Experimental And Theoretical Research On Ultrafast Dynamics Of NO₂

The realization of ultrafast imaging of molecular structure evolution has always been the goal pursued by physicists in the field of strong fields.Researchers have continuously explored the possibility of ultra-fast imaging of molecular structure based on electron scattering and laser diffraction techniques.Emerging tools such as ultrafast electron beams or ultrafast lasers provide more precise time resolution and finer spatial resolution for ultrafast imaging of molecular structure evolution,so that you can have a deeper understanding of the evolution of molecules and their structures.Chemical reactions can be manipulated at a more microscopic level.Real-time observation of the breaking and formation of molecular bonds on the sub-femtosecond time scale is one of the most important research goals at present,and the exploration and development of ultra-fast imaging methods of molecular structure is crucial.It is difficult to quantitatively describe the process of multiple ionization and Coulomb Explosion(CE)of complex molecules.However,with the aid of time-resolved pump-probe detection experimental technology,the Cold Target Recoil Ion Momentum Imaging Spectrometer(COLTRIMS)is used in the study of the molecular dynamics process induced by the fast-intensity femtosecond laser field can carry out detailed analysis of the structural changes of atoms and molecules,which is worthy of in-depth study.In this thesis,two experiments are carried out for the nonlinear symmetrical triatomic molecule NO₂.The first part uses the femtosecond infrared laser with pump-probe detection scheme to perform a time-resolved kinetic coincidence measurement of the nitrogen dioxide molecule.This yield is extracted the highest channel NO₂²⁺?NO⁺+O⁺clearly reproduces the kinetic changes of this dissociation channel with time delay;this dissociation channel contains three main kinetic energy release peaks.We studied the direct secondary ionization induction during the Coulomb Explosion process,the NO₂ dissociation potential energy curve was calculated by the multi-configuration density functional method,and it was found that the three kinetic energy release distributions were derived from the different bond angle configurations of the molecules during the dissociation process and were based on the shape and dissociation of the potential energy curve.The critical nucleus distance of the position proposes the bond breaking mechanism of the N-O bond;further,we studied the evolution of this Coulomb Explosion process and found that when the molecule ionizes to the monovalent parent ion,the bond angle relaxes to the minimum value rapidly.Then a femtosecond bond stretching process occurs.After about 800femtoseconds,the bond length approaches the dissociation limit,and the kinetic energy distribution changes slowly.Based on different potential energy curves,we can qualitatively determine the size of the bond length at different moments,provide intuitive molecular time-resolved dissociation images.At the same time,it is found that the relaxation time scales of different configurations are different,and there are channels that change very slowly,and the kinetic energy is still slowly decreasing after 6ps.This work describes the dissociative ionization process through the time-resolved Coulomb Explosion scheme,combined with the calculation of the molecular dissociation potential energy curve,and clearly reveals the evolution of the dissociative ionization of NO₂ molecules over time.The thesis also studied the experiment of electron heavy scattering induced Coulomb explosion imaging of NO₂ molecules.Through comparative experiments of different polarization conditions,it was found that the KER(Kinetic-energy Release)distribution of ions has certain polarization-dependent characteristics,and the kinetic energy release under linear polarization conditions will be stronger.This characteristic is consistent with the linear molecule OCS is similar,but the polarization dependence of similar V-shaped molecules such as H₂O and D₂O is completely opposite.Combined with the correlation diagram of the angle?between the total kinetic energy and the ion momentum vector,we extracted the momentum of high-energy ion fragments with an energy greater than 30e V under linearly polarized light to reconstruct the configuration of the NO₂molecule during the Coulomb Explosion,and found that the reconstructed bond length did not have a similar sequence In the process of bond length stretching during dissociation,Coulomb Explosion occurs directly through cooperative dissociation.The heavy scattering induced Coulomb Explosion imaging method is used to obtain the transient structure information of NO₂ molecules,and to study the possible ionization-induced structural changes,optimize the experimental conditions and laser parameters,and further study the time-resolved ultraviolet light-induced molecular structural changes.