Flame Bands Research Articles

Chemiluminescence spectra from cool and blue flames: Electronically excited formaldehyde

Resolved emission spectra of electronically excited formaldehyde (1A2 → 1A1) have been obtained from two-stage ignition in a Vertical Tube Reactor combustion flow system, using acetaldehyde and n-butane as fuels. The very low intensity UV-visible emission from both the cool flame reaction zone (200–400°C) and blue flame reaction zone (400–800°C) is due to formaldehyde chemiluminescence. The “continuum” observed by previous researchers is due to formaldehyde band overlap, with typical hot flame emission from radicals, CO flame bands and continuum, and black body radiation being absent except for minor contributions in the acetaldehyde blue flame (800°C). Differences in the spectra band structure and underlying ‘continuum’ are due to the temperatures of the various flames. The cool flame formaldehyde emission yield per reacting fuel molecule is ∼10−8 for the fuels studied.

Shock-tube measurements of specific reaction rates in the branched-chain H 2-CO-O 2 system

Rate constants of four elementary bimolecular reactions have been determined by monitoring the exponential growth of CO flame band emission behind incident shocks in four suitably chosen gas mixtures. Results are as follows: OH+H2→H2O+H, k1=2.1×1013 exp (−5.1 kcal/RT) cm3 mole−1 sec−1 (1100°–1600°K); H+O2→OH+O, k2=1.25×1014 exp(−16.3 kcal/RT) (1150°–1400°K); 0+H2→OH+H, k2=2.96×1013 exp (−9.8 kcal/RT) (1300°–1600°K); and OH+CO→CO2+H, k5=4.2×1011× exp(−1.0 kcal/RT) (1300°–1900°K). Less directly, by analysis of early, linear luminosity and times to 50 percent of peak light intensity, the rate of the initiation reaction, CO+O2→CO2+O is estimated to be k6=1.6×1013exp(−41 kcal/RT).

Calculations of CO2 Energy Levels: The Ã1B2 State

The Ã1B2 state of CO2, identified by Dixon (1963) as the upper level of the ‘carbon monoxide flame bands’, must be of importance in the upper atmospheres of Venus and Mars. New calculations of the high vibrational levels of the ground state, which lead to improved fits of the observed vibration-rotation bands, confirm Dixon's analysis, except that the numbering must be lowered by two, and fix the energy of the v = 0, K = 0 level of 1B2 at 45210 ± 10 cm−1 = 5.605 eV.

Matrix-Isolation Study of the Infrared and Ultraviolet Spectra of the Free Radical HCO. The Hydrocarbon Flame Bands

The photoproduction of H or D atoms from a variety of sources in a carbon monoxide matrix or in an argon matrix to which a small concentration of carbon monoxide has been added leads to the appearance of prominent ultraviolet absorptions between 2100 and 2600 Å, all of which may be assigned to HCO or DCO. Both the CO-stretching and the bending vibrational modes are appreciably excited in the transition. Evidence is presented indicating that the transition observed in the matrix experiments is the same one responsible for the hydrocarbon flame bands. Using the frequencies observed in the matrix experiments, a tentative assignment for the hydrocarbon flame bands has been proposed which is in reasonable agreement with the observed band structure of the emission system. In the upper state, the carbon and oxygen atoms of HCO are approximately singly bonded. Observation of the infrared absorption frequencies of isotopically substituted HCO in an argon matrix has prompted reconsideration of the valence force field appropriate to ground-state HCO. Interaction between the CH-stretching and the CO-stretching modes has been found to play an important role. Factors leading to the stabilization of HCO in an argon matrix in the present experiments, in contrast to the results of previous studies, are discussed.

Rotational structure of some hydrocarbon flame bands

A number of hydrocarbon flame bands have been photographed in high orders of diffraction of a 3.4-m Ebert spectrograph, with an achieved resolving power of about 100,000. The spectrum contains both sharp and diffuse bands, and there are two distinct types of band envelope. Strong evidence is presented that the lower state of the hydrocarbon flame bands is the [graphic omitted], 2A′ ground state of the HCO radical, and that the A and B bands form two separate band systems. The origins are assigned as : [graphic omitted], 2A′ state, T00= 38691 cm–1, [graphic omitted] state, T00= 41270 cm–1. Partial rotational analyses of three bands in the A1 progression show that these are type a bands, and lead to the following structure for HCO in the [graphic omitted], 2A′ state : rCH= 1.16 Å(assumed), rCO= 1.36 ± 0.02 Å, HCO = 111 ± 4°.

