Excitation formaldehyde

The blue luminescence observed during cool flames is said to arise from electronically excited formaldehyde (60,69). The high energy required indicates radical— radical reactions are producing hot molecules. Quantum yields appear to be very low (10 to 10 ) (81). Cool flames never deposit carbon, in contrast to hot flames which emit much more intense, yellowish light and may deposit carbon (82). 

Decomposition of this intermediate, the Criegee split, forms an aldehyde, which is electronically excited formaldehyde in the case of a terminal alkene, and a diradical,... 

Another chemiluminescence method for monitoring ozone involves the production of electronically excited formaldehyde in the 03 reaction with ethene ... 

Light emission was recorded with an IP 21 R.C.A. photomultiplier tube in the range 3000-7000 A. Interference filters (spectral bandwidth A A = 100-140 A.) showed that the light emission of the pic darret arises from the same emitter as that from the slow reaction—probably excited formaldehyde (16). The pic darret appears as a clear pulse on the record of light intensity vs. time (Figure 1). 

Since the burst of illumination is caused by excited formaldehyde, as we have previously shown (15, 16), we should observe an increase in formaldehyde unless it is formed in small amounts in a very energetic reaction such as ... 

The nature of the radiation processes is not fully understood. Ball (10,11), with the aid of a stroboscopic shutter, visually observed cool flames as actual flame fronts moving across the combustion chamber of a motored engine. This was later confirmed by Getz (53). The source of cool flame emission in tube experiments has been attributed to excited formaldehyde by Emeleus (51) and Gaydon (52). Cool flame spectra in engines obtained by Levedahl and Broida (70) and Downs, Street, and Wheeler (35) were reported to be due to excited formaldehyde. The nature of the blue flame spectra has not been fully explored, although some evidence points to carbon monoxide emission (35). 

The light emission is caused by excited formaldehyde, which is believed to be formed in highly exothermic termination reactions such as reactions (46), (47) and (48), since its concentration does not increase as would be... 

The premixed methanol flame [11, 12] does not show the Swan bands of C2, which are prominent in a methane flame [13]. The base of the flame shows strong emission from excited formaldehyde and further up the flame emission from OH and CH occurs. The burning velocity of a stoichiometric methanol—air flame [12] is about 45 cm. sec", and the global activation energy and global order are 43—47 kcal. mole" and unity, respectively [14(a)]. 

The emission spectrum of the first-stage flame is truly that of excited formaldehyde [78, 79] whereas that of the second may be due to either the same emitter or to the formyl radical, depending primarily on the initial mixture ratio. 

In a static system, the decomposition can proceed in three ways, explosion, chemiluminescent reaction and decomposition without glow [131]. The glow which is probably due to excited formaldehyde is detectable even when the nitrate is diluted between 10 and 10 times with inert gas. 

Table I summarizes the primary features of the chemiluminescent emission spectra obtained from the reaction of ozone with 14 simple olefins. The observed spectra fall into three classes which correlate somewhat with the olefin structure. Class A in Table I includes the three terminal olefins studied all gave a broad, weak emission, peaking at about 440 nm. Figure 2 shows the spectrum obtained in the reaction of ozone with ethylene, a typical member of class A, at a total pressure of 0.4 torr. The emission spectrum may result from excited formaldehyde [emission...

An interesting didactic article by Henderson reanalyses the conventional assignment electronic transitions in carbonyl compounds. A study of the molecular dynamics of excited formaldehyde molecules dissolved in water by time-resolved... 

Figure 6.16 Stages in the deactivation of an electronically excited molecule. Schematic description of stages in the deactivation of electronically-excited formaldehyde (a) the conversion of electronic excitation into vibration, initially localized between the C and O atoms (wavy lines), followed by (b) transfer of vibrational eneigy to the CH bonds, and (c) transfer to translational motion of the solvent. The numbers 2, 3a and 3b refer to the processes mentioned in the text and in Figure 6.15. After Ref. [34,b].

The extent of conversion of reactants and the product composition is measured by continuous withdrawal of samples via a very fine probe to a mass spectrometer. Light output associated with oscillatory "cool flames" (a chemiluminescent emission from excited formaldehyde] is detected by a photomultiplier through a window in the wall of the oven. Continuous and simultaneous measurements are thus made, and since the system is "well-stirred", what is measured at one location by the thermocouple, or by withdrawal to the mass spectrometer, is the same as that at any other point. These measurements are time-dependent and so at a stationary state (sn or sf in the phase-plane] they will be invariant at an oscillatory state (uf in the phase-plane] periodic phenomena will be observed. 

Excited diphenyl sulfone and excited formaldehyde might transfer their excitation energy to the fluorescer. 

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