Unveiling the Ionization Energy of the CN Radical.

The cyano radical is a ubiquitous molecule and was, for instance, one of the first species detected in astrophysical media such as comets or diffuse clouds. In photodissociation regions, the reaction rate of CN + CO → CN + CO is one of the critical parameters defining nitrile chemistry. The enthalpy of this charge transfer reaction is defined as the difference of ionization energies (E) between CN and CO. Although E(CO) is known accurately, the E(CN) values are more dispersed and deduced indirectly from thermodynamic thresholds only, all above E(CO), leading to the assumption that the reaction is fast even at low temperature. Using a combination of synchrotron radiation, electron/ion imaging coincidence techniques, and supporting ab initio calculations, we directly determine the first adiabatic ionization energy of CN at 13.956(7) eV, and we demonstrate that E(CN) < E(CO). The findings suggest a very slow reaction in the cold regions of interstellar media.