High temperature studies of singlet excited oxygen, O 2 ( 1 Σ g + ) and O 2 ( 1 Δ g ), with a combined discharge flow/shock tube method

The excited states of oxygen, O 2 ( 1 Δ g ) and O 2 ( 1 Σ g + ), generated in a microwave discharge, were shock heated in order to study their reactions at temperatures in the range 650-1650 K. The increase in the dimol emission (634 nm) from O 2 ( 1 Δ g ) behind the shock front is consistent with the simple collisional model for the production of the emission; the rate of quenching of O 2 ( 1 Δ g ) by O 2 is too slow to measure at high temperatures with the technique. The emission from O 2 ( 1 Σ g + ) increases because of the shock compression and then is further enhanced by a displacement in the steady state concentration which is maintained by the two reactions pooling: 2O 2 ( 1 Δ g )->O 2 ( 1 Σ g + )+O 2 ( 3 Σ g - ) k p ; quenching: O 2 ( 1 Σ g + )+M->O 2 ( 3 Σ g - or 1 Δ g )+M; k q M . The relaxation to the enhanced level of emission permits k M q to be measured directly and then k p is calculated from the enhanced steady state emission level and k M q . There is no evidence for direct, collision induced! enhancement of the emission from O 2 ( 1 Σ g + ). Curved Arrhenius plots of the rate constants were found; some values are given in table 2. The results appear to indicate that in each case there are two mechanisms operating; one involving short range forces, and the other, long range forces or a collision complex. An evaluation is given of the discharge flow-shock tube technique as a method for determining rate constants at high temperatures.