Direct observation of long-lived cyanide anions in superexcited states

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 (1)Σ(+), 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 ((1)Σ(+)) or the superexcited electronic states (3)Σ(+) and (3)Π. 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.