The millimeter/submillimeter rotational spectrum of CaCCH(X²Σ⁺)

The pure rotational spectrum of the CaCCH radical (X²Σ⁺) has been measured in the laboratory using millimeter/submillimeter direct absorption spectroscopy. The species was created in a d.c. discharge by the reaction of calcium vapor and acetylene. Twenty-two rotational transitions of this molecule were recorded in the frequency range 230-380 GHz to an accuracy of ± 150 kHz. Spin-rotation splittings, but no proton hyperfine structure, were observed for CaCCH. The data were analyzed using a ²Σ Hamiltonian, and rotational and fine structure constants were determined. These measurements have resulted in accurate rest frequencies for CaCCH to be used for astronomical searches. A calcium-bearing molecule has yet to be observed in interstellar or circumstellar gas.