Spectroscopic Study of Electronic Flame Temperatures and Energy Distributions

Abstract

A spectroscopic study has been made of electronic temperatures and electronic excitation in flames from an analysis of the intensity distribution of selected parts of the iron spectrum. The iron was added to the flames in the form of ferrocene to serve as a temperature and excitation ``indicator.'' The flames investigated were the stoichiometric H2–O2 and C2H2–O2 flames at atmospheric pressure, a stoichiometric C2H2–O2 flame at low pressure (2.1 mm) and a N+CO atomic flame (3.7 mm). This study shows that the iron spectrum can be used as a spectroscopic ``thermometer'' to determine electronic temperatures in good agreement with adiabatic flame temperatures and OH rotational temperatures in flame regions at thermal equilibrium. The reaction zones of the C2H2–O2 and N+CO flames showed a marked departure from equilibrium as indicated by the nonthermal excitation of the added iron, the observed iron spectrum resembling that of an iron arc. This nonthermal excitation of the iron spectrum, which is readily evidenced by the appearance of iron lines originating from high-energy levels (En′≥50 000 cm—1), can serve as a very sensitive indicator of the departure from equilibrium in a high-temperature reaction system. A brief discussion is given of the possible chemical and energy exchange mechanisms which may give rise to the abnormal excitation of iron in various reaction systems.