Theoretical Study On The Effect Of Orientation On Higher Harmonic Radiation Of Isotope Molecules

In recent years,high-order harmonic generation(HHG)is a hot issue in strong laser-matter interaction.First of all,attosecond pulses can be obtained by HHG,which is widely used in science and technology.In addition,the use of HHG can achieve orbital imaging in the ultrafast time scale,which is of great significance for understanding chemical reactions.From the point of view of real molecules,the movement of nucleus produces a lot of complex effects and mechanisms,for example,the coupling of the movement of nucleus and the electronic motion will affect the intensity and frequency of HHG.In order to understand the mechanism of the effect of the nuclear movement to these phenomena more deeply,people urgently need to explore the correlation mechanism between the nuclear wavepacket and the electronic wavepacket in the vibrational molecular system.Compared with symmetric molecules,asymmetric molecules have larger permanent dipole,and the interaction between the permanent dipole and the laser field has a great influence on the harmonics of polar molecules.Therefore,the high-order harmonic spectra(HHS)of both symmetric and asymmetric vibration molecules may show different phenomena and reflect different internal mechanisms in the process of HHG.In this paper,a molecular HHG model is established,which takes into account the effect of the nuclear motion on the three processes of ionization,propagation and recombination in the process of HHG.The high-order harmonic generation mechanism of symmetric and asymmetric molecules at different orientations and laser parameters is analyzed in depth.In the third chapter,the influence of orientation on the harmonic generation of H2+and T2+ has been studied,and the numerical results show that the influence is dependent on the orientation and laser parameters.In the case of the parallel orientation at the stronger laser intensity or weaker laser intensity with longer wavelength,the harmonic yield of H2+ is higher than that of T2+,while the situation is opposite for the parallel orientation at the weaker laser intensity with shorter wavelength or the vertical orientation.This chapter shows that the molecular transition dipole plays an important role in these phenomena.These results reveal the complex dynamics of the symmetric system of vibration in the intense laser field.In the forth chapter,we study HHG from the oriented asymmetric molecule HeH2+and its isotopic variant HeT2+ in strong few-cycle laser pulses.Simulation results show that the influence of nuclear motion on harmonic spectrum of the asymmetric system is dependent on the molecular orientation and the laser parameters.At relatively weak laser intensities,the harmonic yields of HeH2+ are higher than those of HeT2+ for the parallel orientation and the situation reverses for the perpendicular orientation.However,at high laser intensities,they become comparable for the parallel case.This chapter shows the permanent dipole of the system plays an important role in these phenomena.These results shed light on the complex dynamics of the asymmetric system in strong laser fields.