Flames excited species

Flames are also plasmas, characterized by electron densities of about 10 /cm and electron energies of about 0.5 eV. Many excited species are present in the flame, namely free radicals, ions, excited atoms and molecules, and electrons [43]. Excited species that have been observed include O, OH, NH, NO, and CH [44]. 

Dependence of Flame Species Concentrations upon Additive Concentrations. A method of determining the dependence of various ionic, neutral molecule, and excited species concentrations on the concentration of hydrocarbon added to a hydrogen/oxygen or hydrogen/air flame (based on a principle similar to that of flame ionization detectors... 

Flame photometric (FPD) Light emission of excited species in flame 2-50 pg/s 3 Compounds containing P and S... 

Ca brick-red, Sr carmine-red, etc). If a flame can be kept burning uniformly for an extended period of time and material fed into the flame at a const rate, the intensity of the spectral line or band will be a measure of the concn of the substance. The wave length of the emitted light will permit identification of the excited species... 

This method is based on the emission of light by atoms returning from an electronically excited to the ground state. As in atomic absorption spectrometry, the technique involves introduction of the sample into a hot flame, where at least part of the molecules or atoms are thermally stimulated. The radiation emitted when the excited species returns to the ground state is passed through a monochromator. The emission lines characteristic of the element to be determined can be isolated and their intensities quantitatively correlated with the concentration of the solution. 

CA 83, 82252 (1975) [The system RFNA/ UDMH was examined in a rocket motor of lOOdaN thrust. The optimum pressure for this system was found to be 7 x 106 Pascals. At one atm the H2—02 flame was almost invisible, but under increased pressure a bright, bluish-white light was emitted because of a flame continuum. This flame continuum was concluded as being the emission from transient, excited species characterized by the nonequil state] Compatibility. 1) W.K. Boyd et al, Compatibility of Materials with Rocket Propellants and Oxidizers , DM C Mem 201, Battelle Mem Inst, Columbus, Contract AF 33 (615)-l 121 (1965) CA 67,23666 (1967) [UDMH compatibility data reported is summarized in Table 1. The compatibility data for UDMH/hydrazine (50/50 wt %) is summarized in Table 2. The explanation of the numeric evaluation code used in these tables for metals is presented in Table 3 for nonmetals in Table 4 2) M.J. Spanger T.J. Reinhart, Jr, Development of Filament-Wound Tankage for Rocket... 

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... 

Comparison of emission spectra between 2100 A and 6500A has shown only small differences in relative concns of excited species between low-pressure diffusion flames and explns, whereas during explns peak intensities may be as much as 100 times greater. The time dependence of the free-radical emission during expln indicates the formation sequence to be OH, CH, C2, and evidence for the forbidden CO Cameron bands has been obtained. Similarly the ultraviolet absorption spectrum of the OH radical in acetylene— H2—02 detonations has been measured in conjunction with the associated rarefaction waves (Ref 7). Analysis of the absorption spectrum has indicated average rotational temps greater than 3000°K during the initial 310 microseconds... 

Mention has already been made of beam experiments in which electronic chemiluminescence has been observed from reactions of metal atoms. Palmer s [423-426] and Broida s [427,428] groups have studied reactions of this type in low-pressure diffusion flames. In most cases, the yield of excited species is disappointingly low. The most efficient of the reactions investigated so far (this excludes the reactions of the type X + M2 - MX + M studied by Herschbach s group) appears to be... 

The importance of electronically excited states in reaction kinetics is well established [14]. Electronic excitation leads to qualitative as well as to quantitative modifications in the preceding theories, especially in connection with the intersection of the potential-energy surfaces corresponding to different electronic states. Structures of electronically excited activated complexes have been studied (for example, [55]) and have been used in postulating kinetic mechanisms for the production of nonequilibrium excited species that have been observed (for example, [56]) in hydrogen-oxygen flames. 

The energy of excitation may arise from chemical reaction, by absorption of light, or by thermal excitation. Chemical reaction in flames gives rise to electronically excited species, and it is the emission from these excited states that give rise to the characteristic flame b2mds . ... 

