Vibronic coupling in a potential astronomical molecular ion
Cyanoacetylene, HC3N, is a molecule widespread in the universe, despite its apparent exotic nature. First detected more than fifty years ago in interstellar clouds, it was discovered in proto-planetary disks, other galaxies, and even in the icy clouds of Saturn´s moon Titan. Its ionized variant, HC3N+, though, is much more elusive: it has only rarely been studied in the laboratory, and due to the lack of accurate laboratory spectroscopic fingerprints, has not been detected in space, yet.
Researchers at HMFL-FELIX have now for the first time measured the vibrational spectral fingerprint of the cyanoacetylene cation making use of the wide tunability of the FELIX free-electron lasers from the mid- to far-infrared. For this, they needed to isolate the reactive molecules in an ion trap, and cool them down to cryogenic temperatures. They teamed up with the Theoretical Chemistry group at the Institute for Molecules and Materials (IMM) to understand the observed intricate splitting pattern in the vibrational spectrum due to vibronic coupling.
These splittings are caused by coupling effects of the molecules electronic and vibrational motion, called the Renner-Teller effect, and require a theoretical treatment beyond standard methods. Both the action spectroscopic method used and the theoretical framework developed is quite general and can easily be extended to investigate the vibronic coupling effects in a plethora of other reactive linear open-shell ions that are not amenable to standard spectroscopic techniques.
Apart from the fundamental interest in these molecular systems, many of them are thought to be drivers of interstellar chemistry towards molecular complexity due to their large reactivity. The infrared spectral fingerprints recorded with FELIX match with the observational range of the recently launched James Webb Space Telescope and provide now the basis to search for the cyanoacetylene cation in space.
This joint experimental and theoretical effort is one of the projects within an NWO National Roadmap Grant awarded in 2020 for the development of advanced instrumentation and new experimental techniques at HFML-FELIX.
A vibrational action spectroscopic study of the Renner-Teller and spin-orbit-affected cyanoacetylene radical cation HC3N+
Kim Steenbakkers, Aravindh N. Marimuthu, Britta Redlich, Gerrit C. Groenenboom, and Sandra Brünken
- Chem. Phys. 158 (2023) 084305, https://doi.org/10.1063/5.0135000