Theoretical Study On The Generation Of Ultrafast Magnetic Field Of Molecules In Intense Laser Fields

In recent years,with the development of ultra-short and ultra-intense laser technology,strong field physics and attosecond physics have made great progress.This also makes it possible to visualize and control various microscopic ultrafast dynamical processes on the atomic spatial scale and attosecond time resolution.Recently,the development of attosecond technology has expanded from linear polarization（LP）pulse to circular polarization（CP）pulse.Researchers found that the time-dependented electronic current is generated in the process of electron excitation and ionization under the CP pulse,which leads to the generation of ultrafast magnetic field on the attosecond time scale.This method is more effective than the traditional method of static field,because the laser pulse can control the electronic current ultrafast and coherently without any dissipation.Moreover,it has great potential for studying ultrafast magneto-optics,electron dynamics and molecular chirality,thus arousing widely attention.At present,although many studies have been devoted to the generation of attosecond magnetic pulse in atomic and molecular systems,there are still many aspects to be further explored,such as the physical image from the charge migration to the generation of electronic current and corresponding magnetic field,the deep physical mechanism of the effect of laser parameters on the generation of magnetic field and so on.Based on the above,this thesis carried out the studies of the generation of ultrafast magnetic field from molecules under intense laser fields by solving the two-dimensional time-dependent Schr（?）dinger equation numerically.The main contents are summarized as follows:Firstly,we investigate the generation of ultrafast attosecond magnetic fields in the charge migration process of symmetric molecule H₂⁺and asymmetric molecule He H²⁺under the CP laser pulses.The resonant excitation and direct ionization are considered by using the designed laser pulse.The results show that the electron density distributions of the two molecules both oscillate as functions of time under the resonant excitation,indicating strong electron coherence.In addition,the electronic current and magnetic field generated by the two molecules in the case of resonance excitation are stronger than in the case of direct ionization.This is due to the quantum interference of the electronic states during the resonance excitation,which leads to higher charge migration efficiency,thereby resulting in the strong electronic current and corresponding strong magnetic field.Meanwhile,we also compare the magnetic fields generated by the two molecules in the case of resonance excitation.It was found that the magnetic field generated by H₂⁺is stronger than that by He H²⁺.The time-dependent population analysis of different electronic states shows that such difference in magnetic field of two molecules is mainly due to the fact that H₂⁺does not ionize during the resonance excitation process,while He H²⁺has a small ionization.These findings highlight the importance of coherent resonance between electronic states for the generation of strong magnetic fields and provide a guiding principle for the generation of ultrafast magnetic fields in molecular systemsSecondly,considering that the coherent control of electron dynamics in the strong field includes not only the coherent resonance between electronic states,but also the coherence caused by the quantum interference between two or more laser pulses,we further investigate the generation of the ultrafast magnetic field of H₃²⁺by the multi-frequency trichromamtic CP pulse consisting of co-rotating and counter-rotating dichromatic CP pulses.The results show that the amplitude and wavelength of the laser field are two important factors affecting the magnetic field induced by trichromatic CP pulse.When the laser wavelength is 50 nm,the strength of magnetic field is linearly related to the field amplitude.The main reason is the interference of multi-photon ionization pathways in the modulation process of two dichromatic CP pulses.When the laser wavelength is 70 nm,the strength of magnetic field increases first and then decreases with the field amplitude.This is due to the transition excitation probability from the ground state to the excited state under larger field amplitude is suppressed,resulting in weakening magnetic field.In addition,the phase and helicity of dichromatic CP pulses also have important effects on the magnetic field.In particular,the relationship between the magnetic field and the phase of pulse presents a cosine-like function,and the dependence is mainly due to the interference effect between multiple ionization pathways.These findings provide a guidance for the use of laser pulses to regulate magnetic field in molecular systems,and can be further extended to complex molecular systems with multiple nuclear centers and electrons.Thirdly,considering that previous studies mostly investigated the generation of magnetic fields based on the electronic states with the magnetic quantum number of m=0 for atoms and orbital angular momentum of L_z=0 for molecules,recently the current-carrying states with nonzero orbital angular momentum（L_z≠0）have attracted much attention,and researches have been proved that they have significant effects on electron dynamics.Based on this,the induced electronic current and the corresponding magnetic field by the current-carrying states of ring molecules H₆⁵⁺and H₄³⁺under the CP and LP laser pulses are investigated.The results show that,the electronic ring current and magnetic field generated by CP pulse under resonance excitation are both larger than those generated by LP pulse.This can be attributed to the fact that CP pulse with different helicity can selectively induce the current-carrying state to produce strong electronic current and magnetic field,while LP pulse can not fully excite the current-carrying state due to the lack of helicity.In the case of ionization,the generation of the magnetic field is related to the selected initial state of the molecules.The magnetic field generated by the initial state of the current-carrying state is much larger than that generated by the initial state of the ground state.This is because the current-carrying state itself has a stable electronic ring current,and the generated magnetic field is contributed by the current-carrying state and the laser field.In addition,the magnetic field induced by the current-carrying state does not change obviously with the laser wavelength,which indicates that the current-carrying state is a stable electronic state under the CP pulse.However,this law is not satisfied in the LP pulse.The interaction of the LP pulse and the current-carrying state will lead to the deformation of the current-carrying orbitals,thus weakening the magnetic field.These findings highlight the important role of the current-carrying states and provide a direction for inducing electronic ring currents and strong magnetic fields in molecular systems.In summary,this thesis carried out the theoretical researches on the generation of ultrafast magnetic fields in molecular systems under intense laser fields.Through the explorations of the electronic currents and magnetic fields in linear molecules（symmetric and asymmetric linear molecules）and ring molecules generated by different types of polarization laser fields,we presented the complete physical images from the charge migration to the generation of electronic current and magnetic field,clarified the resonance excitation between different excited states（or ground state and the excited state）of the molecules can induce the strong magnetic field than the case of ionization,proposed a scheme for controlling the generation of magnetic field by multi-color CP pulse,and revealed the important role of molecular current-carrying states in the generation of strong magnetic field.These findings can provide a guidance for the generation of attosecond magnetic field in molecular system and provide theoretical basis for the regulation of ultrafast magnetic field in the following experiments.