Spectroscopic Studies of Low-Pressure Flames. V. Evidence for Abnormally High Electronic Excitation

Spectrum-line reversal measurements, using lines of Fe, Pb, Na and Tl, have been made for the reaction zones of many flames. For most organic flames the reversal temperatures exceed theoretical maximum temperatures, the values rising very high for lines in the ultra-violet. Lines of Fe requiring up to at least 173 kcal./mole For their excitation are emitted from the reaction zones, but not from the interconal gases. Various hydrocarbons (CH4, C2H4, C2H6, C2H2, C2H8), methyl alcohol, cyanogen and ammonia burning in premixed flames with oxygen, air or nitrous oxide all show this abnormally high electronic excitation, but flames of H2, CO, CS2 and formaldehyde do not. The hydrogen/nitrous oxide flame does show some abnormal excitation, but of a different character. Diffusion flames at atmospheric pressure do not show the effect. Processes by which electronic excitation may occur are examined, and the cause of the abnormal excitation is discussed. The spectrum-line reversal method of measuring flame temperatures is criticized.