High temperature studies of singlet excited oxygen, O2(1Σg+) and O2(1Δg), with a combined discharge flow/shock tube method

The excited states of oxygen, O2(1Δg) and O2(1Σg+), 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 O2(1Δg) behind the shock front is consistent with the simple collisional model for the production of the emission; the rate of quenching of O2(1Δg) by O2 is too slow to measure at high temperatures with the technique. The emission from O2(1Σg+) 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: 2O2(1Δg)->O2(1Σg+)+O2(3Σg-) kp; quenching: O2(1Σg+)+M->O2(3Σg- or 1Δg)+M; kqM. The relaxation to the enhanced level of emission permits kMq to be measured directly and then kp is calculated from the enhanced steady state emission level and kMq. There is no evidence for direct, collision induced! enhancement of the emission from O2(1Σg+). 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.