Atomic Ionization In The IR+XUV Two-color Laser Fields

The interaction between matter and an intense laser field can result in lots of important nonlinear phenomena,such as above-threshold ioniztion(ATI),highorder harmonic generation(HHG),high-order ATI(HATI)and nonsequential double ionzation(NSDI).Recently,with the rapid development of laser technology,the infrared(IR)and extreme ultraviolet(XUV)two-color laser fields have become an effective tool to study above nonlinear phenomena in the strong-field physics.This thesis investigates the HATI and NSDI processes of an atom exposed to the IR+XUV two-color laser fields by applying the frequency-domain theory based on the nonperturbative quantum electrodynamics.The HATI and NSDI processes belong to the recollision model,which can be considered as two-step processes in the frequency-domain theory: ATI followed by a laserassisted-collision(LAC).Additionally,the NSDI process in the recollision model includes two types: the collsion-ionization(CI)and the collision-excitationionization(CEI)mechanisms.Our work mainly focuses on the HATI and NSDI processes as follows:Firstly,we investigate an HATI process in IR+XUV two-color laser fields.We present the angular distribution of HATI process with mutiplateau structure,where there exists a dip structure in the second plateau.By the channel analysis,we find that the interference pattern of the angular distribution results from the contributions of these different channels,and the formation of the dip structure is attributed to the fact that atom absorbs another XUV photon in an LAC process.Furthermore,with the help of the saddle-point approximation,we obtain the classical orbit equation and find the source of interference satisfied by the ionized electron in the LAC process.Moreover,we discuss the correspondence between the quantum interference and the classical orbits in the HATI process.Secondly,we investigate the NSDI process caused by the CI mechanism in IR+XUV two-color laser fields,where the photon energy of the XUV laser is higher than the atomic double ionization potential.We obtain the NSDI momentum spectrum,which presents a multiplateau structure.By using channel analysis,we find that the height of the plateau is determined by the number of XUV photons absorbed by electrons.In order to explain the interference pattern of NSDI momentum,we compare the contributions of the forward and backward collisions to the NSDI process.We find that,the forward collision dominates the NSDI when the two electrons are ejected along the same direction;while both the forward and backward collisions make comparable contributions to the NSDI when the two electrons are along the opposite directions.In addition,by applying the saddle-point approximation,we obtain an energy conservation equation and explain the formation of the NSDI in classical view point.Thirdly,we investigate the dependence of momentum spectra on laser intensity in an NSDI process caused by the CI mechanism.We find that the two laser fields play distinct roles in the NSDI process,where the IR laser field mainly determines the width of each band,and the XUV laser field mainly plays a role on the probability of NSDI process.Furthermore,we discuss the effect of two-color laser fields on the ATI and LAC processes.It is found that,the IR laser field is responsible for broadening the peak in the ATI process and providing additional momenta to the two ionized electrons in the LAC process;while the XUV laser field plays a crucial role on the height of the spectrum in the ATI process,and influences the radii of orbits in momentum space in the LAC process.Finally,we investigate the NSDI process caused by the CEI mechanism.The momentum spectrum presents a complex interference pattern and symmetric structure.With the help of the channel analysis,we find that,when the energy of recolliding electron increases,the source of the energy to excite the bound electron changes from absorbing the XUV photons to pure energy transfer between two electrons in the laser-assisted collision excitation process.As a result,both the forward and backward collisions make comparable contributions as the energy of recolliding electron is small,while the forward collision makes a dominant contribution to the NSDI as its energy is large.Moreover,we reconstruct the interference pattern of NSDI spectrum by the two separate ATI spectra,which may be used to analyze the NSDI process.