The flame spectrum of carbon monoxide, II. Application to 'afterburning'

Abstract

In a previous paper it has been shown, from a study of the spectrum of the carbon monoxide flame, that the molecules of CO 2 formed by the combustion are initially in a highly excited state of internal vibration. An attempt is made here to derive approximate values for the lifetimes of these vibrationally activated molecules. The orders of magnitude of the radiative lifetime for the three types of vibration are derived. From data obtained from supersonic dispersion in CO 2 the relaxation times, or collision lifetimes, are discussed with especial emphasis on the effect of moisture on the collision-life of the activated molecules. A section is also devoted to the possibility of the close resonance between the vibrations v 1 and v 2 of CO 2 resulting in a high percentage of dissociation after the combustion of the dry gas. The results give a good interpretation of the experiments of Garner and colleagues on the marked effect of moisture and other catalysts on the infra-red emission of the flame, and the general failure to observe a strong emission band at 14.9 μ . The results of the theory go far towards explaining the latent energy of the combustion observed by David and colleagues. The lifetime of the activated molecules is calculated to be not more than a few tenths of a second, but dissociation and recombination processes might lengthen this time. For moist gases the lifetime will be very much less, and the infra-red radiation from the flame and the latent energy of the products should similarly be much reduced, in agreement with experiment. The vibrationally activated molecules are to be regarded as essentially normal molecules of CO 2 in which the vibrational energy has not had time to reach equipartition with the energy in other degrees of freedom; spectroscopically there is no evidence to show that they are electronically excited or peculiar in any other way. Experiments on the absorption spectrum of a long length of burning gas show strong absorption due to hot oxygen, this being particularly marked when the gases are dry. This is interpreted as being due to transfer of the vibrational energy from the CO 2 to the O 2 molecules, the absorption spectrum of which is thereby shifted to longer wave-lengths.