ROTATIONALLY RESOLVED SPECTRA OF THE B$^2$$\Pi$ - X$^2$$\Pi$ 0$_0$$^0$ AND $\mu$$^2$$\Sigma$ - $\mu$$^2$$\Sigma$ 11$_1$$^1$ TRANSITIONS OF C$_6$H AND C$_6$D

Abstract

The linear carbon chain radicals have been topic of a series of spectroscopic studies. The hexatriynyl radical C$_6$H (and deuterated equivalent C$_6$D), a member of the linear C$_n$H series, has attracted renewed interest in recent years after the astronomical identification of its chemically related anion C$_6$H$^-$ and its low-lying 11$_1$ $\mu$$^2$$\Sigma$ vobronic state. In this talk, rotationally resolved spectra of the B$^2$$\Pi$ - X$^2$$\Pi$ 0$_0$$^0$ and 11$_1$$^1$ transitions of both C$_6$H and C$_6$D are presented. Cavity ringdown spectroscopy is used to record the spectra in direct absorption through a supersonically expanding planar plasma. The $\mu$$^2$$\Sigma$ - $\mu$$^2$$\Sigma$ 11$_1$$^1$ vobronic hot bands are observed for the first time. Heavy rotational perturbations are found in the upper levels of C$_6$D. Precise spectroscopic parameters for the 11$^1$ $\mu$$^2$$\Sigma$ levels of both C$_6$H and C$_6$D are determined for the first time, and the spectroscopic parameters for the B$^2$$\Pi$ 0$^0$ states are also improved. The Renner-Teller interaction is also discussed to estimate the excitation energies of the low-lying 11$_1$ $\mu$$^2$$\Sigma$ vibronic states.