Direct observation of long-lived cyanide anions in superexcited states

Abstract

The cyanide anion (CN^-) has been identified in cometary coma, interstellar medium, planetary atmosphere and circumstellar envelopes, but its origin and abundance are still disputed. An isolated CN^- is stabilized in the vibrational states up to ν = 17 of the electronic ground-state ¹Σ⁺, but it is not thought to survive in the electronic or vibrational states above the electron autodetachment threshold, namely, in superexcited states. Here we report the direct observation of long-lived CN^- yields of the dissociative electron attachment to cyanogen bromide (BrCN), and confirm that some of the CN^- yields are distributed in the superexcited vibrational states ν ≥ 18 (¹Σ⁺) or the superexcited electronic states ³Σ⁺ and ³Π. The triplet state can be accessed directly in the impulsive dissociation of BrCN^- or by an intersystem transition from the superexcited vibrational states of CN^-. The exceptional stability of CN^- in the superexcited states profoundly influences its abundance and is potentially related to the production of other compounds in interstellar space.