| Carrying out ultrafast dynamic measurement of strong-field ionization processes can enhance the understanding of the physical picture of laser-matter interactions.Obtaining resolved information on the photoionization process in attosecond time(10-18 s)can provide a significant reference for testing and improving relevant theoretical models.Strong-field multiphoton transition interferometry(SFMPTI)is a crucial experimental technique for studying the attosecond time-resolved process of strong-field photoionization.It has made significant progress in the dynamic study of many strong-field photoionization phenomena.The main principle and basic process can be summarised as follows:using a strong 400 nm laser field to induce above-threshold multiphoton ionization of atoms and molecules,followed by the introduction of a weaker 800 nm laser.This induces a sequential absorption or release of one 800 nm photons in the ionization continuum,establishing a quantum interference and ultimately generating photoelectron spectra in the ionization channels.A structure of photoelectron peaks known as’sidebands’ is created in the photoelectron spectrum of one or more ionization channels.The sidebands record the formation time or phase of the ionization state.Therefore,the phase information of the sidebands can be used to obtain the relative time information of the corresponding channel in the ionization process.This allows for the discussion of the relative time delay between the channels with a time resolution on the attosecond.Based on the strong-field multiphoton transition interferometry,this thesis focuses on the following main works:Firstly,the time-resolved dynamic information of Ar atoms during strongfield multiphoton ionization is investigated.The study achieves simultaneous measurements of the relative time delays between the multiphoton ionization of Ar atoms and multiple resonance channels.Combined with simulations using timedependent quantum wave packets,demonstrate that the shifts in relative time delays in the multiphoton ionization channels are enhanced with increasing laser field intensity.This suggests that the effect of Coulomb-laser coupling on photoionization time delays becomes more pronounced.It is observed that the relative time delays among the resonance channels exhibit more complex variations than those of the direct ionization channels.Using the 4f channel as a reference point,the time delay between the 5p channel and 4f is approximately 140 as.Similarly,the time delay between 3d and 4f is 460 as,while the time delay between the remaining 5g and 6h channels and 4f is negligible.Given that the photoelectron energies of these channels are in close proximity to the ionization threshold,the delay time should be influenced by a combination of the resonance phase shift of the atomic Rydberg state and the continuum-continuum phase(CC delay)during the ionization process.The use of a near-threshold sideband model enables a qualitative interpretation of the time delay in the multiphoton resonance channel.However,it also highlights the structural complexity present in the multiphoton picture.This research provides an experimental foundation for a comprehensive understanding of the dynamics of Ar atomic ionization.Next,the study investigates the ionization time delays of Kr atoms during strong-field multiphoton ionization,resolved by the spin-orbit coupling spliting states.Kr atoms have two distinct ionization potentials due to the spin-orbit coupling effect.This results in two sets of above-threshold ionization(ATI)peaks with different energies in the photoelectron spectra.Therefore,the iontization time delay for the resolution of the spin-orbit coupling spliting state can be measured experimentally by determining the relative time delay between the formation of spin-orbit splitting states of the photoelectron wave packets emitted from the same electron shell layer.The results indicate that the ionization time delay decreases with increasing photoelectron energy for spin-orbit coupled group states.The ionization time delay is also affected by different laser field intensities due to the influence of resonant intermediate states.For instance,the 4d state has a significant effect on the ionization time delay in low-light intensity,while the 5p state has a different effect in high-light intensity.For instance,at low light intensities,both experimental and TDSE theoretical results indicate that the ionization time delay between spin-orbit coupling spliting states is 280 as for the first-order sidebands.As the electron energies increase,the delay time between the third-order sidebands drops to a near-zero value of 80 as.This result confirms that the time delay arises from the cumulative effect of the atomic Wigner delay and the CC delay,which have opposite signs.Additionally,the time delay comes from the atomic Wigner delay and CC delay.Cumulative effects in the opposite direction were observed,while further analysis of the angular dependence of the time delay indicates that the delay caused by the presence of the 4d intermediate state should be of a small order of magnitude.At high light intensities,the total spin-orbit coupling group state-resolved ionization time delays show an overall increase of 280 as compared to the results at low light intensities.The ionization time delay between spin-orbit coupling spliting states increases due to the involvement of the resonance state 5p during the formation of ionised continuum states into the final Kr-atom ionic states(2P1/2 and 2P3/2).This is because the 5p state influences the process of the electron angular momentum lepton assignment,introducing a higher resonance delay effect.Based on TDSE theoretical simulations,this effect is observed.This result can be corroborated by the results of the time delay angular distribution:the time delay between spin-orbit coupling spliting states is close to 600 as in the direction of the laser field polarization and is positive at all electron emission angles.So the overall increase in the relative time delay between spin-orbit coupling spliting states can be considered to be introduced by the resonance state.The results of the present work reveal the time delay between spin-orbit coupling spliting states of Kr atoms under near-threshold conditions,and the influence of nonlinear effects such as the intensity of the external laser field and intermediate state resonances on the measured time delays is discussed.The work obtains information about the ultrafast dynamics in the formation of different ionic states of Kr atoms.Finally,the object of study was changed to the NO molecule,and unlike the study of the ionization delay of atoms,the molecular photoemission process has to take into account the effect of the motion of the nuclear on the ionization dynamic,and the results of the experiments show that the variation of the internuclear distance,i.e.the motion of the nuclear,has a significant effect on the ionization time delay.Experimental and theoretical simulations show that the resonance-enhanced multiphoton ionization of the NO molecule occurs through the non-adiabatic coupling of two intermediate resonance states(A2∑+(v=2)and B2∏(v’=4)).The phase results show that the main source of the delay time is the change in the size of the molecular internuclear distance corresponding to the departure of the photoelectrons from the molecular Coulomb potential,and the calculation of the molecular vibrational wave function distribution shows that different molecular vibrational dynamics correspond to internuclear distance of 0.1 ?,and that such small changes in internuclear distance cause a time delay of hundreds of attosecond.Furthermore,the effect of non-adiabatic coupling of the two electronic states of the NO molecule on the ionization delay is analysed in terms of the time dependence of the delay.The angular distributions of photoelectrons from different vibrational states ionised from the A2∑+and B2∏ states are structurally the same,but the angular dependence of the delay time behaves differently,with the phase angular distributions of the ionic vibrational states v"=1,2 show the same trend of a gradual increase between 0° and 65°,and a gradual increase between 65° and 90° and decrease to zero,while the phase angle distribution of the ionic vibrational state v"=3 is the opposite,decreasing between 0°and 65° and ending after 65°.This suggests that v"=1,2 are influenced by transition from A2∑+,while v"=3 are dominated by transition from B2∏.The research extends the application of resonance-enhanced multiphoton ionization to obtain attosecond-resolved dynamics information for strong-field molecular ionization. |
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