Origin of C₃ in Comets

Abstract

SEVERAL plausible parent molecules may be proposed for cometary species such as H, OH, C₂, CN and so on, but no parent molecule has been shown to produce C³ by primary photodissociation without the intervention of secondary collisions. Swings¹ has considered the possibility that the parent molecule is diacetylene (H—C≡C—C≡C—H), and Callomon and Ramsay² have observed C₃ in absorption in the flash photolysis of diacetylene diluted with a high pressure of He. They suggest that the primary process may be C₄H₂ + hν→C₃H + CH followed either by decomposition of C₃H to C₃ + H or by reaction of C₃H with CH to form C₃ + CH₂. The first alternative does produce C₃ without secondary collisions, but it could not be responsible for the observations in the flash photolysis experiments in a quartz vessel because the overall process C₄H₂→C₃ + H + CH is not energetically possible above approximately 1057 Å (ref. 3). An additional complication is that product analysis from the continuous photolysis of diacetylene has led to the suggestion⁴ of primary processes such as C₄H₂ + hν→C₂H₂ + C₂ and C₄H₂ + hν→2 C₂H, although C₂ was not seen in absorption in the flash photolysis experiments in isothermal conditions².