Laboratory Rotational Spectroscopy of CaC₃N and CaC₄H

Our understanding of the roles metal-bearing molecules play in interstellar chemistry is limited by the lack of rotational spectroscopic data crucial for astronomical identification/detection. Although calcium is the second most abundant alkaline earth metal in space (after magnesium), only two calcium-bearing molecules have been detected in space, both in the circumstellar envelope of the evolved carbon-rich star IRC+10216. In this work, we report the rotational, fine, and hyperfine structure of two linear calcium-bearing molecules, CaC3N and CaC4H, in their 2Σ+ ground electronic states. These findings both confirm the hypothesized ionic metal–ligand bonding characteristics of this class of molecules that make them potential candidates for optical cooling applications and point the way to detecting these and other calcium-bearing molecules in carbon-rich stars.