The production of O(3P) in the 157 nm photodissociation of CO2.

The branching ratio was measured for the production of O(3P) in the photodissociation of CO2 at 157 nm. A gas mixture consisting of CO2, H2, and Ar was irradiated with an F2 excimer laser, while the relative concentration of O(3P) was monitored continuously using atomic resonance fluorescence. The O(1D) product was removed by either reacting with H2 or by being quenched by CO2. At a high H2/CO2 ratio a residual O(3P) signal persisted which was due to the nascent photofragments of CO2. A Stern–Volmer analysis indicated that the fraction of O(3P) produced is 5.9%. Control experiments using O2 and N2O as precursor molecules confirmed this interpretation of the data. A mechanism is proposed based on curve crossing from the 1B2 to the 3B2 potential energy surfaces of CO2. Since the 1B2 state is bent, a substantial fraction of the absorbed energy is initially in bending motion, resulting in a long-lived chaotic trajectory which has many opportunities to cross over to the triplet surface. A similar model proposed by Tully to explain the quenching of O(1D) by CO is in qualitative agreement with our data. [ABSTRACT FROM AUTHOR]