ORIGIN OF C2 HIGH-PRESSURE BANDS OBSERVED IN THE PRODUCTS OF A MICROWAVE-DISCHARGE THROUGH CO

The origin of C2 high-pressure bands observed in discharges through CO has not been cleared up. The suggestion by Little and Browne of a collisional transfer between the metastable C2(5PI(g), v=0) and the radiative C2(d 3PI(g) v=6) level was progress. However, these authors claimed that only the combination reaction C+C+M-->C2+M should be invoked for the production of the metastable level, excluding the C+C2O-->C2+CO reaction that other authors believed to be at the origin of the C2 HP bands. The correlation diagram of the C+C2O-->C2+CO reaction shows that C2(5PI(g))+CO correlates with ground-state reactants while C2(d 3PI(g))+CO does not. Such a reaction could thus be involved in the C2 HP bands. A kinetic study of these bands i n a fast-flow reactor, downstream from a microwave discharge through CO diluted in He, demonstrated that both react ion s C+C+M and C+C2O were involved. It could be concluded that the C+C2O-->C2+CO reaction has a lower branching ratio for C2(5PI(g)) production than C+C+M-->C2+M. However, the C+C2O reaction has in most discharges, as in ours, a greater reaction rate than that of the C+C+M reaction so its contribution is not negligible and may be the main one if the dissociation rate of CO is low.