Two-photon Double Ionization Of Helium Under The Action Of Extreme-ultraviolet Laser Pulse

Many fundamental processes in nature,such as chemical reactions,superconduc-tivity and giant magnetoresistance,et_c.,are governed by multi-electron dynamics.The interaction between intense ultra-short laser pulses and atoms can produce many non-liner phenomena such as multiphoton ionization,high-order harmonic generation and multi-electron ionization,et_c.Deep understanding of multi-electron dynamics pro-cesses of some simple systems,such as the double ionization process of helium,can help us understand multi-electron dynamics processes in more complex systems.This thesis investigates a two-photon double-ionization(TPDI)process of helium under the ac-tion of an extreme-ultraviolet laser pulse by using numerical solution of time-dependent Schrodinger equation,and the main results are summarized as follows:Firstly,we develop a program for solving the time-dependent Schrodinger equation of helium based on the B-splines method for atomic structure calculations,which can be used to investigate single-photon double ionization and TPDI processes of helium under the action of an extreme-ultraviolet laser pulse.Secondly,there exists a characteristic time t_c for a TPDI process,such that the pattern of energy distribution of two ionized electrons presents a peak or two,depending respectively on whether the pulse duration is shorter or longer than t_c through analyzing the time evolution of two-electron wave packets.Especially,as the pulse duration is greater than t_c,the TPDI spectrum shows a double-peak structure which is attributed to the fact that most of the electron-electron Coulomb interaction energy of the ground state is acquired by single electron during their oscillation around the nucleus before the two electrons leave,and hence the double-peak structure cannot be identified as a signal of sequential ionization.As the pulse duration is less than t_c,the TPDI spectrum shows a single-peak structure which is attributed to the fact that most of the electron-electron Coulomb interaction energy of the ground state is shared equally by two electrons after the end of the laser pulse.Thirdly,the characteristic time t_c for the case of the carrier frequency larger than the ionization energy of the second electron has a key relationship with the Coulomb interaction energy (?)12 between the two electrons in the ground state,which can be expressed as t_c(?)12 ? 4.In addition,the characteristic time t_c can be understood as the minimal evolution time from the ground state to a double ionization state with two electrons carrying different energies according to the concept of quantum speed limit.Fourthly,characteristic time t_c for TPDI process and the Coulomb interaction en-ergy (?)12 between the two electrons in the ground state still satisfy the relationship t_c(?)12 ? 4 when changing the charge of the nucleus,the charge of electrons,the mass of electrons,or the charges of the nucleus and electrons.Finally,when the 2S state is the initial state of the TPDI process,the energy distri-bution of the two ionized electrons will change from a single peak to a double-peak as the pulse duration increases.