The Millimeter-Wave Spectrum of NiC (X¹Σ⁺) and CoC (X²Σ⁺)

The pure rotational spectra of two metal carbide species, NiC (X¹Σ⁺) and CoC (X²Σ⁺), have been measured in the laboratory using millimeter/submillimeter direct absorption methods. The molecules were created by reacting metal vapor with CH₄ in a DC discharge. Four rotational transitions of CoC were recorded, each consisting of 16 hyperfine components arising from the cobalt nuclear spin of I = 7/2. Multiple transitions of the two most abundant nickel isotopomers, ⁵⁸NiC and ⁶⁰NiC, were additionally measured. The spectra were analyzed using the appropriate Hamiltonian and spectroscopic constants accurately determined. For CoC, this study included a complex hyperfine analysis of the magnetic, electric quadrupole, and nuclear spin-rotation terms. Cobalt and nickel are iron peak elements that may be produced via neutron capture processes in asymptotic giant branch stars. Dredge-up events mix these elements to the stellar surface, where they are incorporated into the expanding stellar envelope. CoC and NiC offer a means to trace these elements in carbon-rich circumstellar shells.