Gas phase spectroscopy of alkali carbides: The pure rotational spectrum ofKC(X (4)Sigma(-))

The pure rotational spectrum of the KC radical in its X (4)Sigma(-) ground state has been recorded using millimeter/submillimeter direct absorption sp ectroscopy. This study is the first gas phase observation of potassium carb ide, and of any alkali metal carbide species. The molecule was produced und er d.c. discharge conditions by the reaction of potassium vapor and CH4; th e 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 da ta were analyzed using nonlinear least-squares methods in a Hund's case (b) basis, and rotational and fine structure parameters were accurately determ ined. 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 qu artet multiplicity and higher. The spin-spin interactions in KC appear to h ave a significant direct dipolar component. The bond length and electron co nfiguration in KC have also been established, which suggest some covalent c haracter to its bonding. (C) 1999 American Institute of Physics. [S0021-960 6(99)01909-1].