O2(X,v=8–22) 300 K quenching rate coefficients for O2 and N2, and O2(x) vibrational distribution from 248 nm O3 photodissociation

Abstract

Vibrationally excited oxygen (O‡2) is produced in the atmosphere by ozone photodissociation in the 200–300 nm Hartley band. It has been suggested that photoexcitation of O‡2 in the O2 Schumann–Runge bands will lead to predissociation, and autocatalytic production of O3. The resultant new source of atmospheric O3 could help alleviate current discrepancies between observed and modeled O3 profiles. To evaluate this possibility, we have examined two critical factors—the nascent distribution of O‡2 levels for 248 nm photodissociation, near the peak of the Hartley band, and the rate coefficients for their relaxation by O2 and N2. We find that the distribution extends 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 O2 occurs through single-quantum vibration–vibration (V–V) and vibration–translation (V–T) steps. By modeling the relaxation from the top of the distribution downwards, we simultaneously obtain both the quenching rate coefficients and the nascent vibrational distribution. Agreement with new rate coefficient measurements carried out in a state-specific manner is good, as is also true for the comparison with new V–V and V–T calculations. Data from experiments on O‡2 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 O2. The hypothesized explanation is that two-quantum V–V transfer, peaking at the resonant condition of O2(v=18–19), is the dominant process. As a consequence, atmospheric quenching of O‡2 for levels above v=14 is basically controlled by N2, and at low stratospheric temperatures, the effect of N2 quenching near v=18 is likely to be 2 orders of magnitude greater than quenching by O2. This unexpected effect probably precludes a significant role for O‡2 photodissociation as a new source of stratospheric O3, but the existence of these 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 O2(v=1) and N2(v=1) levels into infrared-active H2O and CO2, respectively. Measurements have also been made for O‡2 quenching by O3, CO2, and He, particularly to establish whether O3 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 and N2. The data on He is particularly interesting, suggesting that considerably more O‡2 is present in the nascent O3 photodissociation products than subsequently appears from O(1D)+O3 interaction. The implications of this finding are discussed.