Vibrationally Excited Ozone in the Pulse Radiolysis and Flash Photolysis of Oxygen

Abstract

The kinetics of ozone formation have been studied in the pulse radiolysis and flash photolysis of gaseous oxygen by measuring the change in optical absorption following the pulse. The absorption spectrum of the ozone present immediately after the pulse is considerably broader and the peak is at longer wavelength (∼2860 Å) than that of ground state ozone (∼2560 Å). The initial absorption is attributed to ozone in the upper vibrational levels produced by O + O2⇋O3‡. It is clearly shown that the over-all third-order reaction O + O2+M→O3 + M occurs by a sequence of steps. The relaxation time for de-excitation from the upper vibrational levels to the ground state is ≥6 × 10−6 sec in O2 at 740 torr and 24°C. De-excitation requires at least 4.5 × 104 collisions, and assuming 20 vibrational levels, the average efficiency is about one quantum in 2200 collisions. The third-order rate constant based on O atom disappearance k(−O) is several times larger than that based on formation of O3 in the ground vibrational state k(O3, υ = 0). Neither rate constant can be evaluated accurately by optical absorption measurements. Measurements at 2800 Å, where all vibrational levels appear to have about the same extinction coefficient, gave a value 2.2 × 108M−2 sec−1. This value should approximate k(−O) for which Kaufman and Kelso determined 2.4 × 108M−2 sec−1 by direct measurement of O atom disappearance. Measurements at wavelengths below 2480 Å gave an upper limit for k(o3.υ = 0) of 1 × 108M−2 sec −1.