Experimental And Theoretical Study On Time-Resolved Electron Momentum Spectroscopy

It is of great importance to observe the energy and charge transfer of molecular system,the evolution of electron cloud,the change of molecular structure,as well as the formation and fracture of chemical bond in order to have deep insight of the chemical reactions and the physical and chemical properties of matters.Since the invention of the pump-probe technology,the ultra-fast dynamics researches have been developing very rapidly that enable people to have a new understanding of the time evolution of electronic structure.Electron momentum spectrum(EMS),which is based on the electron impact ionization experiment,can obtain not only the ionization energy but also the corresponding electron density distribution in momentum space for specific orbital.It has now become as a robust tool for molecular orbital imaging in momentum space.Combining EMS with pump-probe technique to develop time-resolved electron moemtum spectroscopy(TREMS)will provide with us a completely new method to investigate the electronic structures and their evolution dynamics of the excited states of molecules.This thesis is divided into two parts.The frist part introduces our newly built TREMS-and the performance of the apparatus.The new TREMS has been used to investigate the photodissociation of iodomethane molecule.In the second part,the valence orbital electron momentum distributions of dimethyl sulfide and cyclopentene molecules are investigated using high-sensitivity electron momentum spectrometer with 2? azimuthal angle.The thesis comprises of five chapters:In chapter 1 we introduce the ultrafast dynamics and related experimental techiniques,as well as the background,theoretical framework and development of EMS.The combination of EMS and pump-probe techniques is thus a natural choose to develop TREMS.Chapter 2 introduces the structure and the performance of the TREMS,as well as the calibration of time-zero of TREMS.In chapter 3,photodissociation of iodomethane molecule is investigated by the TREMS.The electron momentum distributions of the dissociation limit CH3+ I(I*)are obtained.With the help of theoretical analysis,the rearrangement of the eletron density distribution is observed during the C-I bond breaking.The dissociation is inverse process of bond formation.It is thus helpful for us to understand of C-I bonding from the view of electron density change.Chapter 4 introduces the EMS study on the valence electronic structures of dimethyl sulfide considering vibrational effects.Through the comparison between the momentum distributions measured by experiment and the theoretical calculation,we found that the calculated results considering the vibrational motions reproduce the experimental results better than the one calculated under the equilibrium configuration.The analysis shows that the observed vibrational effect mainly come from vibrational motion breaking the mirror symmetry.Chapter 5 introduces the study on the turn-up effects of electron momentum distribution at low momentum region for valence orbitals of cyclopentene.It is found that the vibration normal modes play a major role while the ring-puckering provide a negative contribution to 9a' orbital.The influence of distorted-wave effect on 3a" and 2a"+3a' orbitals is also demonstrated by changing the energy of the incident electron.Finally,the summary of this thesis and the future prospect of the work of the time-resolved electron momentum spectrometer is presented.