Flames rotational excitation

High speed emission spectroscopy has been used to study free radicals and positive, negative, and multiple ions produced in explosions and flames. Many excited states would exist for many different species from coal subjected to high energy. Complex spectra would result. The combination of electronic-vibration-rotation transitions observable in emission spectroscopy... 

At such a pressure, the excited molecules undergo many collisions before radiating the questions addressed here concern both a description—in terms of state-dependent energy transfer rates—and a diagnostic exploitation of those collisions. In the experiments, OH in the partially burnt gases of a methane-air flame is excited to individual rotational levels of the v7 =0 vibrational level of the A2 state, and measurements of the resulting fluorescence dispersed through a monochromator provide populations of individual levels. 

Figure 4. Portions of the rotationally resolved fluorescent emission in the (0,0) band following excitation of individual rotational levels within v = 0 of the A2X state of OH, in an atmospheric pressure CHk-air flame. Top excitation into N = 10, J = 21/2 bottom excitation into N = 1,...

Rotational Excitation of OH. One of the most surprising aspects of our data was the observation of rotationally hot OH in the flame front of (() = 1.28 and <() = 1.50 flames. Rotational temperatures " 200 K higher than radiation corrected thermocouple measurements were observed these were not expected since rotational energy transfer is so fast at atmospheric pressure. Such excitation was not observed beyond the flame front in any of our ammonia flames and not even within the flame front of a methane... 

Noble Gases. Flames of HN3 and electronically excited Ar, Kr, or Xe emit strong bands of rotationally excited NH(A 11). Additional bands of N2(B Ilg) are found in flames with Ar and Kr [1]. 

It is uncertain to what extent thermal equilibria are achieved in different parts of the flames. — A number of procedures are (in principle) available to determine flame temperatures The immediate measurement, for example by thermocouples, the thermochemical calculation, line reversal methods for electronic excitation temperatures, determination of vibrational or rotational temperatures. In addition more recent methods like advanced Raman techniques may be applied. 

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... 

Figure 5. Fluorescence scans of emission following excitation of N = 4, J — 9/2 of the V = 0 level in OH in a CHk-air flame. Top (1,0) band fluorescence, emitted by molecules collisionally transferred upwards to v = 1 bottom two rotational lines in the (0,0) band, emitted by molecules in the N = = 12 level of V — 0. Both scans are on the same intensity scale.

Figure 22. The fluorescence excitation spectrum of the MgO B A Jn transition under conditions described in Figure 21. Rotational analysis of the spectrum demonstrates that the A 2n metastable is thermalized relative to the bulk flame temperature.

Laser-induced fluorescence is a sensitive, spatially resolved technique for the detection and measurement of a variety of flame radicals. In order to obtain accurate number densities from such measurements, the observed excited state population must be related to total species population therefore the population distribution produced by the exciting laser radiation must be accurately predicted. At high laser intensities, the fluorescence signal saturates (1, 2, 3 ) and the population distribution in molecules becomes independent of laser intensity and much less dependent on the quenching atmosphere (4). Even at saturation, however, the steady state distribution is dependent on the ratio of the electronic quenching to rotational relaxation rates (4, 5, 6, 7). When steady state is not established, the distribution is a complicated function of state-to-state transfer rates. 

Results showing the dependence of the CO collision halfwidth in combustion gases on the vibrational and rotational quantum numbers are shown in Figure 6. The data were obtained with a flame temperature of 1875 K and equivalence ratios in the range 1.2 - 1.4. Although too few data points are available for a detailed analysis, it is clear that 2y decreases with increasing m and that values for 2y are nearly equal (within 5%) for ground state and excited state transitions. 

Excitation of the outer ns electron of the M atom occurs easily and emission spectra are readily observed. We have aheady described the use of the sodium D-line in the emission spectrum of atomic Na for specific rotation measurements (see Section 3.8). When the salt of an alkali metal is treated with concentrated HCl (giving a volatile metal chloride) and is heated strongly in the non-luminous Bunsen flame, a characteristic flame colour is observed (Li, crimson Na, yellow K, lilac Rb, red-violet Cs, blue) and this flame test is used in qualitative analysis to identify the M ion. In quantitative analysis, use is made of the characteristic atomic spectrum in flame photometry or atomic absorption spectroscopy. 

Spark, flame and plasma excitation are applicable to the direct excitation of solution samples. Osumi et al. (1970) used spark excitation to a rotating disc electrode to obtain rare earth detection limits of 2 to 40 ppm in complex-spectra rare earth elements. They used a controlled atmosphere and the rare earth solution contained 60% methanol. The introduction of solutions into spark discharges via porous graphite electrode (Fadeeva and Karpenko, 1972) or as aerosols (Karpenko et al., 1974) has also provided satisfactory analyses. 

RotationaUy resolved spectra of molecules present in the flame allow for temperature determination of the gas phase [3]. The emission intensity, of a rotational-vibrational band of an excited molecule correlates to the temperature of the molecule, T, via the following equation [34]... 

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