The flash photolysis of ozone

Abstract

The photolytic decomposition of ozone in ultra-violet radiation has been studied by kinetic spectroscopy. It has been shown that vibrationally excited oxygen in its ground electronic state plays a most important part in the decomposition. These molecules have sufficient vibrational energy to bring about dissociation of ozone, thus regenerating oxygen atoms which can again produce vibrationally excited oxygen. The importance of this energy chain is emphasized by comparative studies on the explosive decomposition of pure ozone, and the isothermal decomposition when an excess of inert gas is present. In the former case the O* 2 is removed so rapidly, mainly by reaction with ozone, that no absorption due to it can be detected. Using an excess of inert gas to obtain isothermal conditions it has been possible to observe Schumann–Runge absorption of oxygen molecules in their ground electronic state, with up to 16 quanta of vibrational energy. The vibrational energy distribution of the oxygen molecules formed has an apparent maximum at v " = 13 (53.5 kcal/mole) and falls off sharply at v " = 12 and 16 (49.0 and 63.2 kcal/mole). It is shown that the only reasonable reaction for the production of excited oxygen is O + O 3 → O* 2 + O 2 . Studies on the rate of ozone decay with time have also been carried out and the results analyzed in terms of the rate constants of reactions involving the deactivation of excited oxygen and the three-body recombination O + O 2 + M → O 3 + M . It is shown that the spherically symmetrical and chemically inert gases such as A, He and SF 6 are much less efficient in bringing about recombination than N 2 , N 2 O or CO 2 .