Femtosecond Time-resolved Dissociative Ionization Dynamics Of Polyatomic Molecues With Coulomb Explosion Imaging

Femtosecond laser can induce multiple ionization of molecules and a Coulomb explosion.Based on the coincidence measurement method,the momentum distribution of fragment ions can be obtained to reconstruct the structure of the parent molecule.This method is called Coulomb explosion imaging?CEI?.The Coulomb explosion imaging method can realize the direct imaging measurement of molecular structure,and has received extensive attention.This paper includes two aspects:On the one hand,the polarization-dependent Coulomb explosion imaging of the highly symmetric molecule SF₆ is studied.On the other hand,the time-resolved pump-probe?elliptically polarized laser as a pump light,linearly polarized laser as a probe light?of chiral polyatomic molecular Coulomb explosion dynamics process is studied,we get a series of interesting results.Firstly,we use a cold target recoil ion momentum imaging spectrometer?COLTRIMS?to study the Coulomb explosion imaging of SF₆ molecules induced by linearly polarized laser and circularly polarized laser.The ground state of the monovalent ion of SF₆ molecule is unstable,and ultrafast neutral dissociation can occur to form SF₅⁺.This ultrafast dissociation characteristic provides a unique research object for the study of molecular Coulomb explosion,showing the dissociative ionization kinetics characteristics of polyatomic molecules unlike linear molecules.According to the kinetic energy release?KER?and momentum distribution of SF₆⁺?SF⁺₅+F and its neutral dissociation channels,it indicates that the dissociation of neutral F atoms occurs after the secondary ionization of the parent.In addition,the enhanced ionization model that usually explains the Coulomb explosion cannot be used to explain the Coulomb explosion behavior of SF₆.The ultrafast dissociation of the monovalent parent ion suppresses the enhanced ionization process,and the stretching of molecular bonds occurs on the potential energy surface of the bivalent parent ion.Finally,the three-body dissociation process of the molecule reflects when different F atoms dissociate,the molecule will rotate significantly.The Coulomb explosion of linear molecules depends on the polarization of driving light,while the Coulomb explosion of SF₆molecules hardly depends on the polarization characteristics of the laser,which is closely related to the symmetry of the molecule.The high symmetry of the molecule leads to a weak laser collimation effect,which also proves that the molecule collimation effect is the main mechanism that leads to the polarization dependence of dissociative ionization of linear molecules.Secondly,we apply the time-resolved Coulomb explosion imaging technique to study the ion dissociation kinetics of formic acid molecules and their dimer.Using horizontally polarized linearly polarized laser as a pump light,and elliptically polarized lasert with an ellipsometric rate of 0.7 as a probe light,combined with the COLTRIMS detection device,the time-dependent behavior of the channel resolution of formic acid and its dimer is obtained.The energy release of a part of the HCOOH²⁺?CHO⁺+OH⁺channel gradually decreases with the Delay time,indicating that the C-O bond is gradually stretched before the Coulomb explosion,and the kinetic energy approaches equilibrium after 2000 fs,which may reflect a local minimum value of this dissociation potential energy surface,enhanced ionization at this location is the main dissociative ionization mechanism of this channel;the HCOOH²⁺?COOH⁺+H⁺channel is only enhanced at time zero,and no energy reduction with time is observed,indicating molecular ions C-H bond may have undergone rapid dissociation,direct double ionization is the main mechanism of this channel.More interestingly,for the HCOOH²⁺?CHO⁺+OH⁺channel,there is a polarization-dependent dynamic behavior:There is an obvious extreme value distribution in the region where the two lasers have a Delay of-400 to 0fs?negative Delay represents elliptically polarized laser pump,linearly polarized laser probe?and a total energy range of 2.5 to 4.5eV,but the corresponding 0 to 400fs positive Delay?linearly polarized laser pump,elliptically polarized laser probe?region has no obvious enhancement.The extremum distribution around 3.6 eV caused by elliptically polarized laser may reflect the coupling between different potential energy surfaces induced by the laser field,resulting in a local minimum in the C-O stretching process,and elliptically polarized laser will enhance the coupling process.This shows the possibility of applying the polarization characteristics of a strong laser field to control the molecular dissociation dynamics.The next step is to compare the experiments of laser intensity dependence and linearly polarized laser pump-linearly polarized laser probe,and develop a polarization control scheme for molecular dissociation dynamics.