15min:

CARL A. GOTTLIEB, M. C. MCCARTHY AND P. THADDEUS, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; Division of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA 02138..

Following the detection of the linear carbon chain radical C₆H in space, many rotational lines in the X² electronic ground state have been observed in a low pressure discharge at millimeter wavelengths and in a supersonic molecular beam at centimeter wavelengths. In the course of a laboratory search for new reactive hydrocarbon molecules with 6~carbon atoms, several series of harmonically related lines with rotational constants near that of C₆H in the ground vibrational state were observed in the millimeter-wave band. On the basis of the close agreement in rotational constants and intensities, two of the series were assigned to ² and ² states of a low-lying excited bending vibrational level of C₆H. The standard Hamiltonian with five spectroscopic constants reproduces the observed rotational spectrum of the ² state, but several high-order distortion terms in the spin-rotation interaction are needed to reproduce the spectrum of the ² state of C₆H and C₆D. From the measured intensities of the rotational lines it appears that the ² state lies very close to ground but the ² state lies much higher in energy. A brief summary of the laboratory spectrum and applications to the astronomical observations will be presented.