.6 +/- 0.3) x 10^-11 and (7 +/- 3) x 10^-11 cm³s^-1 for ν = 10 and 11, respectively. As the temperature is varied between 150 and 300 K, little or no change is observed in the magnitude of these rate constants. In contrast, the rate for ν = 9 increases by more than a factor of 3 in the same temperature range, from 1.5 to 5.4 x 10^-12 cm³s^-1, perhaps indicating a process with a barrier. These results will be compared with the collisional removal rates of other O₂ electronic states, namely ⁵Π _g and A ³Σ _u^+, and the lower vibrational levels of the b ¹Σ _g^+ state. Overall, the magnitude of the collisional removal rate constants and their temperature dependence revealed interesting details of the specific relaxation processes. Experiments are currently underway to determine the involved relaxation pathways and energy transfer mechanisms to help explain the large differences observed from one vibrational level to the next. This work is supported by the NASA Ionospheric, Thermospheric, and Mesospheric Physics, Supporting Research and Technology, and the Planetary Atmospheres Programs. KVC is grateful for a NSF-REU scholarship under Grant No. PHY-0097861, and ELS thanks the Camille and Henry Dreyfus Foundation for support of a summer internship.