The emission spectrum of hydrocarbon flames

The present investigation was commenced at the suggestion of the late Professor W. A. Bone, as a continuation of the systematic examination of flame spectra which had been carried out in his laboratories (Weston 1925 a, b; Bone and Lamont 1934; Bell 1937). It was decided to extend the work to the case of unsaturated hydrocarbons by a comprehensive study of ethylene flames, accompanied by chemical investigation of the products of reaction. The emission spectrum of hydrocarbon flames generally, in the visible and ultra-violet regions, is characterized by the bands of the OH molecule at λλ3428, 3122, 3064, 2875, 2811 and 2608 A, of the CH́ molecule at λλ4317, 3888 and 3140 A, and of the C2 molecule at λλ 4737, 5165, 5635 A, etc., together with an extensive diffuse band system extending through the visible and near ultra-violet, first observed by Vaidya (1934) in the ethylene flame, and ascribed by him to the molecule CHO. Other observers have suggested the molecules H. CHO (Kondratjew 1936) and CH2 (Bell 1937) as the emitter of the system, without very satisfactory evidence of either chemical or spectroscopic nature. In addition continuous emission spectra of unknown origin are frequently observed, while an extensive diffuse banded system extending through the visible spectrum, and observed in the outer cones of many flames, has been shown by Fowler and Gaydon (1933) to be identical with the afterglow observed with CO in discharge tubes, and to be most probably emitted by excited CO2 molecules formed in the direct combination of CO and O2. The well-known γ bands of NO in the ultra-violet are frequently observed in intense flames when nitrogen is present. Rich, soot-forming flames of hydrocarbons exhibit a continuous spectrum having its intensity maximum in the infra-red, which is undoubtedly emitted by carbon aggregates raised to the temperature of the flame.