Laboratory Spectroscopy and Circumstellar Discovery of New Metal Dicarbides

The chemical evolution of the alkaline earth elements begins in the atmospheres and circumstellar envelopes of evolved AGB stars, but their fate during this period of intense stellar activity is poorly understood. Characterizing the incipient chemistry, molecular properties, and astrophysical distribution of the smallest, earliest formed metal-bearing clusters is thus critical to elucidating the fate of refractory elements more generally. I will discuss a significant advance in our understanding of these processes with a joint laboratory, theoretical, and astronomical study of several new metal dicarbide species. We have synthesized in the laboratory and measured the high-resolution microwave spectra of the alkaline earth metal-bearing molecules MgC₂, CaC₂, and SrC₂, as well as the closely related rare earth species YbC₂. The precise semi-experimental equilibrium geometries, derived from extensive isotopic measurements, show that each molecule has a highly ionic, T-shaped bonding arrangement. The laboratory rest frequencies have enabled the identification of MgC₂ and CaC₂ as the carriers of several strong, previously unassigned radio emission lines in the circumstellar envelope of the well known evolved carbon-rich star IRC+10216. These discoveries yield fundamental insights into the chemical structure and bonding of s- and f-block metal compounds, and place critical new constraints on the postulated astrochemical pathways that incorporate metal atoms into complex polyatomic molecules. This work suggests that larger metal-carbon clusters may now be detectable in the laboratory and in circumstellar environments, providing a unique probe of the chemistry of refractory elements.