Vibrationally excited C6H

Abstract

Rotational spectra of the linear carbon chain radical C6H in two lo w-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 ithin 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 C6H 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-1 or better in equivalent radial velocity. © 2010. The American Astronomical Society. All rights reserved.