Knowledge Bank

$C2H$ is a linear radical found in a variety of terrestrial environments (flames, hydrocarbon photofragments), and is abundant in the interstellar medium. Analysis of its infrared spectrum is complicated by a low-lying electronic state $A~2\Pi$, about $3700cm-1$ above the $X~2\Sigma +$ ground state. Strong interactions between $A~2\Pi$ and $X~2\Sigma +$ make it difficult to assess the vibrational character of the upper states in observed spectra. In the course of near-infrared laser spectroscopic studies of a methane-helium discharge, we observed a $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma -\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma$ band with an origin near $6340cm-1$. Combination difference comparisons confirmed that lower state was the $000$ vibrational level of the ground state of $C2H$. The upper state may be a vibrational state of $X~2\Sigma +$, or of $A~2\Pi$ if a bending vibration is involved. We will present the rotational analysis of this band, and will discuss possible vibrational assignments.