Abstract
THE observation of the C2 Swan bands (A3Πg→X3Πu) in emission in the coma of comets was at one time taken as evidence that the ground state of C2 was the X3Πu state since this cometary emission is known to arise by means of a fluorescence process. However, Ballik and Ramsay1 have shown that the ground state of C2 is x1Σ+g and consequently the observation of the Swan bands would be consistent with the following: (1) C2 is formed initially in the triplet system by decomposition (probably photodecomposition) of a hydrocarbon molecule. (2) The 3Πu→1Σ+g transition does not occur to an appreciable extent prior to fluorescence excitation by solar radiation or in the time between successive excitations. The latter implies that C2 experiences an insufficient number of collisions, or possibly no collisions, to effect the forbidden 3Πu→1Σ+g transition. This is not an unreasonable requirement since it is known that the rotational and vibrational distribution of C2 in cometary atmosphere corresponds to a temperature of about 2,500° K and that C2, therefore, must experience few if any collisions between the time of its formation near the nucleus and the time of its excitation farther out in the coma.
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References
Ballik, E. A., and Ramsay, D. A., J. Chem. Phys., 31, 1138 (1959).
Stief, L. J., DeCarlo, V. J., and Mataloni, R. J., J. Chem. Phys. (in the press).
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STIEF, L., DECARLO, V. Origin of the C (A3Πg)→C2(X3Πu) Emission in Comets. Nature 205, 1197 (1965). https://doi.org/10.1038/2051197a0
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DOI: https://doi.org/10.1038/2051197a0
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