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Published online by Cambridge University Press: 12 April 2016
At present only four polyatomic molecules or ions have been identified in the spectra of comets and their tails. They are C3, CO2 +, NH2, and H2O+. The first two are linear molecules. The C3 radical gives rise to the well-known 4050 group. It was first obtained in the laboratory in an interrupted discharge through CH4; was definitely identified by Douglas as being due to C3, and was later investigated in considerable detail in absorption in the flash photolysis of CH2N2. The complicated vibrational structure of this spectrum was first understood when it was realized that the bending frequency in the ground state is very low (64 cm-1) and that in the excited state the interaction of the vibrational angular momentum with the electronic angular momentum leads to large splittings (Renner-Teller splittings).