Experimental And Theoretical Study Of High Energy Resolution Electron Momentum Spectroscopy

The physical and chemical properties of matter depend on the structure,orbital properties and laws of motion of atoms and molecules in the micro world.Electron momentum spectroscopy(EMS)based on the high energy electron impact(e,2e)reaction,can enable us to directly obtain the orbital electron binding energy spectrum(BES)and the electron density distributions in momentum space for individual orbitals of atoms and molecules.In the past decades,EMS has developed rapidly with its unique advantages and has become an important tool for studying the electronic structures of atoms and molecules.With the development of the experimental techniques and the theoretical methods,EMS has been successfully applied to investigate the electronic structures of atoms,simple molecules,complex biological molecules and solid film samples.The influences of various physical effects on electronic structures,such as molecular conformational effect,vibration effect and electron correlation effect,have been analyzed.However,due to the low energy resolution and detection efficiency of the spectrometer,the physical phenomena such as the Jahn-Teller effect and the relativistic effect have not been studied thoroughly.When molecule ionization occurs,these physical effects will cause the bands of some orbitals to split into different states with small energy separations,and the corresponding electron momentum distributions are different,but the shape difference is relatively small.In order to study the influences of these physical effects on the electronic structure of atoms and molecules in detail,it is necessary to obtain reliable electron momentum distributions(EMDs)of different splitting states,which requires high energy resolution and high statistical experimental measurements.Therefore,it is of importance to develop a high energy resolution and detection efficiency electron momentum spectrometer in the EMS field.The main work of this thesis is to set up a high-energy-resolution electron momentum spectrometer on the basis of the existing 27?-angle EMS spectrometer in the laboratory.On the one hand,the energy resolution of the spectrometer has been improved by using low-energy electron gun and energy monochromator to reduce the incident electron energy dispersion.On the other hand,the detection efficiency of the spectrometer has been further improved by using a new position sensitive detector consisting of two pieces of microchannel plates and a three-layers delay-line anode.The performance of the high-resolution electron momentum spectrometer has been tested and the energy resolution of 0.53 eV is achieved,as well as the high stability and efficiency with a long time working.In this thesis,the electronic structures of methane(CH4),methyl iodide(CH3I)and propane(C3H8)are also studied by using this spectrometer.The Jahn-Teller effect,relativistic effect,vibration effect and interference effect on their electronic structures are analyzed in depth.This thesis includes six chapters as follows:In the first chapter,several typical experimental techniques for exploring the electronic structures in atomic and molecular physics are briefly reviewed and the experimental method and research progresses of EMS are introduced in detail.The purpose and significance of the work in this thesis are also presented.Chapter 2 introduces the theoretical framework of EMS and several theoretical methods for calculating electronic structures involved in this paper,such as Hartree-Fock theory,density functional theory,SAC-CI theory,relativistic calculation and molecular vibration calculation.Chapter 3 presents the development of the high-energy-resolution electron momentum spectrometer,in which the structure of each part and performance tests of the apparatus are described in detail.The instrumental energy and momentum resolution are determined to be 0.53 eV and 0.18 a.u.(?? = 0.8°,?? = 2.2°),respectively,by measuring the BES and EMD of Ar 3p orbital.Chapter 4 gives the experimental results of the highest occupied orbital 1t2 of methane molecule and the Jahn-Teller effect in the ionization process is studied.The measured BES show three maximum structures corresponding to three Jahn-Teller splitting states JT-1,JT-2 and JT-3,and the EMDs for JT-1 and JT-3 states show different characteristics.Couplings between nuclear and electronic motions due to the Jahn-Teller effect will make CH4+ distort from Td geometry of neutral methane to three stable ion geometries C2v,D2d or C3v.The splitting of the BES and the difference of the EMDs for splitting states due to Jahn-Teller effect of 1t2 orbital electron ionization are approximately explained by using the extreme physical picture that methane molecular geometry distorting from Td to C2v,D2d or C3,at the instant of ionization.Chapter 5 shows the measurement of CH3I molecule and the influences of relativistic effect on CH3I are comprehensively analyzed by nonrelativistic and spin-orbit relativistic B3LYP calculations,as well as relativistic pseudo-potential calculations.The EMDs of spin-orbit splitting components 2e3/2 and 2e1/2 are obtained for the first time.The results show that the relativistic effect leads to different EMDs for 2e3/2 and 2e1/2,and has a great influence on the electron distribution of C-I bonding orbital 3a1.For the inner valence orbitals,the measured EMPs for three 2a1 satellites and four la1 satellites are extracted.In addition,the SAC-CI calculation is used to investigate the electron correlation effect.It is revealed that different shapes of EMDs for three 2a1 satellites are caused by the configuration mixture of inner valence orbitals with the same symmetry.Chapter 6 introduces the studies of propane about the vibration effect for the outer valence orbitals and the interference effect for the inner valence orbitals.The results show that the vibrational effect has a great influence on the electronic structure of 2bt,6a,+4b2 and 1a2+3b2 orbitals,but has little influence on 5a1 and lb1 orbits.In addition,the oscillation structures induced by the interference effects in the inner valence orbitals are observed by the ratios of the EMDs for the 3a1 to 2b2 and 3a1 to 4a1 orbitals.The last part is the summary of the thesis and the prospects for future experimental and theoretical work.