Investigations On Ionization Dynamics Of Noble Gas Atoms In Strong Femtosecond Laser Fields

With the development of ultrafast intense laser technology,the interaction of femtosecond intense laser and matter has provided a powerful method to study the ultrafast dynamics of atoms and molecules,and explore the new phenomena in atomic physics under extreme strong field conditions.In the intense femtosecond laser field,light-matter interaction enters a new nonlinear and strong nonperturbative region and a series of novel physical phenomena have been found in experiment,such as multiphoton ionization,above threshold ionization,nonsequential double ionization,and high-order harmonic generation.The study of these new phenomena has become an important forefront of the atomic and molecular physics,promoting the birth and development of many emerging disciplines.For example the generation of the high-order harmonic through the interaction of femtosecond laser and atomic and molecular gas is the preferred way to obtain attosecond pulses,and is also the only way to realize the attosecond pulse,which directly leads to the emergence and development of attosecond physics.Using time-of-flight spectrometer,cold target recoil ion momentum spectroscopy(COLTRIMS),other experimental technologies,and combined with the semiclassical theory,we have investigated the influence of the laser parameters and the internal structure of the atoms on the behavior of strong field ionization.The main works and achievements are listed as follows:1.Wavelength scaling of nonsequential double ionization for xenon in intense laser fields is experimentally studied.The observed wavelength dependence of the ratio Xe2+/Xe+ in experiment deviates significantly from the prediction of the simple-man model,both in the slope of the overall decrease with increasing wavelength and in the existence of some pronounced humps.A simulation based on a semiclassical model,which includes the ionic Coulomb potential,reproduces the changing slope of the overall decrease,which is due to the interplay of wavepacket diffusion,Coulomb focusing,and a closely related Coulomb defocusing effect of the liberated electron.The hump is beyond the scope of the semiclassical model.2.The physical mechanism behind nonsequential double ionization of Xe in intense midinfrared laser fields is experimentally studied.The observed doubly charged ion momentum distribution of Xe at 2400 nm exhibits a distinct transition from a flat-top structure near zero longitudinal momentum at lower laser intensity to the one with two maxima at nonzero longitudinal momentum at higher laser intensities,which is remarkably different from the case of 800 nm.Simulation based on a rescattering model is used to obtain the ratios of contributions from the recollision-impact ionization(RII)and the recollision-induced excitation with subsequent field ionization(RESI)in nonsequential double ionization.Our calculation reveals that the increasing contribution of the RII channel is responsible for the more prominent double-hump structure at longer wavelength or higher laser intensity.Moreover,a simple fitting allows one to reproduce the experimental ion momentum distributions well and obtain contributions from these two channels.3.The atomic polarization effect on photoelectron angular distributions in above-threshold ionization of noble gases in elliptically polarized fields is studied.We theoretically investigate the atomic polarization effect on photoelectron angular distributions(PADs)of Ar,Kr,and Xe with elliptically polarized laser fields.Simulations based on a semiclassical model that includes both the ionic Coulomb potential and the atomic polarization effect show surprisingly hardly any difference between PADs for Ar,Kr,and Xe,which is in good agreement with recent experimental observations.Our calculations reveal that atomic polarization effect increases the distance from the tunnel exit point of the photoelectron to the parent ion and weakens the strength of the interaction between the parent ion and the photoelectron on its subsequent classical propagation.As a result,the production of the forward scattering electrons are substantially suppressed and the production of direct electrons are increased.Our results indicate that the insensitivity of PADs for Ar,Kr,and Xe may be closely related to the influence of the atomic polarization effect.