Stimulated Emission Pumping (SEP)1 Studies of Energy Transfer in Highly Vibratonally Excited Oxygen

Abstract

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.