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True colour optical emission spectrum of an oxyacetylene flame

flame spectrum

Combination of 4 photographs (on colour slide film) taken through a spectrograph. The exit slit is removed and the camera lens as well, so the light falls directly onto the photographic film. The photographs reveal in true colours the emission spectrum of the oxyacetylene (C2H2/O2) flame from the UV to the (visible) red. In contrast to the human eye, the colour slide film used is sensitive to the ultraviolet light (which it turns into blue). All lines visible in these photographs are identified in the graph shown below. Please note that the OH structure around 310 nm is shown twice (far left on a smaller vertical scale).

flame spectrum graph

Spectrum of the natural emission of the flame front, the acetylene feather, and the outer flame of the oxyacetylene flame. The last two are recorded about 2.5 mm below the tip of the flame front. Since the outer flame envelopes the flame front and acetylene feather, its spectral features can be distinguished in their spectra as well. The position for CN emission is indicated as well, but at the low resolution used here, it is masked by CH emission.

flame spectrum graph
Same graph in (almost) black and white (click graph to enlarge).

Short explanation of the (photographed) spectrum

The flame front of the oxyacetylene flame appears blue-green to the naked eye, and the acetylene feather looks a little more whitish. The emission of the flame can be dispersed to obtain a spectrum (Newton1730), which shows that the flame emits light from 250 nm (ultraviolet) to 800 nm (infrared), as is depicted in the figures above. The spectrum of the oxyacetylene flame and the origin of the excited species responsible for it are discussed in detail in the book by Gaydon (Gaydon1974), but the most important characteristics can already be seen in the figures and photographs above. Emission of the flame front consists of the C2 Swan system (between 434 and 720 nm) and the CH (A, B, C) ->X systems (main emission around 431, 389, and 314 nm, respectively). In the acetylene feather, emission of these two species is still the most important, but C3 emission around 405 nm, the Comet-Head group, becomes apparent as well. The most important radiating molecule in the outer flame is OH. Its emission lies between 260 and 350 nm and is so strong it causes actinic conjunctivitis ("welding blindness") if the human eye is not protected. Due to the high temperature of the flame, high rotational and vibrational levels of OH are well populated, which is why emission of the oxyacetylene flame has been used to tabulate OH transition frequencies to a large extent (Dieke1962).
  • I. Newton, Opticks, Book I, Part II, London, 1730
  • A.G. Gaydon, The Spectroscopy of Flames, Chapman & Hall, London, 1974
  • G.H. Dieke and H.M. Crosswhite, J. Quant. Spectrosc. Radiat. Transfer 2, 97, 1962