Mechanism of Chemiluminescence of Atomic-Oxygen—Hydrocarbon Reactions. Formation of the Vaidya Hydrocarbon Flame Band Emitter

A kinetic study of the Vaidya hydrocarbon flame bands emitted in the O–C2H2 reaction has been performed in a conventional fast-flow system at about 300°K. The effect of the free-radical scavengers NO and O2, as well as various other additives, has been studied. The emitter appears to have a direct precursor which is formed in the reaction sequence before the direct precursors of the other near-ultraviolet and visible-light emitters and the chemi-ions. Previously suggested mechanisms for formation of the Vaidya band emitter are critically evaluated. The reactions O+CH2(X3Σg−, vibrationally excited)→CHO*+H and/or O+C2H2O*→CHO*+CHO best explain the results.

Carbon Dioxide Dissociation at 6000° to 11 000°K

The dissociation rate of CO2 has been measured for temperatures from 6000° to 11 000°K. The rate of disappearance of CO2 behind reflected shock waves in a 1%-CO2−99%-Ar gas mixture was monitored by observing infrared radiation from the 2.7-μ CO2 combination bands. The dissociation rates expressed in classical collision theory and an Arrhenius equation are (in cubic centimeters per particle·second), respectively, kAr=(8.95)(10−13)(D/kT)3.21 exp(−D/kT) and kAr=(1.92)(10−13)T12 exp(−2.96 eV/kT) where D=5.5 eV is the CO2 dissociation energy, k is the Boltzmann constant, and T is the absolute temperature. A rate constant was also determined from observations of the CO flame band emission at a wavelength of 3500 Å, which, at equilibrium, is proportional to the product of the CO and O concentrations. The rates determined from these measurements are about a factor of three lower than those obtained from the infrared measurements, and the process has an activation energy of 3.68 eV compared to the 2.96 eV for the infrared measurements. It is suggested that the infrared observations are a measure of the CO2 dissociation, while the flame band emission is controlled by the recombination of CO and O into an excited CO2 electronic energy level and the radiative life time of this CO2 level. A summary is given of CO2 dissociation measurements obtained with both methods over a temperature range from 2600° to 11 000°K.

Isotope effect in hydrocarbon flame bands (deuterium substitution)

By suitably modifying the apparatus for the production of the acetylene glow with atomic oxygen, a large number of deutero-hydrocarbon flame bands have been obtained in the spectrum and it has been possible to complete the analysis of the isotopic bands. These bands have been arranged into v ', v " arrays belonging to four groups. It has been shown that the first group of the isotopic system corresponds to the A group of ordinary hydrocarbon flame bands, while the bands in the second group are due to the CD stretching frequency and those in groups III and IV are due to the bending frequency. The problem of the emitter has been discussed and the evidence is strongly in favour of HCO.

The carbon monoxide flame bands

The carbon monoxide flame bands have been photographed under high resolution from an afterglow source. Bands in the wavelength range 3100 to 3800 Å show a pattern which has been reproduced by calculations of the energies of high vibrational levels of the ground state of CO 2 . The structure of this energy level pattern is strongly affected by extensive Fermi resonance in the 1 Σ + g state. The spectrum is emitted by excited CO 2 molecules which radiate to the ground state from the lowest vibrational level and from the v ´ 2 = 1 level of a B 2 state. This excited state lies approximately 46 000 cm -1 above the lowest level of the ground state, an d has an OCO angle of 122 + 2° and a CO bond length of 1*246 ± 0*008 Å. Combination of these results with the work of other authors shows that the excited state is a 1 B 2 state, and that the carbon monoxide flame bands are associated with the weak absorption system of CO 2 at 1475 Å.

