Probing the Potential Energy Surfaces of BrCN^– by Dissociative Electron Attachment

Abstract

State coupling certainly determines the topologic features of the molecular potential energy surface (PES) and potentially diversifies chemical reaction pathways. Here we report the new PESs of BrCN– in the low-lying electronic states that are distinctly different from the previous predictions in the short Br–CN bond region but validated by the high-resolution ion velocity imaging measurements of low-energy dissociative electron attachment (DEA) to BrCN. Besides the vibrating CN– ions produced in the fast Br–CN bond stretching motions, we confirm that the ro-vibrating CN– ions with a nearly isotropic angular distribution are produced by receiving a torque in the combinational motion of Br–CN bond bending and stretching. The latter process is closely related to the potential well of BrCN– at the first excited state A 2Π3/2 that arises from the Π–Σ state couplings. Our findings not only suggest that the PESs of other anionic cyanogen halides are in dire need of reexamination but also show that ion velocity imaging of the DEA process is a powerful experimental method for evaluating the theoretical PESs of molecular anions.

Direct observation of long-lived cyanide anions in superexcited states

“…In this work, we report an experimental evidence that the CN − ** species are produced in the DEA to BrCN, where the CN − yield is populated in the vibrational states ( ν ≤ 17) at the low electron attachment energy ( E e ), while the CN − ** produced at the high E e value is in the vibrationally or electronically superexcited states. In the previous studies, the CN − ** species cannot be identified in the CN − production efficiency curve 26 and the low vibrational-state ( ν < 9) CN − yields were found in the lower attachment energy range ( E e = 1.07–1.97 eV) 27 . Using the high-resolution time-sliced velocity map imaging (VMI) apparatus 27 – 30 , which was developed on the basis of our previous one 31 , 32 , we record the velocity images of the CN − /CN − ** yields in the higher E e range from 3.57 to 6.57 eV, and the results are shown in Fig.…”

“…Experiments. In our laboratory, anionic high-resolution velocity (or momentum) imaging has been realized [27][28][29][30] , owing to the application of a trochoidal electron monochromator. In the experiments, within a crossed-beam arrangement, the monochromatized pulsed electrons (with an energy spread about 100 meV here, along y axis) are guided to the reaction region with a homogenous magnetic field (70 G) which is produced with a pair of Helmholtz coils; then the anionic yields are pushed out, accelerated, and flying (along z axis) through the VMI lens system.…”

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.).…”

“…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 .…”