High-Resolution Spectroscopy and Astronomical Detection of Molecular Anions

The importance of negative ions (anions) in astronomy was demonstrated in 1939 by Rupert Wildt who showed that H_ is the major source of optical opacity in the solar atmosphere, and therefore the material which one mainly sees when looking at the sun and similar stars. It is remarkable that in the many years since, during which nearly 130 neutral molecules and 14 positive molecular ions have been found in a variety of astronomical sources, no molecular anion had been identified, owing in large part to the absence of laboratory rotational spectra. This puzzle has been solved with our recent laboratory identification of the large molecular anion C6H_ as the carrier of a series of unidentified rotational emission lines in the circumstellar shell of the late carbon star IRC+10216, and its detection in the cold molecular cloud TMC-12 . Since then, high resolution spectroscopy of molecular anions and searches for them in astronomical sources is advancing at an astonishing pace. Five additional, closely related carbon chain anions (CCH_, C4H_, C8H_, CN_, and C3N_) were in the last year detected in our laboratory, and three of them have in the meantime also been detected with surprisingly high abundances in space using highly sensitive radio telescopes3 . In this talk I will briefly describe the spectroscopic techniques that we use to detect and characterize molecular radicals and ions via their high-resolution gas-phase spectra, and how these data can be used to identify these molecules in astronomical sources. I will then summarize the recent results on molecular anions in detail and outline how these new findings might improve our knowledge about the interstellar and circumstellar medium.