C2 in Comet Halley - evidence for its being third generation and resolution of the vibrational population discrepancy

Abstract

The authors have used long-slit spectrophotometric observations of Comet Halley on five nights at heliocentric distances of 0.90 - 2.21 AU to determine the surface brightness distribution of the coma in the strongest C₂ Swan bands and the scattered light continuum. The C₂ observations indicate that C₂ must be the product of the decay of two precursors, rather than the one stage of decay previously accepted, or that one of the more complex chemical formation models applies. The authors have obtained the first clear-cut determination of the variation of the C₂ band sequence flux ratios with heliocentric distance. Those data indicate that the transition moment of the ground-state single-triplet transitions is 2.5×10^-6.