CAVITY RINGDOWN SPECTROSCOPY OF $HC_{7}H$ AND $HC_{9}H$

$^{a}$ J. Fulara, P. Freivogel, D. Forney, and J.P. Maier, J. Chem. Phys. 103, 8805 (1995).Author Institution: Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138 and Division of Engineering and Applied SciencesVibronic bands of the $^{3}\Sigma_{-} \leftarrow X^{3}\Sigma_{g}^{-}$ transitions of linear $HC_{7}H$ and $HC_{9}H$ have been detected in the gas phase using cavity ringdown spectroscopy in the wavelength range 500-600 nm. The $0^{0}_{0}$ bands of both molecules, as well as an additional vibronic band of $HC_{9}H$, 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 $data^{a}$. 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 $HC_{9}H$, the wavelength of the $0^{0}_{0}$ band agrees to within one part in $10^{4}$ 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 $v_{3}$ band at 521.91 nm