Discussion of the ``Forbidden'' Intercombination Transition of C2 in Comets

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THE only comet emissions known to arise from the C2 radical are the Swan bands of the triplet electronic system (A 3IIg-->X'3 IIu) (ref. 1). The absence of definitive evidence for singlet system emissions has led to speculation that C2 must be formed directly in the triplet state either by a primary photochemical process or by direct sublimation from the icy nucleus2,3. These speculations have been supported by the widely accepted view that the singlet and triplet states of C2 cannot be exchanged by either a radiative or a collsional process. Because the radiative intercombination transition for C2, X'3 IIu<-->x 1Σ+g, is ``forbidden'', it has been assumed not to occur. More specifically, Ballik and Ramsay4, state that a lifetime of several days would not be unlikely for this transition. Because the gas density in the coma is very low, collsional processes have also been assumed not to occur. The exclusive observation of triplet C2 is particularly disturbing because the x 1Σ+g state has been shown to be the ground state and the observed triplet system originates from the X' 3IIu state which is 610 cm-1 above the ground state4. This emission is in contrast to all other emissions which are observed in fluorescence from the ground state.

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