15min:

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

Vibronic bands of the ³ _u^- X³ _g^- transitions of linear HC₇H and HC₉H have been detected in the gas phase using cavity ringdown spectroscopy in the wavelength range 500-600 nm. The 0₀⁰ bands of both molecules, as well as an additional vibronic band of HC₉H, were measured. Both molecules were produced in a DC discharge through a dilute mixture of diacetylene in Ar in the throat of a supersonic nozzle. The identifications were confirmed by detecting the singly and doubly deuterated species of both molecules, and by comparison with previous matrix data. Neither molecule could be detected by laser-induced fluorescence. Each band exhibits expected P- and R-branches, and the band origins have been measured to 0.1 cm^-1 or better. For HC₉H, the wavelength of the 0₀⁰ band agrees to within one part in 10⁴ with that of the weak diffuse interstellar band at 581.86 nm. Astronomical verification of this assignment could be achieved by detecting the expected P- and R-branches in the DIB, or by detecting the somewhat weaker ₃ band at 521.91 nm.