The kinetics of the carbon monoxide flame bands

The intensity Ic of the carbon monoxide flame bands observed in the low-temperature reaction of O with CO was found to have similar kinetics to those of the O+NO and H+NO emissions, Ic=I0c[O][CO], where I0c depended upon the nature but not on the pressure of the carrier gas used as third body. I0c, and the rate constants k1c for overall combination of O and CO were found to be much less at 293°K than I0a and k1a (the analogous quantities in the O+NO reaction), but while k1a, and I0a showed small negative temperature coefficients, I0c was found to have a positive Arrhenius activation energy of +3.7 kcal/mole. Extrapolated values of I0a and I0c at 1/T=0 were similar. Most of the excited CO2 molecules formed in the third order combination process are quenched collisionally rather than radiatively. The absence of any effect upon the values of I0c when the third body used is in a triplet state (O2) shows that a spin reversal reaction is not rate determining. The observation of similar pre-exponential factors for I0a and I0c shows that the emission process in the O+CO reaction does not involve spin reversal. It is considered that the flame band emission is due to a transition from an excited singlet state to the (singlet) ground 1σ0+ state of CO2. Molecules in this excited singlet state are formed by a rapid radiationless transition from the triplet state of CO2 in which they are initially formed. The low value observed for I0c is then due to the presence of an energy barrier in forming the excited triplet and singlet state, and not to the spinforbidden nature of the overall combination reaction.

Carbon monoxide flame bands

Carbon monoxide flame bands

THE ABSORPTION SPECTRUM OF BO2

The boron flame bands have been observed in absorption during the flash photolysis of mixtures of boron trichloride and oxygen. Detailed analysis of the spectrum has shown that the bands arise from two electronic transitions in the linear symmetric molecule BO2, [Formula: see text] and A2Πu−X2Πg. The main molecular constants, in cm−1 except for r0, are summarized below:[Formula: see text]Both 2Π states show the Renner effect. In the ground state the Renner parameter, εω2, was found to be −92.2, whereas in the first excited state it is much smaller, −13.1 cm−1.

The emitter of hydrocarbon flame bands

The emitter of hydrocarbon flame bands

AN EMISSION SYSTEM OF THE IO MOLECULE

The "methyl iodide flame bands", lying in the region 4100 to 6300 Å, have been photographed using a 21-ft concave grating spectrograph. The bands are shown to arise from an A2Π → X2Π transition of the IO molecule. Rotational and vibrational analysis of the bands has been carried out and the molecular constants of IO obtained.

Emission and absorption spectra of flat flames

The multiple reflection absorption technique has been applied to flat flames. The presence of oxidation and pyrolysis intermediates in rich-limit flat flames has been shown by their ultraviolet absorption spectrum. Formaldehyde and benzene have been detected in flames of ether and hexane with air. There is also weak absorption in the region 2,750in the case of the hexane flame. This could be attributed to absorption by a cyclic radical or molecule. Early results of the application of the multiple reflection technique to low-pressure flames have given the absorption bands due to OH and NH. The emission spectra of the Egerton-Powlingflames of various fuels near the rich and weak limits of combustion are recorded. It is found that the emission spectrum of the rich diethyl-ether flame varies in a similar way to that indicated by Agnew and agnew but some details of a qualitative nature are added. The occurrence of the hydrocarbon flame bands and the possible relationship with the conditions of occurrence of the cool flame bands are briefly discussed. Observations of the emission and absorptionspectra of flames burnt on a porous plate burner are mentioned.

