Light Emission from Flames

STATE SELECTIVITY IN LIGHT EMISSION FROM FLAMES... 

Despite the many observations of these phenomena, there remain many unanswered mechanistic questions. Nearly all of the identifiable light emission from flames is from small fiee radicals, and the formation of the radicals in electronically excited states demands energetic precursors. Thus the reactants are also free radicals many of the likely reactions form the highly stable CO molecule as one of the products. Only fairly recently [6] were any mechanistic pathways established for the production of excited OH and CH in one low-pressure flame, using absorption measurements of ground state radical concentrations but, for at least CH, even that mechanism has been questioned in a different series of experiments [7]. 

Figure 29.13 Time-averaged visible light emission from excited afterburning jets. The shaded area shows the fluctuation in the natural flame intensity...

The light emission from ether cool flames has been studied by Ouellet and Ouellet [73]. The total emission increases with reactant concentration, and the duration increases with diameter of the reaction vessel. Raising the ambient temperature modifies the way the emission varies with time. 

Merely from the fact that even a true black body would yield less than half this amount (Table 7) at an estimated flame temperature of about 2500it becomes obvious that in intense pyrotechnic radiation thermal grey-body emission is heavily augmented by selective radiation and luminescent phenomena. This can be experimentally demonstrated by a comparison of light emission from binary mixtures where various alkali salts act as oxidizers. Table 9 (from Table 13.7, Shidlovsky ) shows these relations. He uses a fixed ratio of 40 % metal fuel and 60% of the nitrates of sodium, potassium, or barium. It is of course not... 

A second physical method that has been used for analysis of SO2 in air is flame photometry. The sample is burned in a hydrogen-rich flame with light emission from the S2 species at 394 nm being detected by a PM tube. However, since this wavelength is characteristic of sulfur, any other sulfur-containing species can interfere, and this method has largely been superseded by the gas-phase fluorescence method described above. 

Incandescence - light emission from a heated object, such as a lamp filament or hot soot in a candle flame. See also black-body radiation in Section 1.4. 

Historically, the first spectroscopic studies involved characterizing the emission of visible light from the sun, from flames, and from salts added to flames. Our survey of spectroscopy, however, begins with absorption because it is the more important technique in modern analytical spectroscopy. 

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