Vibrational Excitation of Molecule Oxygen During Recombination of O Atoms*

Vibrationally excited oxygen is of critical importance in nonequilibrium high-enthalpy flows, encountered behind hypersonic shock waves. To date, there are few direct experimental verifications of vibrational state resolved dissociation rates. We present a framework where state specific recombination rates can be inferred from the time-resolved measurements of O₂ vibrational populations, such that the state specific dissociation rates can be obtained from the detailed balance. In the present approach, recombination reactions of atomic oxygen are monitored after a ns pulse discharge burst in an O₂-Ar mixture. The time evolution of O₂(v) populations in the mixture are measured by ps Laser LIF in the Schumann-Runge bands. Levels from v”=7 to 21 have been detected. TALIF at 226 nm has been used to measure the atomic oxygen concentration. Within ∼1 ms after the discharge burst, a rapid decay of O₂(v) is observed, indicating V-V and V-T relaxation of vibrational states populated by electron impact and by quenching of the excited Ar. After the rapid initial decay, the vibrational populations level off and remain nearly constant, or exhibit a transient rise, on the timescale of about 10 ms, suggesting the presence of a persistent source of vibrational excitation from chemical reactions.