Discussion of the “Forbidden” Intercombination Transition of C₂ in Comets

Abstract

THE only comet emissions known to arise from the C₂ radical are the Swan bands of the triplet electronic system (A ³II_g→X′³ II_u) (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 nucleus^2,3. 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 II_u↔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′ ³II_u 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.