Development Of A S-Type Electron Energy Loss Spectrometer And Study Of Selection Rules For Electric Multipole Transition Of Molecule

As a subdiscipline,atomic and molecular physics plays an important role in physics,and it is crucial importance to understand the properties and motions of the microscopic world.Energy level structures and dynamic parameters of atoms and molecules are the essential contents of the atomic and molecular physics,and the collision methods us-ing photons,electrons and neutrons as probes are important methods for studying the energy level structures and dynamic parameters of atoms and molecules.Electron en-ergy loss spectroscopy,as one of the collision methods,is a powerful tool to study the energy level structures and dynamic parameters of atoms and molecules.Nowadays,improving the energy resolution and efficiency is still the trend of development of the electron energy loss spectrometer.In this thesis,a new high-resolution electron energy loss spectrometer is devel-oped.S-type double-hemisphere monochromator and analyzer are employed by this spectrometer,which can reduce the space charge effects,increase the electron beam in-tensity,reduce the background,and improve the signal-to-noise ratio.In the new built electron energy loss spectrometer,the beam intensity at the collision point can reach 30 nA with a best energy resolution of 64 meV,whose performance is one of the best in the world.The measured optical oscillator strengths of He and Ar agree well with the previous results,so the properties of this spectrometer is calibrated.In addition,the electron collision can excite the electric multipole transition.But,for molecules,the corresponding selection rules for electric multipole transition have only been fragmentally discussed in the previous literatures.Therefore,in this thesis the selection rules for the electric multipole transitions of molecule are derived and sum-marized.Firstly,the selection rules for the electric multipole transitions of diatomic molecule is studied.The summarized selection rules are listed in two tables.Based on these selection rules,the valence-shell excitation spectra of H2,N2 and CO by the fast electron impact and inelastic X-ray scattering are explained.Based on the selec-tion rules of diatomic molecule,the selection rules for the electric multipole transitions of polyatomic molecule,especially triatomic molecules,are also derived.Triatomic molecule have multiple vibrational modes,and not any two vibronic transitions be-tween the allowed transitions are allowed.Another factor that needs to be considered is that the molecular geometry(symmetry)may change during the electronic transition.Therefore,when studying the selection rules for multipole transitions of polyatomic molecules,we must not only consider the selection rules of vibrational transition,but also consider the symmetry change.The selection rules for the multipole transition of triatomic molecule are derived under taking these issues into account.Based on the summarized selection rules,the electron energy losss pectra of H2O,CO2 and N2O are identified,and the momentum transfer dependence behaviors of their valence-shell excitations are explained.