Mechanisms of Populating Electronically Excited CN in Active Nitrogen Flames

Abstract

CN red (A ²Π⇀X ²Σ) and violet (B ²Σ⇀X ²Σ) bands radiated from active nitrogen flames with halogenated hydrocarbons and with cyanogen derivatives have been studied at pressures from 0.013 to 100 torr. Intensity measurements have shown that there are three distinct vibrational population distributions P₁, P₂, and P₃ in the A ²Π state and corresponding distributions P₁′, P₂′, and P₃′ in the B ²Σ state of CN. At very low pressures, P₁ distributions are characteristic of cyanogen derivative flames and P₂ distributions are characteristic of halogenated hydrocarbon flames. P₃ distributions are most easily observed at pressures above 1 torr and with trace amounts of fuels containing carbon. Three distinct mechanisms responsible for producing these three types of distributions are proposed; two mechanisms involve the transfer of energy derived from the recombination of atomic nitrogen and the third is a chemical reaction using atomic nitrogen. Vibrational relaxation is observed for the P₂ and P₁′ distributions. Small amounts of oxygen are found to increase greatly the CN emission at low pressures. Intensity measurements of rotational lines in the red bands and in the violet 0, 0 band have been made as a function of pressure in CH₂Cl₂ flames. In the A ²Π state at pressures above 4 torr, a rotational temperature of 340°K can be defined, but below 4 torr the rotational distribution is best represented by a sum of two Boltzmann distributions. The effect of pressure on the amount of CN in the two distributions leads to a measured cross section of 15 Ų for the effective collisional relaxation of rotation. From a study of the violet 0, 0 band, several new pairs of rotationally perturbed lines are found at low pressures. The formation rate of the A ²Π state relative to the B ²Σ state and to the X ²Σ state is obtained from the intensity ratio of main lines relative to extra lines in rotational perturbed pairs. CN is found to be formed in the A ²Π state at least 30 times more readily than in the B ²Σ state and about four times faster than in the X ²Σ state.