Vibrationally excited oxygen (O2double dagger) is produced in the atmosphere by ozone photodissociation in the 200-300 nm Hartley band. It has been suggested that photoexcitation of O2double dagger in the 02 Schumann-Runge bands will lead to predissociation, and autocatalytic production Of 03. The resultant new source of atmospheric 03 could help alleviate current discrepancies between observed and modeled 03 profiles. To evaluate this possibility, we have examined two critical factors-the nascent distribution of O2double dagger levels for 248 nm photodissociation, near the peak of the Hartley band, and the rate coefficients for their relaxation by 02 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 02 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 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. The hypothesized explanation is that two-quantum V-V transfer, peaking at the resonant condition of 02 (v = 18-19), is the dominant process. As a consequence, atmospheric quenching of O2double dagger 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 02 . This unexpected effect probably precludes a significant role for O2double dagger photodissociation as a new source of stratospheric 03, 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 02 (V = 1 ) and N2 (V = 1 ) levels into infrared-active H2O and CO2, respectively. Measurements have also been made for O2double dagger quenching by O3, CO2, and He, particularly 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 and N2. The data on He is particularly interesting, suggesting that considerably more O2double dagger is present in the nascent 03 photodissociation products than subsequently appears from O(1D) + O3 interaction. The implications of this finding are discussed.