Carbon dioxide thermal decomposition: Observation of incubation

Incubation prior to the thermal decomposition Of CO2 is observed for the first time behind shock waves, confirming the expected bottleneck in collisional activation of this triatomic molecule. The thermal decomposition of carbon dioxide has been investigated behind reflected shock waves at temperatures of 3200-4600K and pressures of 45-10kPa. Ultraviolet laser absorption was used to monitor the CO2 concentration with microsecond time resolution, allowing observation of a pronounced incubation period prior to steady CO2 dissociation. Master equation simulations, with a simple model for collisional energy transfer, were carried out to describe the measured incubation times and dissociation rate coefficient. The second order rate coefficient for CO2 dissociation was found to be 3.14 x 10(14) exp(-51 300 K/T) cm(3) mol(-1) s(-1). The number of incubation collisions was found to range from 7 x 10(3) at 4600 K to 3.5 x 10(4) at 3200 K. The master equation simulations suggest that the energy transferred per collision must have a greater than linear dependence on energy.