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.

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

“…Experiments . In our high-resolution VMI apparatus, a trochoidal electron monochromator is used to reduce the energy spread of thermally emitted electrons. − The narrow energy spread can be maintained for the pulsed electron beam (along the y axis), which is essential for obtaining anion velocity images with a high resolution . The anion detector (in the x–y plane) consists of two multichannel plates and a set of delay-line anodes (commercially available from Roentdek, Inc.).…”

“…Nevertheless, more sophisticated computations about the resonant states above 1,3 B 1 of NO 2 – are still needed. What we found here is largely owed to the development of the high-resolution technique for the DEA studies. − Perhaps the molecular dissociations in the anionic resonant states are the last unmatured research field of unimolecular reaction, while more and more of their dynamics will be revealed with the high-resolution measurements.…”

“…The energy spread (100–150 meV) of the monochromatized electrons for the high-resolution VMI measurements is much smaller than that (500–600 meV) of the thermal electrons emitted directly from the filament, and it is not worsened seriously under the electron-pulsed mode . Using the high-resolution VMI apparatus, − here we report an experimental study of the near-threshold DEA process of nitrogen dioxide [e – + NO 2 (X 2 A 1 ) → NO 2 – → NO (X 2 Π) + O – ( 2 P)]. This is the energetically lowest pathway and has an energy threshold ( E th ) of 1.65 eV .…”

“…In contrast to the conventional turntable ion detection (e.g., see ref ), VMI has exhibited merit in detecting the charged yields; , more noticeably, a very high resolution was achieved with the slow photoelectron VMI technique . In the DEA dynamics studies, we initiated the high-resolution VMI project a few years ago − and recently achieved near-threshold VMI measurements . The energy spread (100–150 meV) of the monochromatized electrons for the high-resolution VMI measurements is much smaller than that (500–600 meV) of the thermal electrons emitted directly from the filament, and it is not worsened seriously under the electron-pulsed mode .…”

“…Before recording the production efficiency curve of the O – yields, we need to calibrate the electron attachment energy and obtain the attachment energy dependence of the electron beam intensity. The details about these routine operations can be found elsewhere. − As shown in Figure , the solid black circles represent our experimental data without the electron beam intensity normalization. These data exhibit a profile analogous to that reported by Nandi and Krishnakumar, except for the broadness of the first band.…”