O2(X,V=8-22) 300-K QUENCHING RATE COEFFICIENTS FOR O2 AND N2, AND O2(X) VIBRATIONAL DISTRIBUTION FROM 248-NM O3 PHOTODISSOCIATION

Vibrationally excited oxygen (O2double dagger) is produced in the atmo sphere by ozone photodissociation in the 200-300 nm Hartley band. It h as been suggested that photoexcitation of O2double dagger in the 02 Sc humann-Runge bands will lead to predissociation, and autocatalytic pro duction Of 03. The resultant new source of atmospheric 03 could help a lleviate current discrepancies between observed and modeled 03 profile s. To evaluate this possibility, we have examined two critical factors -the nascent distribution of O2double dagger levels for 248 nm photodi ssociation, near the peak of the Hartley band, and the rate coefficien ts for their relaxation by 02 and N2. We find that the distribution ex tends to v = 22, close to the thermodynamic limit, with a peak near v = 8. The 300 K quenching rate coefficients have been evaluated using a cascade model, in which it is assumed that relaxation by 02 occurs th rough single-quantum vibration-vibration (V-V) and vibration-translati on (V-T) steps. By modeling the relaxation from the top of the distrib ution downwards, we simultaneously obtain both the quenching rate coef ficients and the nascent vibrational distribution. Agreement with new rate coefficient measurements carried out in a state-specific manner i s good, as is also true for the comparison with new V-V and V-T calcul ations. Data from experiments on O2double dagger quenching by N2 show that in the v = 16-22 range, potentially important in the atmosphere, quenching proceeds up to five times faster than for the case of 02. Th e hypothesized explanation is that two-quantum V-V transfer, peaking a t the resonant condition of 02 (v = 18-19), is the dominant process. A s a consequence, atmospheric quenching of O2double dagger for levels a bove v = 14 is basically controlled by N2, and at low stratospheric te mperatures, the effect of N2 quenching near v = 18 is likely to be 2 o rders of magnitude greater than quenching by 02 . This unexpected effe ct probably precludes a significant role for O2double dagger photodiss ociation as a new source of stratospheric 03, but the existence of the se high-energy entities can have other consequences, among them being enhanced activity with minor species, and the possibility that energy may flow from the relatively stable 02 (V = 1 ) and N2 (V = 1 ) levels into infrared-active H2O and CO2, respectively. Measurements have als o been made for O2double dagger quenching by O3, CO2, and He, particul arly to establish whether 03 and CO2 can Play a competitive quenching role in the atmosphere. Although O3 is a fast quencher, with CO2 being 2 orders of magnitude slower, they are unlikely to compete with O2 an d N2. The data on He is particularly interesting, suggesting that cons iderably more O2double dagger is present in the nascent 03 photodissoc iation products than subsequently appears from O(1D) + O3 interaction. The implications of this finding are discussed.