Coulomb Explosion Imaging Of SO₂ And N₂O Molecules In Femtosecond Laser Field

The molecules undergo ionization dissociation and Coulomb explosion in the femtosecond laser field,which has become a hot issue in atomic and molecular fields.The momentum distribution of fragment ion generated by the Coulomb explosion contains abundant molecular configuration and ionization dissociation information,and Coulomb explosion is widely used to study molecular ionization and molecular structure imaging.In this dissertation,the dual?multi?ionization dissociation kinetics of V-type molecule SO₂ and linear molecule N₂O under the action of femtosecond laser field was experimentally studied by COLTRIMS instrument and the coincidence measurement method,and the molecular imaging was performed using the Coulomb explosion imaging method to study the changes of the molecular configuration during molecular ionization.We first studied the relationship between the yield of Coulomb tunnels and the light intensity of SO₂ molecules,and found that due to the existence of sequence dissociation in the three-body dissociation,there is a competition relationship between the three-body dissociation channel and the corresponding two-body dissociation channel.In addition,we also observed the existence of O₂⁺in the SO₂²⁺dissociation product,but there was no competition relationship to the SO₂²⁺?SO⁺+O⁺channel with light intensity change.In combination with the SO₂²⁺potential surface,it is believed that the generation of the channel S⁺+O₂⁺may originate from the structure with a smaller bond angle.Combining the angular distribution of the two dissociation channels and the total kinetic energy release,further studies have shown that the high and low excited states of the SO₂²⁺ion originate from the ionization of angles of 180°and 90°between molecular O-O axis and the laser polarization,respectively.This shows that the V-molecule SO₂ exists in two directions of collimation in the laser field,while the linear molecule N₂O only ionizes in the HOMO-1 orbit along the direction of laser polarization.The study of high valence three-body Coulomb explosions of SO₂ and N₂O indicate that three-body dissociation can occur through sequence dissociation and non-sequence dissociation.We use the Coulomb potential to simulate the Coulomb explosion kinetics of two molecules,and find that the angle?_cmm between the two terminal ion momentum vectors is independent of the bond length and is only related to the initial bond angle.Therefore,the initial bond angle distribution before molecular dissociation is reconstructed according to the?_cm distribution,and the bond length distribution is reconstructed in combination with the KER distribution of each channel.The distribution of the angular momentum?_cm of the O-O ion of the SO₂molecule in the experiment is lower than the minimum value of the?_cm simulated by the Coulomb potential.This may be due to the fact that the special electron layout of the V-type structure of the SO₂ molecule makes the Coulomb explosion become non-coulomb at the initial time,while it is not the case for linear molecules N₂O.The reconstructed results show that the results of the bond angle reconstitution of the V-type molecule SO₂ are both 100°,the bond length gradually increased with the increase of the valence state,which is somewhat different from the 120°angle of the neutral molecule.This may be due to the fact that the laser field causes the molecules to bend and the electronic layout has an influence on the nuclear motion,which makes a certain error in Coulomb's approximate simulation..The bond angles of the valence states of N₂O linear molecules are both 165°,and the bond length is also gradually stretched as the valence state increases.According to the stretching law and laser pulse duration,we estimate the bond length and the neutral molecular bond length of the two molecules under ionization conditions under our experimental conditions.The R_SO bond is about 4.3×10³ m/s,and the R_NO bond is about 5×10³ m/s.The R_SO of the neutral bond length of the SO₂ molecule is 2?,which is much longer than the actual bond length of 1.43?.The R_NO bond of the N₂O neutral molecule is 1.24?,which corresponds to the actual bond length of 1.19?,indicating that the bond length stretching of SO₂ molecules before trivalent is more violent than the higher valence states and the evolution of the molecular configuration of SO₂ molecules at low valences is non-coulomb potential.However,the bond length stretching of the entire multi-ionization process of the N₂O molecule moves along the Coulomb potential.