Pulse duration effect on photoelectron spectrum of atom irradiated by strong high frequency laser

Abstract

With the development of high harmonic generation and the free electron laser,one can obtain the laser pulses whose frequencies range from XUV to X ray.Using these novel light sources,one can investigate the electron dynamics with attosecond resolution.With the increase of intensity,a lot of nonlinear processes have been found,such as high harmonic generation, above threshold ionization and dynamic stabilization of atomic ionization.When the atom is irradiated by an ultra-intense short laser pulse,many additional sub-peaks appear in the original photoelectron peaks.The original peaks of the photoelectron spectra are formed by the ionization interference from different optical cycles.The formation of sub-peaks are attributed to the shift energy level by the action of strong laser electric field.In previous studies,the sub-peak phenomenon was mainly observed in the short pulse.In this work,we investigate the duration effect of laser pulse on this phenomenon.The photoelectron is calculated from the time-dependent wavefunction in momentum by using generalized time dependent pseudo spectral scheme.At small laser intensity,there is only main photoelectron peak near the position whose energy is the difference between the central frequency of the laser and ionization energy.As the laser duration decreases,the width of the photoelectron peak gradually increases.For the higher laser intensity,many sub-peaks appear in the photoelectron spectra.The width of the sub-peak is also decreasing with the increase of the laser pulse's duration. The amplitude of these sub-peaks is decreasing with the increasing of the duration of laser pulse.For the longer pulse (50 optical cycles),these sub-peaks disappear.The variation of the amplitude and energy position for the first sub-peak with the laser intensity is analyzed.As the increase of laser pulse width,the energy of the sub-peak increased.Comparing with the longer pulse,the short pulse has a larger enhancement.In order to understand the profiles of the photoelectron spectra,we investigate the time-dependent ionization profile of the atom.The results show that the ionization occurs in the whole duration of the laser pulse for small incident intensity.The ionization mainly occurs at the raising edge of the laser pulse for the large laser intensity.For the longer pulse,the gradient of laser intensity is small.Its energy level shift effects on the ground state of the atom is small.Thus, one can not observe any sub-peak in the photoelectron spectrum of atom irradiated by the long laser pulse.