Origin of the C (A³Πg)→C₂(X³Πu) Emission in Comets

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.