No abstract

Experimental evidence is given that supports the possibility of a previously unknown non‐LTE mechanism for stratospheric ozone formation, which could have a significant impact on the stratospheric ozone budget even if the quantum yield for production of highly vibrationally excited O2 in reaction (1), (averaged over all wavelengths shorter than 243nm) were as low as 0.2%. Stimulated emission pumping enabled preparation of individual vibrational states of O2(X³Σg−,19≤v≤27) and laser induced fluorescence was used to follow the time evolution of the prepared states and thereby determine the vibrational‐state‐specific total‐removal rate‐constants for relaxation by O2 and N2 at 295K. Self‐relaxation shows a sharp threshold for enhanced relaxation near the energy of O2(X³Σg−, v=26) which is coincident with the energetic threshold for reaction (2). The magnitudes of the self‐relaxation rate constants for O2(X³Σg−, v=26 and 27) are quantitatively consistent with the kinetic parameters of reaction (2). Relaxation by N2, while important for lower O2 vibrational states, is shown to be about 10 and 200 times slower than self relaxation for v=26 and v=27, respectively. These are the first two vibrational states of O2 that could form O3 via reaction (2).

Recent results have shown that highly vibrationally excited O2 is formed in significant quantities from the ultraviolet photolyis of ground state ozone. An important question for understanding the balance of O3 in the upper atmosphere is the lifetime of these highly vibrationally excited molecules which are proposed2 to be a photolytic source of odd oxygen atoms. In this work we report the rate constants for the collisional deactivation of SEP prepared O2(v"=18-25) by O2(v"=0), at temperatures of 295 and 395 K. The experiments are analogous to the "Pump", "Dump" and "Probe" studies carried out by Yang et al on NO.3 A pulsed tunable Argon Fluoride laser is used to "PUMP" O2 from X 3 Σ u − ground electronic state to a specific rovibrational level of the B 3 Σ g − excited electronic state via the well known Schumann-Runge bands4. A Xenon-Chloride pumped tunable dye laser system then stimulates, or "DUMPS" the O2 back to a specific excited rovibrational level of the ground electronic state. A second tunable dye laser system then "PROBES" the vibrationally excited O2 population by Laser Induced Fluorescence. By varying the time delay between the DUMP and PROBE lasers, the time dependant occupation of the prepared vibrational level is monitored. The collisional quenching rate constant for a given vibrational level is then determined from the pressure dependance of the lifetime. Implications of the measured rates for atmospheric chemical reactions are discussed.

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.