| With the rapid development of laser technology,the interaction of ultrashort intense laser pulses with atoms and molecules can be utilized to generate various nonlinear phenomena,such as high harmonic generation,nonsequential double ionization,and above-threshold ionization.Among these,the photoelectron emission spectrum of above-threshold ionization of atoms carries information about the quantum state of the system and information about the driving laser,making it a valuable tool for diagnosing the parameters of incident ultra-short laser pulses and probing the electron structure inside the atoms.Due to the limitation of light sources,previous studies of photoelectron emission spectroscopy typically employed near-infrared laser pulses as the driving source,and the primary ionization mechanism of atoms was tunnel ionization.However,with the advancement of free-electron lasers and high-harmonic generation techniques,it has become possible to obtain extremely short-wavelength light sources in the extreme ultraviolet(XUV)region.Few-photon ionization becomes the predominant ionization process when such light sources interact with atoms.In this process,the electrons may first resonantly transition to the lower excited state of the system and then ionize,making the process of their photoelectron emission more complex.To analyze the role of excited states in this process,we employed a time-dependent pseudo-spectral approach in momentum space and numerically solved the time-dependent Schr(?)dinger equation.We systematically investigated the above-threshold ionization process of hydrogen atoms under different laser conditions,with a focus on the effect of the excitation process on the photoelectron emission spectrum.The specific work includes the following three parts:Firstly,the effect of the duration of the laser pulse on atomic above-threshold ionization was investigated.It was observed in the photoelectron emission spectrum of atoms that,in addition to the above-threshold ionization peaks generated by multiple cycles and the sideband peaks resulting from the interference of the ionization electron wave packet at different times during the rising edge of the pulse envelope,the photoelectron emission peaks with energy that remained almost unchanged with varying laser pulse width are also observed.Based on strong-field approximation calculations and analysis of the population of bound states,the reason for the appearance of this photoelectron peak was elucidated.It arises from electrons resonantly transitioning from the ground state to an excited state,followed by ionization from the excited state.As the pulse width of the driving laser increased,oscillatory structures were observed in the peak intensity of this photoelectron peak.The analysis revealed that the reason for this oscillation structure is the enhancement effect caused by the overlap of the peak energy position and the sideband energy position.Furthermore,it was discovered that the energy positions with the highest intensity in the sideband peaks of the photoelectron spectra are shifted to higher energies with increasing pulse width.The analysis indicated that this shift is due to changes in the moment of atomic ionization primarily as the pulse width increased.The atomic photoelectron emission spectrum provides a new for real-time monitoring of atomic ionization processes.Secondly,the effect of the intensity of the driving laser on atomic above-threshold ionization was studied.Initially,the photoelectron emission spectrum of hydrogen atoms under the influence of a 317 nm wavelength driving laser was calculated as a function of laser intensity.It was observed that,in the photoelectron emission spectrum,not only were photoelectron peaks generated by direct ionization from the ground state with different cycles observed,but also new photoelectron peaks were observed.The peak energy of these new peaks shifted towards higher energies with increasing laser intensity,in the opposite direction to the shift of the above-threshold ionization peak.Through the strong-field approximation model,the time-dependent excited state population and the analysis of the superposition state,it was found that these photoelectron peaks were generated due to interference between electron wave packets ionized directly from the ground state and those ionized after resonantly transitioning from the ground state to the 2p excited state.Furthermore,the ionization process of hydrogen atoms under the influence of a shorter wavelength(152 nm)driving laser was investigated.It was observed that as the intensity of the driving laser increased,the ionization probability rapidly increased before showing a decrease,indicating the presence of ionization suppression characteristics.Within the range of ionization suppression at certain laser intensities,no multi-peak structures resulting from dynamic interference were observed in the photoelectron emission spectrum.Through analysis of the photoelectron emission spectra from different excited states,it was found that within this range of laser intensities,electrons were first excited in the laser field before ionization.The ionization suppression intensity varied for different energy excited states,leading to the disappearance of the typically expected dynamic interference phenomena in the photoelectron emission spectrum.Thirdly,the study focused on the photoelectron emission spectrum of atoms under the influence of shaped driving laser pulses.It was observed that changing the parameter A in the Sin phase function of the shaped pulse could control the number of sub-pulses in the shaped laser electric field and the relative amplitudes between different sub-pulses.Using this approach,the photoelectron emission spectra of atoms were investigated under the same Keldysh factor conditions(i.e.,Ponderomotive energy)with driving laser wavelengths of 200 nm,400 nm,and 800 nm for the shaped pulses.The research revealed that under the conditions of an 800 nm wavelength driving laser,the influence of excited states on the ionization process was relatively small.However,under conditions of 400 nm and 200 nm wavelengths,the excited states of atoms had a significant impact on the photoelectron emission spectrum.Under these conditions,the photoelectron emission spectrum of atoms exhibited a highly complex peak structure.In addition to observing above-threshold ionization peaks generated by different sub-pulses,new photoelectron peaks were observed due to electrons resonantly transitioning from the ground state to the excited state.Furthermore,sub-peak structures resulting from interference between ionized electron wave packets from different sub-pulses in the shaped pulse were also observed.Through strong-field approximation calculations and analysis of the photoelectron emission results from different sub-pulses,it was shown that these structures were generated due to interference between electron wave packets ionized from the ground state under multiple sub-pulses in the shaped driving laser pulse,as well as the contribution of excited states.A comparison between different wavelengths revealed that it was easier to control the excitation of atoms under the influence of short wavelength driving laser pulses,enabling more precise modulation of atomic photoelectron emission. |
