Experimental Investigations On Atomic Ultrafast Ionization And Rydberg State Excitation Dynamics In Strong Laser Fields

With the development of ultrafast laser technology,people can study the dynamic processes and structures of atoms and molecules with sub-angstrom spatial and attosecond temporal resolution.Recently,a series of interesting ultrafast nonlinear phenomena have been found in the study of the interactions of intense laser fields and atoms and molecules,such as above-threshold ionization(ATI),nonsequential double ionization(NSDI),high-order harmonic generation(HHG),Rydberg states excitation(RSE),and so on.The understanding of these strong-field atomic physics has laid a solid foundation for the development of emerging disciplines such as molecular ultrafast imaging and attosecond science.The strong-field ionization and RSE processes have attracted widespread attention in recent years.In this thesis,using the time-of-flight spectrometer and cold target recoil ion momentum spectroscopy(COLTRIMS)and combining with the home-made laser intensity recording system and semiclassical model,we have investigated the dynamics of atomic ionization and RSE processes induced by a strong laser field.The main works and achievements are listed as follows:1.Investigations on long-range Coulomb effect in above-threshold ionization subject to few-cycle and multicycle laser fields.We have performed a joint experimental and theoretical work investigating the low-energy part of the ionic momentum for Ne subject to few-cycle and multicycle laser fields.It is found experimentally that at 800 nm the low-energy structure(LES)shows itself as a double-hump structure(DHS)in momentum distribution of singly charged ion for Ne,and moreover,this structure is more prominent for multicycle laser fields than for few-cycle cases.This result can be reproduced and explained qualitatively with a semiclassical model and attributed to the paramount role of long-range Coulomb effect(LRCE).That is to say,after the laser field vanishes,the electrons decelerate while flying away from the core by the long-range tail of Coulomb potential,which eventually makes DHS less notable.2.Investigations of a transition in the dynamics of strong-field atomic excitation.The intensity dependence of Ar*and Ar+ yields in an intense laser field has been investigated experimentally and theoretically.At 400 nm,the measured intensity dependence of Ar*yields shows a prominent sequence of steps.The first enhancement is accompanied by the increase of Ar+yields and can be understood by multiphoton resonance mechanism,while the second enhancement appears at the intensity where a local minimum of Ar+yields comes out.Moreover,at 800 nm,the yields of Ar*exhibit significant modulations as a function of laser intensity,which are out of phase with respect to the yields of Ar+.With the analysis by a dedicated quantum model of RSE,it is shown that the coherent recapture mechanism plays an important role in the high-intensity regime at 400 nm and also the whole intensity range explored in this work at 800 nm.Our results reveal that the mechanism of RSE experiences a transition from multiphoton resonance to coherent recapture mechanism.This finding provides a comprehensive physical picture behind the strong-field RSE process.3.Investigations on the wavelength scaling of argon atomic excitation induced by intense laser fields.We have performed a joint experimental and theoretical investigation on the wavelength dependence of the atomic RSE process.It has been found that both the yields of Ar+and Ar*decrease with the rising laser wavelength for a certain laser intensity.With a nonadiabatic CTMC method,the experimental results can be qualitatively reproduced,which validates the FTI mechanism at mid-infrared wavelengthes.Our result implies that longer wavelength will induce narrower electron momentum distribution and larger spatial distribution right after the laser pulse ends.Correspondingly,the proportion of the high principal quantum number of the Rydberg state distribution increases.