The Study On The Photoelectron Velocity Map Imaging Of Laser Induced Ionization Of Nitric Oxide Molecules

In this thesis,taking heteronuclear diatomic molecules as an example,we experimentally measure the photoelectron velocity images from nitric oxide molecules ionized by a strong femtosecond laser using velocity-map-imaging technique.Based on this measurement,we analysis the influence of laser intensity on resonant excitation ionization paths and discuss the contribution of different Rydberg states in the ionization process,meanwhile extract the characteristic information about the photoelectron angular distributions of these Rydberg states involved.The kinetic energy and angular distributions of released electrons can provide direct experimental evidence for getting a further understanding of the molecular ionization processes in laser field.Compared with atoms,the energy structures of molecule are more complex,there is still a lot of phenomena unexplained in strong field ionization and dissociation processes of molecules.For solving these problems,the innovation of experimental techniques and new theoretical models,which are more close to the actual situation of the molecule,are required.Our laboratory has built up an experimental apparatus combined a hexapole with a velocity map imaging of electrons system.After focusing be hexapole,molecular beam interact with the femtosecond laser to produce electrons.Then,we perform velocity-map imaging on the photoelectrons of the nitric oxide molecules.Nitric oxide molecules ionized by a linearly polarized laser field have been measured at different laser intensities ranging from 1.1×10¹³ to 7.8×10¹³W/cm² at 800 nm or 2.0×10¹² to 1.4×10¹³ W/cm² at 400 nm,respectively.Through the analysis of the kinetic energy spectra and their switching information with various laser intensities,we have identified the source of electrons and certained the contribution of different Rydberg states at different laser intensities.Compared with 800 nm,the structure of photoelectrons energy spectra for 400 nm is more oversimplified,there is only one obvious resonant peak with its corresponding ATI peaks.As varying the 800 nm laser intensity,we observe the channel switching phenomenon as well as the changed contribution rate of different electronic Rydberg states in corresponding photoelectron images.The reason for these phenomena is the stark shift of electronic Rydberg states so that certain Rydberg state is easily moved into the resonance region which induced stronger signal and vice versa.While,by comparing the photoelectron angular distribution of each Rydberg state at different laser intensities,it is found that angular distribution reflects the natural characteristics of Rydberg states.It would help us to get an overall view of the structure of Rydberg states involved in molecule,an in-depth understanding of complex resonance enhanced multiphoton ionization process,as well as obvious chinnel switching phenomenon and photoelectron angular distributions through detecting photoelectron images of nitric oxide molecules.This work provides experimental basis for achieving the control of molecular quantum state under strong laser field by analyzing the field impact on the molecular Rydberg states involved.