The production of excited species in flames has already been mentioned in Section 1.3. A hydrogen-oxygen flame exhibits a well defined band system in the near ultraviolet, which has been shown to originate from electronically excited hydroxyl radicals . A very wide variety of emitting species has been identified in flames we are not concerned here with the chemical implications of the results obttiined, and the reader must be referred to more specific articles which have appetu ed elsewhere . 

A further requirement for measurement of absolute concentrations of excited species in flames is that the volume from which emission is collected be known. The simplest experimental arrangement for flames at atmospheric pressures is to focus the radiation from the flame onto the entrance slit of a spectrograph. Reasonable assumptions can be made about the thickness of the emitting layer, and Ausloos and van Tiggelen have used the arrangement successfully in semi-quantitative determinations of excited OH, NH, NO and NH2 in flames emitting the bands of these species. 

Studies of low-pressure flames offer several advantages. In particular, the flame can be maintained flat, and the light from different parts of the reaction zone studied separately the reaction volume from which light is collected is determined with much greater accuracy for such flames. At low pressures, chemiluminescent processes are more important than thermal excitation, collisional quenching of excited species is reduced, and self-absorption is diminished. A typical investigation of the low pressure flame is that of Gaydon and Wolfhard quantitative measurements of the C2 emission were made. 

So far, we have been concerned mainly with emission of radiation from electronically excited states. Emission may also arise from vibrational transitions in various reaction systems. The species HO2 has long been postulated as an important chain carrier in combustion reactions, although emission from electronically excited HO2 has yet to be demonstrated unequivocally. However, Tagirov has observed radiation in flames at a frequency of 1305 cm which he ascribes to transitions from vibrationally excited HO2. Investigations of vibrational quenching processes are of great interest, and if the vibrationally excited species emit infrared radiation, then emission spectrometry may be the most satisfactory way of following the reaction. Davidson et describe a shock-tube study of the relaxation of... 

Table 28-2 lists the common fuels and oxidants employed in flame spectroscopy and the approximate range of temperatures realized with each of these mixtures. Note that temperatures of 1700°C to 2400°C are obtained with the various fuels when air serves as the oxidant. At these temperatures, only easily excitable species such as the alkali and alkaline earth metals produce usable emission spectra. For... 

In atomic emission spectroscopy, the analytical signal is produced by the relatively small number of excited atoms or ions, whereas in atomic absorption the signal results from absorption by the much larger number of unexcited species. Any small change in flame conditions dramatically influences the number of excited species, whereas such changes have a much smaller effect on the number of unexcited species. 

Flame Photometric (FPD) Flame excitation produces chemiluminescent species of S-and P-hydrocarbons 1-20 pg 103 for S lO-iforP High selective forS,P... 

By virtue of their enormous size and comparatively high stability against radiative decay, high Rydberg states are very susceptible to collisions. Indeed, some of the detectors which are used to observe them, such as the thermionic diode (see section 8.16) depend for their operation on the presence of collisions. Collisions between excited species affect ionisation and recombination rates in such diverse environments as gaseous nebulae [41], laboratory plasmas [42] and flames [43] their study is therefore of some considerable intrinsic interest. 

The detector is extremely sensitive to temperature and flow changes as well as the Hj/02 ratio in the fuel-rich flame. Deactivation of the excited species by collision with organic radicals can cause signal quenching when high hydrocarbon concentrations co-elute. 

Flame photometric detector FPD, a selective GC detector for sulphur and phosphorus containing compounds. Separated components pass into a hydrogen-rich flame where they undergo a series of reactions to produce excited species HPO and S2. The resulting atomic emission spectrum is monitored using narrow band pass filters (526 and 394 nm, respectively) and a photomultiplier detector, sensitivity is 10 to 10 " gs . ... 

In Eq 1, sulfur compounds are combusted to sulfur monoxide (SO) and other products. In Eq 2, the second step of the mechanism, light energy (hv) in the blue region of the spectrum is emitted from the excited species resulting from the ozone reaction. The basic mechanism of the DP-SCD is the same as that described above, but two plasmas (flames) instead of one are provided to improve selectivity and the ability to measure lower sulfur levels without hydrocarbon interferences. A conceptual drawing of the flow dynamics used in the Dual Plasma burner is shown (Fig. 1). 

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