Study On Resonance-Like Enhancement In HATI Of Atom In Intense Laser Fields

Investigation of the interaction between ultrafast intense laser field and atoms and molecules is one of the most active frontier research areas currently. In this extreme condition, many novel non-perturbative physical phenomena may happen to atoms and molecules, including multiphoton ionization (MPI), tunneling ionization (TI), above threshold ionization (ATI) and nonsequential double ionization (NSDI) as well as the high-order harmonic generation(HHG). Among all, above threshold ionization phenomenon has always been one of the hot topics in the research of atomic and molecular physics in the strong laser field since it was discovered. Due to the complexity of the interaction between the strong laser field and atoms and molecules, the consensus has not been achieved for the physical mechanism behind the some experimental phenomenon of the above threshold ionization. One of them is the resonance-like enhanced structure in the photoelectron spectroscopy of high order above threshold ionization, and it needs to be further investigated theoretically.On the other hand, due to the significant development of the ultrashort intense laser technology in tunable mid-infrared fields in recent years, the research of atomic and molecular physics in the strong field has been extended gradually from near infrared to mid-infrared fields. At present, much attention has been attracted on the mid-infrared laser field physics in the world.Quantum S-matrix theory and "uniform approximation" method are used to study the resonance-like enhancement structures in photoelectron spectrum of high-order above-threshold ionization for argon atoms in the near infrared and mid-infrared fields in this thesis. Our research show that both in the near infrared and mid-infrared fields, the resonance-like enhancement structures that a group of several adjacent high-order ionization on the threshold peaks exhibits a significant enhancement by up to an order of magnitude when the laser intensity is increased by only a few percent will appear in the spectrums and the corresponding laser intensities satisfy the channel closings conditions, which further confirms the explanation of channel-closing mechanism of the resonance-like enhancement. In addition, we also find that with increasing laser wavelength and intensity, the enhancement and suppression will appear alternately in the photoelectron energy spectrum. This phenomenon may be attributed to the interference of "quantum orbital" of electrons which collide with the core at different return times. It may be used to explain the experimentally observed extension of the RLE energy region at longer wavelength. Moreover, we also show that in strong mid-infrared laser fields, the type-I and type-II RLE structures will also appear, which is similar to that in the near infrared laser fields.