15min:

C. A. GOTTLIEB, M. C. MCCARTHY, M. J. TRAVERS AND P. THADDEUS, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138 and Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138.

Four new carbon chain radicals C₁₀H, C₁₂H, C₁₃H, and C₁₄H have been observed in a pulsed supersonic molecular beam with a Fourier transform microwave spectrometer. The radicals were produced in a discharge through a dilute diacetylene/neon mixture in the throat of a supersonic nozzle. All are found to be linear with ² electronic ground states, and all except C₁₄H have resolved lambda-type doubling. At least 10~rotational transitions, between 6 and 16~GHz, were measured in the lowest spin component --- ²_3/2 of C₁₀H, C₁₂H, and C₁₄H, and the ²_1/2 component of C₁₃H. Only three spectroscopic constants in the standard Hamiltonian for a molecule in a ² state were required to reproduce the spectra to a few parts in 10⁷: an effective rotational constant, a centrifugal distortion constant, and a lambda-type doubling constant. Detection of these highly unsaturated carbon chains establishes that C_nH radicals containing up to 14~carbon atoms are readily produced in a supersonic molecular beam. The relative abundance of C_nH radicals with an even number of carbon atoms is fairly constant from C₆H through C₁₂H. Although the new radicals are about two orders of magnitude less abundant than C₄H, the strong predicted ² - ² electronic transitions may be detectable in a supersonic jet by standard laser spectroscopic techniques.