Hydrocarbon flame bands

It is possible, but has by no means been proved, that the lower state of the hydrocarbon flame bands is the same as that of the absorption bands just described. If the two systems do have a common lower state, the upper state of the flame bands would represent a third, more highly excited, electronic state of HCO. 

Laboratory and industrial-scale processes show that acetylene is one intermediate in carbon formation in the combustion of petroleum hydrocarbons. This is not only a result of thermal decomposition but a part of the complex of reactions occurring in the oxidation system. Steps resulting in the immediate production of acetylene seem to be molecular decomposition of molecules, or free radicals, or dehydrogenation, followed by combination or addition of oxygen. Peroxide formation may occur also. These reactions may be a general source of the hydrocarbon flame bands. 

In recent wrork particular emphasis has been given to studies of flame spectra and the evidence as to the formation and reaction of excited species such as C2, CH, OH, and HCO from acetylene and oxygen (17, 29, 31, 41, 43, 54). The occurrence of excited hydrocarbon flame bands attributable to HCO radicals led Herman, Hombeck, and Laidler (31) to suggest the reaction... 

Emission of the hydrocarbon flame bands, due to HCO, has recently been studied, along with chemi-ionization271. Again the mechanism is uncertain. 

D. E. Milligan and M. E. Jacox, Matrix isolation study of the infrared and ultraviolet spectra of the free radical HCO. The hydrocarbon flame bands, J. Chem. Phys. 51 277 (1969). 

As shown in Figure 9-2. important regions of a flame include the primary combustion zone, the interzonal region, and the secondary combustion zone. The appearance and relative size of these regions vary considerably with the fuel-to-oxidant ratio as well as with the type of fuel and oxidant. The primary combustion zone in a hydrocarbon flame is recognizable by its blue luminescence arising from the band emission of C-, CH, and other radicals. Thermal equilibrium is usually not achieved in this region, and it is, therefore, rarely used for flame spectroscopy. 

Similarly, indecisive results are obtained by the spectroscopic study of the luminous zone. A continuous band of colour is observed, and this would result whether the luminous particles were solids or dense hydrocarbon vapours. As is mentioned later (see p. 84), even the flame of hydrogen burning m oxygen under high pressures yields a continuous spectrum, and in this case the possibility of solid particles being present is entirely ruled out. 

The flame photometric detector is the principal component in the determination of sulphur compounds for which it offers a selectivity of about five orders of magnitude with respect to hydrocarbons. The selective sulphur detection is based on the formation of electronically excited S2 molecules in a hydrogen-rich flame. These short-lived species revert to their ground state and emit characteristic molecular band spectra with peak wavelengths at 384 and 394 nm. This chemiluminescent radiation passes an optical filter and is monitored by a UV-sensitive photomultiplier. 

---