| The interaction of intense laser with atoms and molecules can generate high-order harmonic radiation that drives the laser frequency.Using high-order harmonics,coherent light sources from XUV to soft X-ray bands can be generated,which is also the main way to generate attosecond ultrashort pulses.The high-order harmonic generation is due to the recombination of the ionized electrons with the parent ions,which carries the structural characteristics of the parent ions.Therefore,it is also used to image the electronic structure of atoms and molecules.The harmonic spectrum can be well calculated by the numerical solution of the time-dependent Schr?dinger equation,but the physical information that is difficult to obtain from it cannot provide more understanding of the structural information of the harmonic emission spectrum.To this end,we developed the Bohmian trajectory scheme and analyzed the space-time emission information of different orders of higher harmonics,including the following work :(1)In this paper,Using a few Bohmian trajectories obtained by numerically solving the time-dependent Schr?dinger equation,the high-order harmonic emission spectrum of helium atom is calculated,which is quantitatively consistent with the accurate results.Using the space-time wavelet transform scheme,the space-time behavior of harmonic emission in different energy regions is analyzed.It is found that the harmonic emission time in different energy regions corresponds to the time-frequency behavior of harmonic emission spectrum.The harmonic emission spectrum in the cut-off energy and platform region is mainly emitted near the nucleus.The low-order harmonic emission region is on both sides of the nuclear region.The harmonic emission time near the resonance energy is after the peak of the laser pulse.On this basis,the influence of wavelength and light intensity on harmonic spatiotemporal emission is further analyzed.The harmonic space-time emission behavior of different wavelengths under the same light intensity is studied.It is found that when the wavelength is longer,the space of electron emission is more localized.The reason for this phenomenon is that the ionization is smaller under long-wave conditions,and the distribution of bound state wave packets is closer to the nuclear region.By analyzing the time behavior at the same wavelength and different light intensities,it is found that under weak light intensity,the ionization mechanism is no longer dominated by tunneling ionization,and multi-photon ionization plays an important role,so harmonic emission can also be observed at the rising edge of the pulse.(2)The high-order harmonic emission spectrum process of hydrogen molecular ions is studied by using the Bohmian trajectory scheme.Compared with atomic harmonics,the harmonic emission space of molecules becomes more nonlocal.Under the condition of small nuclear spacing,the main spatial position of harmonic emission is not limited to the middle of the two nuclei,but oscillates near the two nuclei with time.Under the condition of nuclear spacing of 4,the spatial position of harmonic emission is also different in a single cycle,and it will be transferred from one nucleus to another.Under this condition,the spatial range of harmonic emission spectrum is larger,and the main emission time of harmonic is at the falling edge of driving laser pulse. |
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