Theoretical Study Of Ultrafast Dynamics Of Atoms And Molecules In Ultrashort Laser Pulses

Since the laser field strength becomes strong enough to distort the binding Coulom-b field inside the atom,the interactions of intense fields with atoms and molecules pro-duce many highly nonlinear phenomena,such as high-harmonic generation（HHG）,above-threshold ionization,and nonsequential double ionization.Especially,sub-femtosecond pulses are generated,which allowed for investigating the dynamic light-matter interactions on a few-femtosecond or even sub-laser-cycle timescale.It is of great significance for the development of science and technology to probe and manipulate the electron dynamics within attosecond temporal resolution.In this paper,we study the ultrafast dynamics of the atoms and molecules by the use of attosecond and few-cycle pulses.Firstly,we simulate the transient absorption of attosecond pulses of near-infrared-laser-dressed atoms by considering a three-level system in the adiabatic approximation.The delay-dependent interference features are investigated from the perspective of the co-herent interaction processes between the attosecond pulse and the quasi-harmonics.We find that many features of the interference fringes in the absorption spectra of the attosec-ond pulse can be attributed to the coherence phase difference.However,the modulation signals of laser-induced sidebands of the dark state are found to be related to the population dynamics of the dark state by the dressing field.Secondly,we apply the MCTDHF approach to two-electron problems in one dimen-sion.For testing its applicability and investigating the correlated two-electron dynamics,we study the ionic excitation of helium system in strong laser fields.The simulation result-s show that the ionic excitation experiences a transition from the recollisional excitation to the field-induced direct excitation with the increasing laser intensity,indicating the in-terplay of the electron-electron correlation,singly and doubly ionization and rescattering dynamics.We show that the high-harmonic generation in the recollision process can be utilized to manifest the multi-electron ionization dynamics.We also study the rescattering dynamics of the one-dimension hydrogen molecule with the nuclei fixed at various inter-nuclear distances.By analysis of the laser intensity dependence of the yields of the H₂⁺ions,we show that the rescattering process strongly depends on the internuclear distance.In addition,we propose the internuclear collisional excitation mechanism to interpret the enhanced excitation of the H₂⁺ion at large nuclear distance.Thirdly,we perform an ab initio study of high-order harmonic generation of CO₂molecules by means of the TDDFT theory.The molecules are exposed in the intense linearly polarized laser pulses with arbitrary molecular orientation.Our study shows that the dynamical minimum in the HHG spectrum is not only dependent on the intensity of the laser field,but also the energy difference between the different orbitals.The results also show that the orientation dependence of HHG spectrum is determined by multiple orbital contributions.