Negative Ion Velocity Mapping Apparatus For Dissociative Electron Attachments And Its Application

Electron-molecule reaction has been an essential subject of molecular physics and chemical physics. Dissociative electron attachment (DEA) is a typical processe occurring at low energy, with yields of the neutral and negatively charged. Along with the cross sections of the fragment ions, the kinetic energy and angular distributions of these ions provide the most details of DEA dynamics. Especially, the angular momentum distributions of the anionic product of the DEA process provide us information on the angular momentum of the incident electron and the symmetry of the quantum states involved. Measurements of the kinetic energy and angular distributions of fragment ion have been achieved by several experiments. The ion detections were usually restricted in some angular range, in particular, the forward (near0°) and backward (near180°) directions were difficult to be accessed due to the geometrical limitations of the conventional turn-table arrangement. In the photodissociation velocity map imaging (VMI) and velocity sliced imaging (VSI) techniques are used. The basic idea of VMI and VSI was more recently introduced into the DEA studies by Krishnakumar’s group in India.Here we report our newly developed apparatus of velocity mapping for the electron-molecule reactions. This apparatus consists of a pulsed electron gun, a set of ion velocity mapping optic lenses, a two-dimensional position detector including two pieces of micro-channel plates and a phosphor screen, and a charge-coupled-device camera for data acquisition. In contrast to the previous ones, our apparatus have such characteristics:the VSIs can be recorded more easily, using a quite simple data-acquisition system; a longer flight tube enables us to detect more ionic fragments and gain the VSIs of ions from DEA to polyatomic molecule; two working modes, the positive and negative ion detections, can be easily switched, thus our apparatus can be extended for the electron-impact dissociative ionization (DI) studies. The efficient performance of this system is evaluated by measuring the angular distribution of O from the electron attachments to NO at7.3and8.3eV and O+from the electron collision with CO at40.0eV. We also tested the time-of-flight (TOF) spectrometer of positive and negative ion.Stereo-dynamics of dissociative electron attachments to CO2is investigated by the O anion velocity imaging experiments combined with the R-matrix calculations.2Πg as a Feshbach resonant state of CO2-is confirmed to play roles in the dissociations around8.0eV. We find that the dynamic evolutions of the complex potential energy surface due to the Renner-Teller effect result in the dramatically different anisotropic O-momentum distributions. The sliced image of O-recorded at the incident energy of4.4eV, the O ions with very low kinetic energies (<0.2eV).We performed the DEA experiments for CO in the electron incident energy range of9.5-10.6eV by using our home-made anion velocity image mapping apparatus. At the incident energies of9.5eV, the O-ions with very low kinetic energies (<0.3eV). The others clearly exhibit the backward-scattering characters. O-ions at10.0eV can be interpreted very well using this two-state combination mechanism. However, it is out of our expectation that this mechanism, even if including more resonant states, failed in reproducing the backward-scattering pattern observed at10.6eV. In this work we confidently show that the phase interference of three outgoing waves results in the coherent backward-scattering pattern of O-momentum distributions observed at the incident energy of10.6eV. The kinetic energies of near-zero eV at10.6eV can be produced via the tunneling dissociation.