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\u2009=\u200917 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\u2009≥\u200918 ( 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. Cyanide anions exist in interstellar space, but the vibrationally or electronically excited states above the electron autodetachment threshold of CN - are assumed to have no contributions to cosmic CN - . Here, the authors report long-lived, superexcited CN - from the dissociative electron attachment to cyanogen bromide.