Swan band

In addition to these laboratory-based experiments it is interesting to note that the Swan bands of C2 are important in astrophysics. They have been observed in the emission spectra of comets and also in the absorption spectra of stellar atmospheres, including that of the sun, in which the interior of the star acts as the continuum source. 

Fig. 7. Sonoluminescence of excited state Emission from the At = +1 manifold of the d Hg — H transition (Swan band) of Reproduced with...

Carbon stars earlier than C3 cannot be well detected due to the weakness of C2 Swan bands. A majority of the stars belong to C4 and C5 stars with high carbon abundance, while there are a few C8 and C9 stars. 

In diffusion flames of alkali metals with organic halides, emission from C2 (Swan bands) has been explained238 by... 

Dirty Flames. At this point one could well ask so what happens in real combustors which are turbulent, soot and particle laden and are highly luminous By the end of this morning s session you should be convinced that CARS can be applied to these systems. I don t want to steal all of Alan Eckbreth s slides so I will show only two more. Figure 13 shows the BOXCARS spectrum of N- with a computer fit to a temperature of 2000°K in a laminar sooting propane diffusion flame (12). Figure 14 shows the vertical temperature profile for this same flame system. It should be pointed out that care must be taken under these conditions to account for the laser interaction with carbon in the flame which can generate laser induced Swan Band emission from C2-... 

Figure 19. The laser-induced fluorescence excitation spectrum of the Ct swan band system in an acetylene-air flame (21)...

Figure 24 shows a plot of fluorescence intensity vs. laser power for the Swan Band system of C which we showed in Figure 19. It is apparent that fluorescence response becomes nonlinear at laser powers on the order of 1 joule/itr. However, it is equally apparent that the signal never reaches completg saturation (independent of laser power) even at 15 joules /m. ...

Existence. The existence of C13 was first proved by King and Birge2 in 1929 with a remarkable spectrogram of carbon vapor obtained by heating carbon up to 2,500°C in an evacuated furnace. A banded spectrum was observed with its head at 4,737.0 A. This is the so-called Swan band which is prominent in all emission spectra of carbon flames and which is due to the radical C2. Just at the side of this band King and Birge found a much weaker band identical in pattern but with its head at 4,744.5 A. They interpreted the band properly as being due to the diatomic carbon molecule Cl2-C13. 

The A sUgXmu Swan bands of C2 are emitted from several complex reaction systems. The potential curves for C2 [153] are shown in Figure 1.10. 

Palmer and his co-workers have observed more complex C2 emission from diffusion flames of alkali metals (Na or K) in haloforms and carbon tetra-halides [160-165], The Swan bands show a much broader v distribution than those emitted from other systems, and although the v = 6 level is sometimes excited preferentially, it is clear that more than one process excites the A 3I1B state in the diffusion flame reactions. Detailed interpretation is difficult and is hampered by a lack of precise thermochemical data for some of the species involved. Tewanson, Naegeli, and Palmer [165] have suggested that three quite distinct mechanisms may cause excitation (i) the association of C... 

The upper states of both emission bands can be populated by either direct excitation from the ground state of the corresponding molecule or by dissociative excitation from methane or higher hydrocarbons. Figure 4.9 shows a compilation of emission rate coefficients for the A2A — X 277 CH band (v = 0,1,2, 3 — v" = 0,1, 2, 3) and the C2 Swan band (v = 0 — v" = 0) [41]. There is experimental evidence that for CD the same rate coefficients than for CH can be applied [42]. The emission rate coefficient for direct excitation is several orders of magnitude higher than the emission rate coefficient for dissociative excitation. The dominant excitation mechanism depends on the... 

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)]. 

Photolysis of C3O2 at 193 nm shown to produce C2 514 Swan band emission in addition to C2O, in contrast to behaviour observed following 248 nm excitation. Lifetime studies indicate that the Cj may be formed in secondary processes... 

At long time delays, a broad background emission is observed with superimposed features characteristic of the Swan bands of Cj... 

Hydroxyl emission sources have been applied also in shock tube studies of hydroxyl radical reactions " . In this case the required time resolution (see refs. 18, 19) was obtained by using a flash discharge in water vapour. Similar methods have been described in which the Cj Swan bands are excited by a discharge in butane . 

The oldest of the spectroscopic radiation sources, a flame, has a low temperature (see Section 4.3.1) but therefore good spatial and temporal stability. It easily takes up wet aerosols produced by pneumatic nebulization. Flame atomic emission spectrometry [265] is still a most sensitive technique for the determination of the alkali elements, as eg. is applied for serum analysis. With the aid of hot flames such as the nitrous oxide-acetylene flame, a number of elements can be determined, however, not down to low concentrations [349]. Moreover, interferences arising from the formation of stable compounds are high. Further spectral interferences can also occur. They are due to the emission of intense rotation-vibration band spectra, including the OH (310-330 nm), NH (around 340 nm), N2 bands (around 390 nm), C2 bands (Swan bands around 450 nm, etc.) [20], Also analyte bands may occur. The S2 bands and the CS bands around 390 nm [350] can even be used for the determination of these elements while performing element-specific detection in gas chromatography. However, SiO and other bands may hamper analyses considerably. 

In carbon tetrachloride/sodium (or potassium) flames, emission of the C2 Swan bands is observed [119, 120]. Naegeli and Palmer [121] have directed attention to the routes for production of excited Cj and propose... 

---