The Development Of Low-Energy Symmetric(e,2e)Spectrometer And Electron Momentum Spectroscopy Study On Molecules

Electron collision with atoms and molecules is one of the fundamental processes in nature,which plays an important role in many different fields.Investigation on the collision process is critical to understand the behavior and changes of the internal structures of atoms and molecules.Various types of processes can happen when electron impact on the target.Among them,the electron impact single ionization,or(e,2e)reaction,has become an important system for studying multi-particle correlation dynamics,since it involves more than two final state particles.Recent low-energy(e,2e)study employing symmetric geometry indicates that large discrepancies usually exist between measured triple differatial cross sections(TDCSs)and theoretical calculations.On one hand,precised theory for general description of(e,2e)process still remains a challenge.On the other hand,the previous data in low-energy symmetric(e,2e)study were usually obtained by using two hemispherical ananlysers,the statistics of which was usualy very poor due to the low efficiency of the instrument.Meanwhile,the fluctuations of electron current and gas sample intensity would bring extra uncertainties in angular distributions of TDCSs.One of the motivations of this thesis is to develope a low-energy(e,2e)spectrometer performing at symmetric geometry.The spectromter employs one toroidal analyser with two-dimensional position sensitive detector,able to detect two outgoing electrons in coincidence with simultaneous measurements of angle and energy is a wide range.This will not only improve the efficiency of the spectrometer,but can diminish the influence of the fluctuations of electron current and gas pressure on angular distributions as well.Electron momentum spectroscopy(EMS)studies on molecules are also carried out using a high-sensitivity angle and energy dispersive EMS spectrometer.The influence of distorted wave effect on electron momentum distributions of fluorobenzene and isomerization on those of dichloroethylene is investigated.The first chapter of this thesis is an introduction.Firstly,the principle and kinematic geometry of(e,2e)experiment are briefly introduced,followed by a summary of the development of experimental techniques.In the introduction chapter we focus on the current status of the low-energy(e,2e)studies employing symmetric geometry.The second chapter mainly introduces the construction of a low-energy symmetric(e,2e)spectrometer.The overall structure of the spectrometer and some major components are briefly introduced.The preliminary performance of the spectrometer is tested,including the commissioning of the electron gun and detector,the calibration of the energy and the determination of the energy resolution.The commissioning of the coincidence is also performed.The third chapter briefly introduces the high-sensitivity angle and energy dispersive EMS spectrometer and thoretical background of EMS.In fourth chapter,EMS study on 11 outer valence and 6 inner valence orbitals of fluorobenzene molecule at two incident electron energies(1200 eV and 800 eV)is carried out.The electron momentum distributions for these orbits is obtained.By comparing the theoretical calculations based on plane wave impulse approximation with the experimental results,the influence of the distortion wave effect on the measured electron momentum distributions for valence orbitals of fluorobenzene is revealed.In fifth chapter,EMS is used to study the electron momentum distributions of valence orbitlas of three isomeric molecules of dichloroethylene,i.e.1,1-dichloroethylene(iso-C2H2Cl2),cis 1,2-dichloroethylene(cis-C2H2Cl2)and trans 1,2-dichloroethylene(trans-C2H2Cl2),at 1200 eV incident energy.The Cl lone-pair orbital of 1,1-dichloroethylene molecule is analyzed by considering bond oscillation.The influence of isomeration on electron momentum distributions is analysed and discussed by comparing the two-dimensional electron density maps of the binding energy and relative azimuthal angle as well as orbital charge density maps for the three isomers.The thesis is ended with a summary and some future perspectives on the low-energy symmetric(e,2e)spectrometer.