Excitation And Ionization Dynamics Of Atomic Rydberg States In Strong Laser Field

With special properties such as extremely large orbital radius,long lifetime,and an exaggerated response to electric and magnetic fields,Rydberg state excitation of atoms and molecules has particular applications in precision measurements,quantum nonlinear dynamics,and quantum information.Rydberg state particles are prevalent in the interaction between strong laser fields and atoms/molecules.The study of the excitation and ionization processes of Rydberg state atoms is an important piece of the puzzle to complete the picture of strong field ultrafast physics.Recently,more and more topics have been focused on the study of the Rydberg state.With the development of ultrafast and ultra-intense laser technology,Rydberg state atoms driven by strong laser fields has been used to accelerate neutral-particle,reveal multiphoton Rabi oscillations,understand the photoelectron spectral features,and the generation of near-threshold harmonics.However,there are still many unresolved questions about the excitation and ionization of Rydberg state atoms in strong fields,such as the Rydberg state excitation mechanism and its dependence on the laser parameters,the interference of electron wave packets on the Rydberg state,and the circular dichroism of Rydberg state atoms ionized in circularly polarised laser.Therefore,based on the above research status,this paper presents a systematic study of the excitation and ionisation of Rydberg state atoms in strong laser field.The main work is as follows:(1)Using classical trajectory Monte Carlo(CTMC)model,this paper theoretically investigated frustrated tunneling ionization(FTI)in the interaction of atoms with elliptically polarized laser pulses.Our results show that the yield of frustrated tunneling ionization events exhibits an anomalous behavior which maximizes at the nonzero ellipticity.By tracing back the initial tunneling coordinates,we show that this anomalous behavior is due to the fact that the initial transverse velocity at tunneling of the FTI events is nonzero in the linear laser pulses and it moves across zero as the ellipticity increases.The FTI yield maximizes at the ellipticity when the initial transverse momentum for being trapped is zero.Moreover,the angular momentum distribution of the FTI events and its ellipticity dependence are also explored.The anomalous behavior revealed in our work is very similar to the previously observed ellipticity dependence of the near-and below-threshold harmonics,and thus our work may uncover the mechanism of the below-threshold harmonics which is still a controversial issue.(2)By numerically solving the three-dimensional time-dependent Schr(?)dinger equation(3D-TDSE),we theoretically investigated the dynamic interference in multiphoton excitation and ionization of hydrogen irradiated by linearly polarized laser pulses.At 800 nm laser pulses,dynamic interference is found in the Rydberg state.Interference of the multiphoton excitation paths at the rising and falling edges of the laser pulses gives rise to the fast oscillations of the intensity-dependent yield of the excited states.At 400 nm laser pulses,our results show that the period of the oscillation increases abruptly when the laser intensity increases above the 8-photon ionization threshold.By tracing the population evolution of the ground state,we find that the depletion of the ground state on the rising edge of the laser pulse closes the multiphoton excitation path at the falling edge of the pulse.The slower oscillation comes from the interference of the excitation paths at different instants during the rising edge of the laser pulses.The change of the interference paths at the high laser intensity region also occurs in multiphoton ionization.The photoelectrons released at different instants of the rising edge of the laser pulses possess different parities.This is imprinted in the up-down asymmetry along the laser polarization direction in the photoelectron momentum distribution.The dynamic interference of the Rydberg states provide rich information of the dynamics in strong-field ionization.(3)By numerically solving the 3D-TDSE,we investigated the circular dichroism of circular Rydberg states ionized by circularly polarized laser pulses.It has been shown that the ionization yield of the Rydberg state which are corotating with respect to the laser field is higher than the counter-rotating Rydberg state in the tunneling ionization region.While in the multi-photon region,the ionization yield of the counter-rotating Rydberg state is higher.For the co-rotating excited state,our results show an obvious ionization suppression when the laser frequency is beyond the one-photon ionization threshold,and it is followed by a strong ionization enhancement when the laser frequency further increases.These ionization suppression and enhancement are absent for the counter-rotating excited state.By tracing the ionization process,we found that resonance between the initial state and a lower-energy state occurs at the frequency of the ionization enhancement.Ionization from this lower-energy state is responsible for this ionization enhancement.At the frequency of ionization suppression,Rabi oscillation between the initial and the lower-energy state occurs,which prevents further ionization,and thus suppresses the final ionization yield.By adjusting the laser intensity and pulse width at the resonance frequency,this paper proposes a method for the wide range of modulation of circular dichroism,which provides a new idea for the preparation of spin-polarized pulses.(4)Using the classical model,this paper investigated the ionization properties of Rydberg state atoms in circularly polarised laser.It is found that due to the "slingshot effect",electrons in elliptical orbits with larger centroid ratios are more likely to ionize,especially when they pass near the nucleus,suggesting that Coulomb scattering of electrons with the nucleus is the "catalyst" for ionization.In addition,corotating electrons are more likely to ionize by absorbing energy from the electric field as they scatter with the nucleus because they match the frequency of the electric field more efficiently and the’slingshot effect’ is therefore more efficient.Taking advantage of the classical model,this paper traces the trajectory of electrons and the statistical distribution of energy,and successfully explains the trend of ionization yield of corotating electrons as a function of laser frequency.This work further improves the understanding of the details of the ionization process of the Rydberg state atoms.