Collisional Relaxation of Highly Vibrationally Excited O2 and a Bimodal Vibrational Distribution of O2 from O3 Photolysis: A Possible Explanation of the "Ozone Deficit Problem"

Stimulated emission pumping was used to investigate the collisional relaxation of vibrationally state selected O 2 ( X 3 Σ g − , 19 ≤ v ≤ 28 ) . Strong evidence was obtained suggesting that O 2 ( X 3 Σ g − , v ≥ 26 ) reacts with O2 to form O3 + O. Collisional relaxation at lower vibrational excitation appears to agree well with theoretical models which derive effective information about the interaction potential from ab initio calculations of the (O2)2 van der Waals molecule. This remarkable result shows how existing theories designed to explain vibrational energy transfer at low excitation may be extended to the "chemical energy regime." Results of recent experiments on the photolysis of ozone at 226 nm show that the vibrational distribution of the O 2 ( X 3 Σ g − , v ) is markedly bimodal, with one peak near v = 14 and another at v = 27. The explanation of this is, as yet, completely lacking and represents an interesting fundamental problem for ab initio theory. The production of highly vibrationally excited O2 by ozone photolysis together with the reactivity of highly vibrationally excited O2 may have significant atmospheric consequences. Initial modelling results suggest that the inclusion of highly vibrationally excited O2 may reconcile the long-standing discrepancy between the predicted and observed concentrations of stratospheric ozone.