The Development Of Time-Resolved Electron Momentum Spectrometer And Related Experiments

The microscopic structure of matter,especially the spatial arrangement of nuclei and the electron density distributions,determines its physical and chemical properties.The understanding of electronic structures or orbital wavefunctions of atoms and molecules,especially the frontier orbitals related to chemical properties and chemical reactions,directly affect the understanding of chemical properties and the mechanism of chemical reactions.With the application of pump-probe technique,people have already entered the door of real-time detection of the evolution of electronic structures,making outstanding achievements for ultrafast dynamics.Electron momentum spectroscopy(EMS),a unique technique of investigating electronic structures,can obtain electron density distributions of molecular orbitals in momentum space.After several decades of development,it shows its great advantage in molecular orbital imaging in momentum space.The main work of this thesis is to develop a nanosecond time-resolved electron momentum spectrometer by combining the pump-probe technique and EMS in order to study molecular excited state dynamics in terms of the ionization energy and electron momentum distributions.The spectrometer has been designed and built,and the preliminary performance tests are obtained.]n addition,the first singlet excited state S1(?,?*)of toluene was studied by the time-resolved electron momentum spectrometer during the research period in Prof.Takahashi's Group in Tohoku University as a joint PhD student,getting binding energy spectra and momentum distributions of the S1 state.The other work of this thesis is the study of the vibrational effects on electron momentum distributions of molecular orbitals.EMS studies on ethanol and oxetane show that electron momentum distributions for some orbitals are strongly influenced by molecular vibrations.Careful consideration of the influence of various factors,including vibrational effects,on electron momentum distributions should be taken when performing both tranditional and time-resolved EMS studies.The content of this thesis is divided into six chapters,and the content of each chapter is as follows.Chapter 1 presents the concept of ultrafast dynamics imaging and the related experimental techniques,the principles of EMS,the development of the spectrometers and the problems to be considered in the evaluation of electron momentum distributions.The idea of developing time-resolved electron momentum spectrometer is also introduced.Chapter 2 shows the design of time-resolved electron momentum spectrometer,including the general design,optical system,pulsed electron gun,energy analyzer,detector;electronic system,power supply system,vacuum system and magnetic shielding.Chapter 3 gives the preliminary performance tests of the time-resolved electron momentum spectrometer.Chapter 4 exhibits the detailed results of the S1(?,?*)excited state of toluene investigated by the time-resolved electron momentum spectrometer of Prof.Takahashi's Group in Tohoku University when as a joint PhD student in the group.Chapter 5 is the EMS experimental and theoretical studies on the conformation of ethanol.Considering vibrational effects,the confliction of experimental electron momentum distributions and equilibrium geometry calculations is not well solved.Chapter 6 studies vibrational effects on electron momentum distributions of outer valence orbitals oxetane in detail.It is found that the agreement between experiment and theory is greatly improved when taking vibrational effects into account.Finally,the summary of the thesis and the future work on the time-resolved electron momentum spectroscopy is presented.