Dynamics Studies Of The Low-energy Electron Dissociative Attachments To Molecules By Ion Velocity Map Imaging

Electron-molecule reaction has been an important subject of physical chemistry. When incident electron energy is below the ionization threshold of target molecule, an electron-molecule resonant state can be formed. Due to the state instability, the temporary negative ion will dissociate into a negatively charged ion plus one or more neutral fragments. Cross section, angular distribution, and kinetic energy of the fragments provide us for information about the resonant states involved. Since the angle-resolved differential cross sections measured with the rotating detector in a conventional turn-table arrangement are usually limited in a small angular range, we established an apparatus employing the velocity sliced map imaging technique which enables us to record the momentum distributions of anionic fragment in2π angular range. Recently, this apparatus has been successfully applied in dynamics studies of dissociative electron attachment (DEA).(1) Using this apparatus, we performed the DEA study for N2O in the electron attachment energy range from0.70eV to2.60eV. With the help of ab initio molecular dynamics calculations, the evolution of momentum distributions of the O-fragment in terms of the electron energy is identified as the result of a competition between two distinctly different indirect pathways, namely, climbing over and bypassing the energy ridge after the molecular structure bending. These two pathways prefer leaving the N2fragment at the high vibrational and rotational stats.(2) We observed the orientation effect in the low-energy electron to apolar molecule CF4which is randomly oriented in a field-free environment during attachment. The orientation effect we observed implies that the electron attachment prefers along local C-F bond, and this orientation effect cab be retained due to the impulsive dissociations of CF4(3) In dissociations of temporary negative ion BrCN-formed by electron attachment around5.0eV, we observed the backward distribution of CN-with the higher kinetic energy and the forward distribution of this fragment with the lower kinetic energy. Such striking backward-forward asymmetry is interpreted as a result of asymptotic matter-wave interference of CN-produced via spin-orbit state dissociations of BrCN-at the first2Π state into CN-plus Br (2P3/2) and into CN-plus Br*(2P1/2). (4) We carried out the DEA experiments for ICl at the electron energies around1.5eV and3.7eV. Within the adiabatic scheme, the angular momentum distributions of these fragments should be presumably in typical Π symmetry. However, the angular distributions of I-measured here clearly indicate the∏-∑mixing symmetry. Such a dramatic symmetry transformation arises from the nonadiabatic dissociation dynamics at the large internuclear distances.