Studies On Atomic And Molecular Ultrafast Dynamics Based On Strong-Field Photoelectron And Multi-Dimensional Coherent Spectroscopy

Laser-matter interactions have provided a powerful tool to reveal the law of motion of microscopic particles.With the development of ultrashort pulsed lasers,femtosecond laser allows us to get insight into the complex dynamics processes of the microworld with an unprecedented timescale,such as intraatomic/intramolecular ultrafast dynamics and the dynamics of interatomic many-body interactions.In this thesis,we present experimental and theoretical investigations on the ultrafast dynamics of atoms and molecules in femtosecond laser fields by using strong-field photoelectron spectroscopy and multi-dimensional coherent spectroscopy.The major achievements of our work are listed as follows:Firstly,based on strong-field photoelectron spectroscopy,the achievements on intraatomic/intramolecular ultrafast dynamics in strong femtosecond laser fields have been carried out.1.We present joint experimental and theoretical investigationa on ultrafast dynamics of atomic ionization in the orthogonally polarized two-color pulses(OTC).Firstly,we theoretically investigate the photoelectron momentum distributions(PMDs)in OTC by the strong-field approximation(SFA)and the Coulomb-Volkov distorted-wave approximation(CVA)theories.Our results show that in comparison with the SFA simulations,the PMDs in CVA are in better agreement with the experimental observations.We find that the change of the PMDs in CVA can be ascribed to the different Coulomb corrections of the phases for the forward-rescattering electron orbits and the direct electron orbits.Furthermore,we present a joint experimental and theoretical investigation on the intracycle interferences of electron wavepacket by using OTC laser fields.We find a hitherto unobserved interference in the PMD.Analysis of SFA calculations reveals that the interference is caused by quantum pathways from nonadjacent quarter cycles.2.We present a theoretical investigation about the ultrafast dynamics of molecular ionization in a sculpted linearly polarized two-color laser field.By solving the time-dependent Schrodinger equation(TDSE),a novel interference pattern is found in the two-dimensional PMD of N2 molecule.Resorting to a simple semiclassical analysis and the SFA theory,we show that this new interference pattern is a rarely observed PH pattern from the second-return backscattering electron orbits.Secondly,based on mutli-dimensional coherent spectroscopy,we have investigated the dynamics of interatomic many-body interactions.The main conclusions are:1.We have built a mutli-dimensional coherent spectrometer which can be used for experimental study of the dynamics of interatomic many-body interactions.The spectrometer includes laser system,time-ordering control system,phase modulation-selection-demodulation and data acquisition modules.The resolution and signal-to-noise ratio of the spectrometer have reached the internationally advanced level.This lays a solid foundation for the study of the dynamics of interatomic many-body interactions.2.The dynamics of interatomic long-range dipole-dipole(DD)interactions is experimentally studied.Double-quantum two-dimensional coherent spectroscopy(2DCS)was implemented to probe interatomic DD interactions in both K and Rb atomic vapors at cold-atom density range.Based on the optical Bloch equations,the results confirm that the long-range nature of the DD interaction gives rise to the double-quantum signal.This strongly supports the theory to account for DD interactions at all density.Furtherore,we demonstrated that double-quantum 2DCS is sufficiently sensitive to probe DD interaction at cold atoms in a magneto-optical trap,paving the way for many-body interactions studies of cold atoms and molecules.3.The ultrafast dynamics of multi-atom Dicke states is experimentally studied.Multi-quantum 2DCS are implemented in a K atomic vapor and for the first time,n-quantum 2D spectra with n up to seven are obtained.In each spectrum,the spectral peaks match exactly the corresponding Dicke states in the multi-quantum frequency,which directly demonstrate the existence of Dicke states with a deterministic number of atoms in an atomic vapor.Moreover,the decoherence rate of n-quantum coherence was measured and found to increase linearly with the number of atoms n,and further the collision decoherence rate was extracted.