The kinetics of the carbon monoxide flame bands

Abstract

The intensity Ic of the carbon monoxide flame bands observed in the low-temperature reaction of O with CO was found to have similar kinetics to those of the O+NO and H+NO emissions, Ic=I0c[O][CO], where I0c depended upon the nature but not on the pressure of the carrier gas used as third body. I0c, and the rate constants k1c for overall combination of O and CO were found to be much less at 293°K than I0a and k1a (the analogous quantities in the O+NO reaction), but while k1a, and I0a showed small negative temperature coefficients, I0c was found to have a positive Arrhenius activation energy of +3.7 kcal/mole. Extrapolated values of I0a and I0c at 1/T=0 were similar. Most of the excited CO2 molecules formed in the third order combination process are quenched collisionally rather than radiatively. The absence of any effect upon the values of I0c when the third body used is in a triplet state (O2) shows that a spin reversal reaction is not rate determining. The observation of similar pre-exponential factors for I0a and I0c shows that the emission process in the O+CO reaction does not involve spin reversal. It is considered that the flame band emission is due to a transition from an excited singlet state to the (singlet) ground 1σ0+ state of CO2. Molecules in this excited singlet state are formed by a rapid radiationless transition from the triplet state of CO2 in which they are initially formed. The low value observed for I0c is then due to the presence of an energy barrier in forming the excited triplet and singlet state, and not to the spinforbidden nature of the overall combination reaction.