The Theoretical Investigation Of The High-order Harmonic Generation And Nuclear Dynamics For Molecular Systems In Intense Laser Field

Many nonlinear physical phenomena can be observed when anatom or a molecular ion is exposed to an intense laser field, of whichthe high-order harmonic generation (HHG) with high time-resolutioncan be used to monitor the molecular structure and the ultrafastelectronic dynamics. At present, the relevant researches about HHGhave been transited from the atomic systems to the molecularsystems. Moreover, extensive attention has been paid to the harmonicemission from asymmetric molecular ions duo to the important role ofthe first excited state with longer lifetime in HHG process.Additionally, the researchers have also focused on the nucleardynamics in dissociation and Coulomb explosion processes in strongfield.In this work, based on the non-Born-Oppenheimer approximation,the one dimensional time-dependent Schr dinger equation of themolecular system in intense laser field has been simulated toinvestigate the characters of the harmonic emission from HeH2+andthe nuclear dynamics of H2+and HD+in dissociation and Coulombexplosion processes with the nuclear kinetic-energy-released (KER)spectra based on the flux operator. The concrete investigations are as follows:(1) Distinguish the multiple recombination channels in the harmonicemission from HeH2+. In this thesis, by establishing the multichannelrecombination model, six recombination channels may contribute tothe harmonic emission. Combing with the laser-dressed energy, thesesix recombination channels have been found and distinguished byadjusting the laser intensities. Additionally, the role of the individualrecombination channel in the harmonic emission has been alsoevaluated. The results indicate that the homonuclear (heteronuclear)recombination channel dominates in the HHG process for excitedstate-based-ionization (ground state-based-ionization).(2) Manipulate the multichannel recombination in the harmonicemission from HeH2+via combining the monochromatic trapezoidallaser field with a static electric field. When the laser polarizeddirection is antiparallel to the permanent dipole of HeH2+, the positivepeak intensity can be used to control the electronic localizationaround the He2+. With proper laser intensity of the static electric field,the recombination channels based on the He2+can be controlled,moreover, an isolated48as pulse can be obtained by superposingappropriate harmonic orders.(3) Discuss the effect of the electron distribution around two nucleion the harmonic emission. By comparing the HHG from HeH2+andHeD2+at different vibrational states, it shows that the ionizationprocess of the electron localized around one nucleus compete withthe transfer process to the other nucleus. By increasing the initialvibrational level, more electrons localized around the D+tend totransfer to the He2+so that the ionization probabilities of electrons localized around the He2+are increased. In this case, the difference ofharmonic efficiency between HeH2+and HeD2+is decreased while thedifference of harmonic spectral structure is increased. Due to thestrong interference of multiple recombination channels, evidentminimum can be observed in harmonic spectrum of HeH2+.(4) Discuss the dissociation and Coulomb explosion processes of H2+in chirped laser pulse. When the chirped laser pulse interacts with H2+at lower vibrational state, both the dissociation and Coulombexplosion processes can be enhanced with proper chirp parameter byobserving the variations of the KER spectra. For lower laser intensity,the dissociation process is more sensitive to the chirp parameter dueto the enhancement of the bond soften.(5) Investigate the effect of the permanent dipole moment onCoulomb explosion process. By comparing the variations of the KERspectral structures for H2+with those for HD+at different vibrationalstates, it indicates that the permanent dipole moment affects greatlyon the multi-photon process from the ground state to the excited statein Coulomb explosion process, as a result, the KER spectra of HD+aredifferent from those of H2+despite of close ionization probabilities athigher vibrational state.