Exploring And Controlling Electron Dynamics In Strong-Field Nonsequential Double Ionization

Nonsequential double ionization in strong field is a basic process of the interaction between the intense laser and atom or molecule.The two electrons from nonsequential double ionization show a strong correlation,which provides an important channel for people to study electronic correlation at the atomic and molecular level and thus makes the underlying electronic dynamics of nonsequential double ionization have been a hot topic in strong-field physics.Nowadays it has been known that nonsequential double ionization occurs through “recollision”,but the complexity in the recollision process of correlated electrons hampers a full exploration of the details in nonsequential double ionization.More interestingly,the microscopic dynamics in nonsequential double ionization changes with the driving laser field.With the classical ensemble model and semiclassical ensemble model,in this thesis the microscopic electron dynamics in nonsequential double ionization at the cases of low laser intensity and the mid-infrared laser pulses is studied.Especially,this thesis investigates the controlling of the two correlated electrons' dynamics in strong field nonsequential double ionization.Moreover,this thesis has innovatively proposed and demonstrated probing the release time of two electrons in nonsequential double ionization with counter-rotating circularly polarized two-color laser pulses.The main content and innovation points of our work include:(1)Multiple-recollision process in nonsequential double ionization at low laser intensity is investigated.At low laser intensities,the recollision energy of the first electron is not enough to ionize the second electron directly by recollision so that in this case the multiple-recollision process is the main dynamics in nonsequential double ionization.Because of the multiple recollisions,the ion gain more energies from the laser field,forming higher doubly excited state and then the double ionization occurs.The calculation results indicate that multiple-recollision trajectories of nonsequential double ionization result in the different correlated electron momentum spectrum distributions with single-recollision events.The proportion of double-recollision trajectories in total nonsequential double ionization events increases with the decreasing laser wavelength and decreasing laser intensity.(2)Nonsequential double ionization with laser pulses in the mid-infrared region is investigated and the multiple-returning recollision process is found.With the semiclassical ensemble model,the number of returnings depends on the first electron's initial transverse velocity at tunneling.The recollision electron occupies the higher energy in the mid-infrared region,thus the asymmetric energy sharing between electrons at recollision is more obvious than that in the near-infrared region,which is responsible for the strong repulsion behavior with respect to the diagonal in the correlation electron momentum distribution.(3)On this basis,controlling the dynamics of the correlated electron in nonsequential double ionization by parallel two-color fields is investigated.The recollision of returning electron is well confined in a time window of a few hundred of attoseconds with the two-color fields,and the position of which can be controlled with attosecond precision by tuning the relative phase.Thus,the recollision energy and the time delay between ionization times of the two electrons after recollision is accurately controlled with the two-color fields.According to this theoretical result,we launched experimental study and controlling the nonsequential double ionization by parallel two-color laser pulses has been achieved,which is reflected in the experimental observed ion momentum distributions.(4)A scheme that probing the emitting times of the two correlated electrons in strong-field nonsequential double ionization by the counter-rotating circularly polarized two-color laser pulses is proposed.Usually people probe the strong-field electron's ionization time with an elliptically or circularly polarized laser field based on the angular streaking technique.But recollision is forbidden in the laser field with the large ellipticity and thus the nonsequential double ionization is strongly suppressed.Thus the technique can't be used to record the release times of the correlated electron pairs in the nonsequential double ionization.With the counter-rotating circularly polarized two-color laser pulses,the emitting times of the two correlated electrons in strong-field nonsequential double ionization is investigated for the first time.In the counter-rotating circularly polarized two-color laser fields,the first electron can return and recollision with the parents ion and the nonsequential double ionization happens.The counter-rotating circularly polarized two-color laser field also possesses the property of angular streaking and thus it can be used to trace the release times of the emitting times of the correlated electrons in nonsequential double ionization.By theoretically calculation,probing the two correlated electrons' ionization times in nonsequential double ionization with this scheme is demonstrated and the dependence of the ionization time on the laser field is also displayed.