Dissociative Electron Attachment to Molecular Acetonitrile

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.

Vibrationally resolved photoemissions of N2 (C3Πu → B3Πg) and CO (b3Σ+ → a3Π) by low-energy electron impacts

Vibrationally resolved photoemission spectra of the electronic-state transitions C³Π_u → B³Π_g of N₂ and b³Σ⁺ → a³Π of CO following low-energy electron impacts are measured with a crossed-beam experimental arrangement. The absolute cross sections of C³Π_u (ν') → B³Π_g (ν″) of N₂ are presented for the vibrational state-to-state transitions (ν',ν″) = (0,0), (0,1), (1,0), (1,2), and (2,1). The excitation cross sections of the metastable state C³Π_u of N₂ show the maxima at the electron-impact energies 14.10 (ν' = 0) eV and 14.50 (ν' = 1) eV, which are potentially related to the core-excited vibrational Feshbach resonant state ²Σ_u ⁺ of N₂ ^- formed by electron attachment. The absolute cross sections of b³Σ⁺ (ν' = 0) → a³Π (ν″ = 0, 1, 2, 3, 4) of CO are given by the calibrations with those of N₂ measured in this work. Besides the maximum excitation cross section 5.85 × 10^-18 cm² at 10.74 eV of the CO b³Σ⁺ (ν' = 0) state, some fine structures on the excitation function profile are attributed to different shapes and Feshbach resonant states of CO^- formed by electron attachment, while the others arise from the direct electron-impact excitation. Some discrepancies, particularly for N₂, between the present data and the results available in the literature studies arise from different experimental techniques and data-processing procedures. Furthermore, contributions of physical processes such as wave-packet evolution and non-Franck-Condon dynamics are highlighted here.

Synchronous and asynchronous dynamics of the concerted three-body dissociations of temporary negative ion CH2F2. J Chem Phys 2020;152:084305. [PMID: 32113364 DOI: 10.1063/1.5135609]

Molecular concerted three-body dissociation is a fast process, but still can be classified into synchronous and asynchronous pathways. It is challenging in experiments to evaluate different contributions of the aforementioned mechanisms. Here, we report an experimental identification of the synchronous and asynchronous concerted three-body dissociations of temporary negative ion CH₂F₂ ^- at an electron-molecule resonant state formed by electron attachment. The synchronous-asynchronous branching ratios indicate that the asynchronous process is predominant although the synchronous contribution is slightly enhanced with the increase in the electron attachment energy. This study provides two intuitive pictures of the concerted three-body dissociations, in particular for the nonequivalent-bond cleavages of a polyatomic molecule.

Ion momentum imaging study of the ion–molecule reaction Ar+ + O2 → Ar + O2+

O₂⁺ products of the charge exchange reactions between Ar⁺ and O₂ are distributed in the wider range of scattering angle at higher collision energy.