Atoms And Molecules In Intense Elliptically Polarized Laser Fields: Above Threshold Ionization And Double Ionization

The progress of laser technology in past decades has greatly promoted the development of atomic and molecular physics in ultrafast strong optical fields.The interaction between atom and molecule and ultrafast strong optical field can produce many novel and interesting phenomena.Studying these phenomena not only increases the understanding of the interaction mechanism between laser fields and atoms or molecules,but also brings many new applied technologies.Among them,ionization of atoms and molecules in ultrafast optical fields is the basic phenomenon in strong field physics,and acts as the first step of many subsequent strong field processes,and thus,it has always been at the focus of researchers in the worldwide.Studying the effects of laser field or atomic and molecular properties on the strong field ionization and revealing its intrinsic mechanism playing a part in the process of ionization dynamics are of great significance to the development of the theory and application of strong field physics,including the study of the initial distribution of tunneling electrons,molecular orbitals and structure imaging,generation of attosecond and extreme ultraviolet light sources,etc.In this dissertation,we mainly study the single and double ionization behaviors of atoms and molecules in elliptically or circularly polarized laser fields by coincidence measurement method.Firstly,we have achieved the construction and test of COLTRIMS required for coincident measurements.As a powerful tool,COLTRIMS can detect experimentally the ionization and dissociation of atoms and molecules in strong fields because of its advantages of three-dimensional momentum imaging and multi-particle coincidence measurement.This setup guarantees our follow-up experimental research.In the elliptically polarized laser fields,on the one hand,we can use the rotating electric field and dependence on ellipticity to study the law of strong field ionization,on the other hand,we can also reconstruct the molecular frame,by which we found some new phenomena experimentally.The ionization and double ionization behaviors of atoms and molecules in femtosecond strong laser field are the research theme of this dissertation,specifically detecting the above threshold ionization of Xe atoms in elliptically polarized laser fields,the dissociation double ionization of CO molecules in circular ly polarized fields,and the strong-field double ionization of Kr atoms by using a coincident measurement.We firstly measure the above threshold ionization of Xe in elliptically polarized near-infrared laser fields by using COLTRIMS.Electron interference structures modulated by the laser ellipticity were observed in the experiment.By extracting the differential distribution of the electron momentum width in the direction perpendicular to the optical polarization plane and studying the corresponding electronic trajectories of different structures,and observing their response to the ellipticity variation,we can analyze and identify the origin of these structures.When the ellipticity is small,the electron low-energy structure produced by Coulomb focusing can be observed,which direction points to the major axis of the polarized ellipse and does not change with the ellipticity.And it disappears when the ellipticity is larger than 0.6.But the interference structures of forward-scattering and direct-ionized electrons strongly depend on the ellipticity.From linear polarization to small ellipticities,not only the yields of the forward-scattering interference structure increase or decrease for different electron momentum regions,but also its phenomenon of tilt occurs in the whole range of electron momentum.In addition,we propose an in-situ measurement method to extract the accurate ellipticity of near-circularly polarized laser pulses by using two elliptically polarized pulses with opposite helicities.Comparing the experimental measurements and theoretical calculations,it is confirmed that the ATI electron spectra and ion yields,which both varies periodically with the time delay of two elliptical pulses,can accurately give the ellipticity of the laser pulses.Nextly,dissociative and sequential double ionization of CO molecules in circularly polarized strong fields are studied with four-body coincidence measurements.The measurement for the channels with high kinetic energy release show the important contribution of ionization of low-occupied molecular orbitals and post-ionization field-driven excitation.It is also shown that the method extracting the individual molecular frame distribution of two electrons by circularly polarized laser can be used to analyze the complicated ionization,excitation and dissociation dynamics in molecular dissociative double ionization.Furthermore,the measurement of channels with low kinetic energy release show an observation of unusual low-energy electrons.We assign this channel to the autoionization state of CO⁺that is produced by strong laser fields.And the product ions on the ation state are firstly ionized from the autoionization state in the circularly polarized near-infrared fields and then dissociated from CO²⁺along its potential energy curve.By further analyzing the molecular frame angular distribution of these low-energy electrons,it is found that the angular distribution changes continuously from double peak to single peak as the kinetic energy decreases.This is quite different from the weakly anisotropic angular distribution of autoionizing electrons,suggesting that the low-energy electrons mainly come from field ionization rather than autoionization.The variation of angular distribution indicates the nuclear-distance-dependent ionization dynamics of CO⁺autoionization states in circularly polarized fields.In addition,by changing the group delay dispersion of laser pulses,it is found that the positively chirped pulses can significantly enhance the yield of dissociation channels related to the autoionization state.Finally,we illustrate a study on the electron correlation in Kr strong field double ionization by fully differential measurements and semi-classical simulation.The evolution from non-sequential to sequential double ionization mechanism was firstly confirmed by the measurements of intensity-dependent ion yield and ion momentum.By comparing the semi-classical calculation results based on helium-like model and GSZ potential model,which includes the inner-shell screening effect,the electron momentum correlation spectra obtained under typical laser intensities show the important role of dynamic screening effect in nonsequential double ionization.In the sequence double ionization regime,the momentum correlation spectra and the sum-energy distributions of the two electrons show the imprint of electron correlation.The semi-classical calculation with GSZ model reveals the key role of Coulomb focusing and Coulomb singularities in this phenomenon.All in all,by using coincidence measurements and some theoretical calculations,we study the influences of many factors,including elliptically polarized laser fields,Coulomb focusing,molecular multiorbital effect,and screening effect of inner-shell electrons,on the atomic and molecular ionization.And we also study the phenomena of electronic interference,field-induced excitation and population of molecular autoionization state during or after ionization.