2010ApJS..189..261G

Rotational spectra of the linear carbon chain radical C₆ H in two low-lying excited vibrational states were observed both at millimeter wavelengths in a low-pressure glow discharge and at centimeter wavelengths in a supersonic molecular beam. Two series of harmonically related lines with rotational constants within 0.3% of the ²Π ground state were assigned to the ²Σ and ²Δ vibronic components of an excited bending vibrational level. Measurements of the intensities of the lines in the glow discharge indicate that the ²Σ component lies very close to ground, but the ²Δ component is much higher in energy. The standard Hamiltonian for an isolated ²Δ state with five spectroscopic constants reproduces the observed rotational spectrum, but several high-order distortion terms in the spin-rotation interaction are needed to reproduce the spectrum of the ²Σ component in C₆ H and C₆D. The derived spectroscopic constants allow astronomers to calculate the rotational spectra of the ²Σ and ²Δ states up to 260 GHz to within 0.1 km/s or better in equivalent radial velocity.