Dissociative Electron Attachment Dynamics And The Development Of High-resolution Anion Velocity Map Imaging Apparatus

As an important physicochemical process in atmosphere chemistry,interstellar chemistry,plasma and ionization irradiation,dissociative electron attachment(DEA)to molecule produces anionic and neutral fragment radicals.The momentum distributions of anionic fragment using anion velocity map imaging can reveal the quantum physics properties of electron collision with molecule,the properties of electron-molecule resonant states and dissociation dynamics of temporary negative ion.Using velocity map imaging technique,we progressively carried out a series of dissociative electron attachment studies.Our research work in this thesis can be devided into two parts.The fisrt part of our research work was done by our previous anion velocity map imaging apparatus.Making full use of the apparatus with high detection efficiency,high anion momentum resolution and stable performance,we studied dissiciative electron attachment to different molecules which including from simple diatomic molecules(CO)to complex polyatomic molecules(C02,ethanol,acetaldehyde).(1)Succeeding our previous finding about coherent interference of the resonant states of CO-formed by electron attachment,we observed two dissociation channels e-+CO ?CO-?C(1D,1S)+O-in the high electron energy range of 11.3?12.6eV.The dissociation channel to produce C(1S)is found in the measurement at 12.3 eV,which was not reported before.More importantly,the angular distributions of O-fragments in momentum images are the completely backward distributions.In contrast to our previous finding,the state configuration of the coherent interference is changed in higher electron energy,implying the existence of two different coherent resonances.(2)Due to the complexity of the processes,the DEA to polyatomic molecule is still a challenge.In addition,the study of DEA to ethanol,acetaldehyde is critical to the understanding of the DEAs to biomolecules.We investigated the dynamics of DEAs to ethanol and acetaldehyde,using a combination of anion VMI experiments and ab initio molecular dynamics simulations.A cascade process to the O-yield of the DEA to ethanol is proposed.The anisotropic angular distributions of the fast O-ions of the DEA to acetaldehyde indicate that the higher order partial waves should play more important roles at the higher electron attachment energies.(3)Four billion years ago,compositions of Earth's atmosphere were almost C02 and a little bit of molecular oxygen.In the textbook,only one reaction pathway led to the production of these molecular oxygen:a three-body recombination reaction(O+O+M?O2+M).Considering the abundances of the low-energy electrons in the upper atmosphere,especially some peaks of distribution in energy range from 15 to 20eV,we proposed a new mechanism that dissociative electron attachment CO2 to produce oxygen molecule(e-+CO2?O2+C-).We observed a new channel of DEA to CO2 that produces O2(X 3?g-)+C-in experiment,which coexists with a competitive three-body dissociation to O + O+ C-.The cross section of the former channel is larger.So this indicates that dissociative electron attachment to CO2 is a new mechanism on origin of molecular oxygen in Earth's prebiotic primitive atmosphere.Although we found vibrational excited O2 molecule in DEA to CO2,we can't get more ditailed information about vibrational-state resolved dynamics due to the low resolution.Considering the developmet trend in DEA research,we proposed to develop the high-resolution VMI apparatus.So the second part of my research work is the development and application of this apparatus.(1)Our newly developed the high-resolution VMI apparatus consists of trochoidal electron monochromator,a skimmed supersonic gas jet,a well-designed anion lens system,ion detector system and a CCD camera for data acquisition.In our apparatus,the momentum resolution in a time-sliced image is enhanced remarkably by introducing a trochoidal monochromator of low-energy electrons and modifying the VMI lens system.We succeeded in building a high-resolution ion VMI apparatus,in particular,for the first time by combination with the electron trochoidal monochromator.(2)Using this apparatus,we performed the DEA study for CO2 in the electron attachment energy range from 4.15 to 4.95eV.The lifetime of CO2-at this resonant state is about 10-11s and it's the first time to derive such a short lifetime of ion from the experiment.The intramolecular nuclear motions of transient anion CO2 at a low-lying shape resonant state can be clearly identified from the high-resolution momentum images of O-product.We clarify a long-standing controversy about the dissociation dynamics of CO2-at this resonant state.The high-resolution velocity map imaging apparatus is a promising reaction microscope providing more insights into the DEA dynamics,especially molecular vibrational effect.