Electron Correlation In Strong Field Double Ionization: From Multi-photon To Few-photon

Electron correlation is a ubiquitous phenomenon in multi-electron systems and plays an important role in many strong field processes.When intense laser field interacts with atoms and molecules,the electron correlation effect is particularly prominent in two processes: double ionization and doubly excited state resonance.In the early double ionization experiments with infrared and nir-infrared lasers,it was found that the yield of double ionization increased at low laser intensity.Electron correlation plays a decisive role in this phenomenon.In recent years,benefiting from the rapid development of laser technology,few-photon double ionization in ultraviolet and extreme-ultraviolet laser pulses has drawn a lot of attention.Typically,few-photon nonsequential double ionization has led to a research upsurge because it reflects strong electron correlation effect.The sequential and nonsequential double ionization mechanisms at long and short wavelengths are quite different.The knowledge of the physics in these phenomena are still not adequately clear,especially for the electron dynamics of doubly excited states in double ionization.Thus we studied the physics in double ionization in these two wave bands,and the electron correlation phenomena with doubly excited states involved.In infrared and ultraviolet bands,the double ionization processes correspond to multi-and fewphoton mechanisms.We applied different models for the theoretical research of the two processes.The contents and innovations of this thesis are as follows,(1)In infrared band,we systematically study the dependence on pulse width of multi-photon nonsequential double ionization of argon atoms.In this regime,electrons may return to the parent nucleus by re-collision to form doubly excited states.We use classical ensemble model to simulate the double ionization and get the momentum spectrum.The results show that there is a strong electron correlation in short wavelengths but not in long wavelengths.We statistically analyzed the time of each event in double ionization and found that the collision-double ionization time delay is the decisive factor for different electron correlation behavior in short and long wavelengths.We find that when the doubly excited states tunnel dominates the double ionization,the delay and the proportion of double-excited-state double-ionization are the main factors affecting the phenomenon of double-ionized electron correlation.(2)In ultraviolet band,we systematically study the electron dynamics in few-photon double ionization of helium atom.By numerical simulation with the quantum model,we find in the momentum spectra the double and triple splitting phenomena.Through analysis of the energy levels in the dressed-atom picture,we find that when the electrons are doubly excited by resonance from the ground state,the energy level of the doubly excited state may be doubly split.When there is a third level which can be simultaneously resonantly excited,triple splitting of the double excitation level may occur.This level splitting results in joint Rabi oscillations of states participating the resonant double ionization processes.In addition,we point out that the triple splitting of energy levels results from the detuning effect in the excitation process.Thus a new way to regulate the electronic correlation behavior by light intensity is proposed.(3)We study the two-photon resonance two-ionization process in the extreme ultraviolet region modulated by infrared light.We observed radial and angular interference structures in the momentum spectrum.By means of perturbation analysis,we determine that they have different formation mechanisms from those observed in the two-photon double ionization momentum spectrum under two ultraviolet pulses.By modifying the perturbation model,we repeat these interferences.It is believed that the interference is mainly due to the ac Stark effect of the infrared laser on the doubly-ionized continuum state.We propose that under short pulses,the asymmetry of interference structure may result from the streaking effect of infrared field or the interference between ionization tunnels involving double excited states.