Gas phase spectroscopy of alkali carbides: The pure rotational spectrum of KC (X ⁴Σ^-)

The pure rotational spectrum of the KC radical in its X ⁴Σ^- ground state has been recorded using millimeter/submillimeter direct absorption spectroscopy. This study is the first gas phase observation of potassium carbide, and of any alkali metal carbide species. The molecule was produced under d.c. discharge conditions by the reaction of potassium vapor and CH₄; the vapor was generated in a Broida-type oven. Eleven rotational transitions were measured for KC in the frequency range 344-515 GHz; fine structure was resolved in every transition, which consisted of a quartet pattern. The data were analyzed using nonlinear least-squares methods in a Hund's case (b) basis, and rotational and fine structure parameters were accurately determined. The third-order contribution to the spin-rotation term was not found necessary for the data fit, although it has been suggested for states of quartet multiplicity and higher. The spin-spin interactions in KC appear to have a significant direct dipolar component. The bond length and electron configuration in KC have also been established, which suggest some covalent character to its bonding.