Temperature Dependence of OH(υ) Vibrational Relaxation by Atmospheric Gases

Emission from vibrationally excited OH is a key observable in the terrestrial middle atmosphere. A detailed understanding of vibrational relaxation by collisions with other atmospheric species is required so that we can quantify the importance of these processes in controlling atmospheric OH emissions. The SABER instrument aboard the TIMED satellite monitors OH emissions that are used to evaluate the energy deposition rate for the H + O3 reaction in the mesosphere. In order to accurately retrieve this information from the SABER OH measurements, reliable measurements are needed of collisional energy transfer rate constants for the key colliders, namely, oxygen molecules, nitrogen molecules, and oxygen atoms. The dependence of the rate constants on the OH vibrational level as well as the temperature dependence must be known. We report laboratory measurements on the removal of OH(υ = 9) by oxygen atoms, oxygen molecules, and nitrogen molecules at room temperature and relative rates at low temperatures. In addition, we present the first measurements of OH vibrational energy transfer below 200 K for the OH vibrational levels 3 and 4. We note a strong temperature dependence of the rate constant for OH(υ = 4) + O2, whose value more than doubles when the temperature is decreased from approximately 300 K to 160 K. In the experiments, we generate O(3 P) and OH(υ) by photodissociation of ozone at 248 nm in a mixture of ozone, nitrogen, oxygen, and hydrogen. We monitor the temporal evolution of the OH(υ) population by laser induced fluorescence. By controlling the initial conditions of the experiments, we can extract the rate constant for OH(υ) removal by the gases in the system. We will present our experimental results and discuss their atmospheric implications based on the most current modeling calculations. This work was supported by the NASA Geospace Sciences and Planetary Atmospheres Programs. The participation of Z. M. Geballe was supported the NSF Research Experiences for Undergraduates Program.