Probing the Potential Energy Surfaces of BrCN- by Dissociative Electron Attachment

Pathways of two-body dissociation of BrCNq+ (q = 2, 3) induced by electron collision

Pathways of two-body fragmentation of BrCNq+ (q = 2, 3) have been explored by combined experimental and theoretical studies. In the experiment, the BrCN molecule is ionized by 1 keV electron impact and the created fragment ions are detected using an ion momentum imaging spectrometer. Six two-body fragmentation channels are identified. By measuring the momentum vectors of the fragment ions, the kinetic energy release (KER) distributions for these channels have been determined. Theoretically, the potential energy curves of BrCNq+ (q = 2, 3) as a function of Br-C and C-N internuclear distances are calculated by the complete active space self-consistent field method. By comparing the measured KER and theoretical predictions, pathways for the fragmentation channels are assigned. The relative branching ratios of the channels are also determined.

Dissociation Dynamics of Anionic Nitrogen Dioxide in the Low-Lying Resonant States

Experimental studies of the dynamics near the molecular dissociation threshold are frequently frustrated, due to the small cross sections and the demand for identification of close-lying electronic states or nuclear motions with multiple degrees of freedom. Using the high-resolution anion velocity map imaging technique, here we report a dynamics study of the dissociations of anionic nitrogen dioxide in low-lying resonant states formed by electron attachment [e^- + NO₂ (X²A₁) → NO₂^- → NO (X²Π) + O^- (²P)]. The long-term puzzling issues about the near-threshold dissociations of NO₂^- are settled. We suggest that three low-lying resonant states (¹B₁, ³B₁, and ³B₂) of NO₂^- contribute to the production of O^- at attachment energies of <5 eV. Furthermore, B₁ and B₂ symmetries of the resonant states lead to different anisotropies of the O^- angular distribution. At the relatively high electron attachment energies, a prompt fragmentation in the molecular bent conformation competes with an indirect dissociation in the straightened conformation.

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 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.