Laboratory detection and pure rotational spectrum of the CaC radical (X³Σ^-)

The calcium carbide radical, CaC, has been observed in the laboratory for the first time using millimeter/submillimeter direct absorption spectroscopy. The species was created in the gas phase by the reaction of calcium vapor and methane under extreme DC discharge conditions. Eleven rotational transitions of CaC in its X³Σ^- ground state were recorded in the frequency range 247-536 GHz. Each transition of CaC was found to consist of triplets due to fine-structure interactions that indicate that the ground state is ³Σ^-, as opposed to ⁵Σ ^-, both of which have been suggested by theory. The data were analyzed in a Hund's case (b) basis, and rotational, spin-spin, and spin-rotation constants have been accurately determined. CaC may be detectable in circumstellar envelopes of asymptotic giant branch stars, in particular those that are carbon-rich.