Spectra of cool flames and pre-ignition glows

Cool flames and pre-ignition glows of rich and weak mixtures of propane with air and oxygen have been studied in a simple flow system at atmospheric pressure. In the cool-flame region the spectrum shows the usual formaldehyde emission bands, but in the pre-ignition glow near the high-temperature ignition limit the spectrum is quite different, consisting mainly of hydrocarbon flame bands, due to HCO. With weak mixtures the HCO bands are accompanied by strong OH and some CH and CO-flame bands and the spectrum is rather similar to that of the hot flame. With rich mixtures, the HCO bands are accompanied by some CH and formaldehyde bands and weak OH but no C 2 , so that the spectrum is quite different from that of the rich hot flame, which is dominated by C 2 and CH emission and shows little HCO. The unusual observation of strong HCO with rich mixtures is discussed. It is suggested that the C 2 and CH emission occurs under conditions when free atoms and radicals diffuse back from the burnt mixture, while the HCO emission is associated with chain termination or thermal initiation processes. This supports the view that radical diffusion is important in flame propagation. No pre-ignition glow was observed with methane or formaldehyde. The pre-ignition glow of CO occurs readily at both rich and weak limits at atmospheric and low pressure and shows CO-flame bands (attributed to CO 2 ). Addition of methane or formaldehyde to the CO modifies the ignition limits and reduces the glow region, but with a little methane added to rich CO + O 2 , clear spectra were obtained which showed the cool flame bands of formaldehyde superposed on the CO-flame bands. Addition of formaldehyde instead of methane did not produce the formaldehyde bands. The reaction processes are discussed, and it is suggested that during oxidation of CO, free O atoms or O 3 are produced which may react with methane to result in cool flame processes. The relation of these observations to previous work on the auto-ignition of methane is discussed.

Green and orange band spectra of CaOH, CaOD and calcium oxide

The green and orange flame bands have also been obtained in a ‘vacuum’ arc in water vapour and heavy-water vapour. An isotope shift has been observed, supporting James & Sugden’s assignment of the flame bands to CaOH. Absence of additional bands from an arc in mixed H 2 O and D 2 O eliminates Ca(OH) 2 . The fine and gross structure of the CaOH and CaOD bands is described and discussed; the molecule is non-linear; the excitation process is believed to be thermal rather than chemiluminescent. These flame bands are not the same as the green and orange bands obtained with calcium salts in an arc in air. These latter are probably due to an oxide, but their structure is too complex for them to be due, as has been suggested, to transitions between singlet electronic states of CaO. It is possible that they may be due to transitions between triplet states of CaO, but under large dispersion the structure is so very complex that a polyatomic emitter appears more likely, and the dimer Ca 2 O 2 is favoured. The relevance of these results to determinations of the dissociation energies of alkaline earth oxides, to the use of the bands in spectro-chemical analysis and to the absence of CaO bands from stellar spectra is discussed.

Investigation of the Spectrum of the High Voltage Arc in Carbon Dioxide: the CO Flame Spectrum

The spectrum emitted by a high voltage arc in CO2 at atmospheric pressure has been investigated in the region 2000 A to 9000 A. It was found to consist, mainly, of a continuum and the `CO flame bands'. A study of the spectrum at different dispersions and on photographic plates of various kinds shows that although the Schumann-Runge O2 bands do form a part of the spectrum they are relatively weak compared with the bands attributed to CO2. The excitation of the Schumann-Runge O2 system in flames is discussed.

The spectra of chilled hydrocarbon flames

It has been found that the spectrum of the outer cone of hydrocarbon flames struck back in a water-cooled steel burner shows the hydrocarbon flame bands in the ultra-violet which are usually known as the ethylene flame bands. Under suitable conditions these bands are relatively very strong, being almost the sole feature of the spectrum. Chemical tests show that the occurrence of these hydrocarbon flame bands is closely associated with the formation of a peroxide, probably an alkyl peroxide. The bands are not necessarily associated with the presence of aldehydes in the interconal gases. Various catalysts have little effect on the spectrum. A brief discussion on the nature of the emitter of the hydrocarbon flame bands is given. The present observations favour the radical HCO as the emitter, this being formed perhaps by the breaking up of peroxides in much the same way as CH is known spectroscopically to be formed in the breaking up of hydrocarbons.

Spectroscopic observations on hydrocarbon flames in atomic oxygen

Flames of hydrocarbons, burning in atomic oxygen, have been reexamined, in order to find out whether the ethylene flame bands occur in such flames. The results indicate that they are strong in benzene and acetylene, but weak and diffuse in ethylene. C 2 , CH and HO are also present. Methyl alcohol gives the HO and CH bands and also ‘cool flame’ bands rather faintly, while formaldehyde shows only the HO band at λ 3064. The ethylene flame bands are absent from the flame of benzene burning in atomic hydrogen, which yields only C 2 and CH bands. The Balmer lines also appear, however, due to stray light from the main discharge.