Spectroscopic studies of low-pressure flames IV. Measurements of light yield for C 2 bands

Abstract

The light yield for the Swan bands is measured for a number of hydrocarbons. For C$_{2}$H$_{2}$ + 2$\frac{1}{2}$O$_{2}$ flames 3 $\times $ 10$^{-5}$ photons are emitted for each O$_{2}$ consumed at 1 atm. pressure, rising to 13$\cdot $5 $\times $ 10$^{-5}$ at 3$\cdot $5 mm. pressure. For propane the effect of pressure is more marked. The effect of mixture strength and of diluents is studied. For partial replacement of hydrocarbon by H$_{2}$ or CO (the mixture being held stoichiometric) the yield falls with surprising rapidity, being proportional to a high power of the hydrocarbon concentration (square to sixth power). Preliminary results for CH are similar. The C$_{2}$ emission does not appear to be due to true chemiluminescence, but there is abnormally high excitation by some active species in the flame, and the concentration of C$_{2}$ exceeds the values found by Brewer, Gilles & Jenkins for equilibrium with solid carbon at the flame temperature. Polymerization and formation of unsaturated compounds appears to be important. The relation between C$_{2}$ emission and the formation of solid carbon is discussed; polymerization of C$_{2}$ cannot account quantitatively for the formation of solid particles.