强XUV光下原子分子电离过程的研究进展(特邀)

Photoionization of Atoms and Molecules by Strong XUV Pulses(Invited)

随着近年来自由电子激光技术的发展,强极紫外(XUV)激光脉冲作用下的原子分子电离过程逐渐成为强场物理领域的研究热点之一。强XUV光下的电离过程中存在很多新奇,甚至反直觉的非微扰效应。主要介绍近年来在XUV光下的原子稳定化、Autler-Townes(AT)分裂和动态干涉上的研究进展。主要涉及原子稳定化出现的机制及探测方法,原子稳定化区域光电子角分布的变化,拉比振荡与AT分裂的联系,AT分裂的峰间距、相对峰高、双峰之间的次峰结构,阈上和阈下电离过程中动态干涉出现的条件,单电离与双电离能谱中的动态干涉构型,动态干涉与原子稳定化的联系,动态干涉与AT分裂之间的联系,以及非偶极修正与电子关联的影响等。

Significance With the rapid development of free-electron laser(FEL)sources,the ionization of atoms and molecules by strong extreme ultraviolet(XUV)pulses has become a major topic in strong-field physics.A range of novel and even counterintuitive non-perturbative phenomena can be observed in photoionization with strong XUV pulses.This review focuses on non-perturbative effects in photoionization processes,including atomic stabilization,Autler-Townes(AT)splitting,and dynamic interference.Progress At high laser intensities,instead of being ionized,the electron tends to remain bound near the nucleus.This suppression of ionization in the high-frequency and high-intensity regime is often referred to as atomic stabilization.In the Kramers-Henneberger(KH)frame,the atomic bound electron wave function evolves into a dichotomous state at high laser intensity,leading to a reduced ionization rate.The formation of dichotomic KH states suggests that stabilization can be monitored by the angle-resolved spectra of the photoelectron.Molecule-like two-center interference fringes are predicted to be observable in a pump-probe scheme.Experimentally,atomic stabilization can be interpreted as the temporal destructive interference of photoelectrons ejected at different times.The angular distribution of the photoelectron in the stabilization regime differs significantly from that in the perturbative regime.Early studies based on reduced-dimensional time-dependent Schrödinger equation(TDSE)calculations suggest that stabilization can be disrupted by nondipole effects or electron-correlation effects.More recent full-dimensional TDSE calculations show that nondipole corrections can even enhance stabilization if a suitable pulse duration is chosen.When the photon energy matches the energy difference between two bound states,Rabi oscillations of the populations can be induced in a two-level system.Rabi oscillations have been identified in a variety of physical systems,including ultracold atoms,Rydberg atoms,molecules,metal nanostructures,Josephson-junction circuits,and quantum dots,across a wide range of frequencies from radio to ultraviolet.Recently,Rabi oscillations have also been observed in the XUV regime.Real-time observation of Rabi oscillations has been achieved by detecting resonance fluorescence,state-dependent refractive index,birefringence,differential reflectivity spectra,and photoelectron spectra.Alongside Rabi oscillations,AT splittings appear in the photoelectron energy spectrum.AT splittings have been observed in various settings,including multiphoton ionization,double ionization,and even in the absence of population oscillation of the initial ground state.The AT doublet is sensitive to many factors,such as the number of Rabi cycles,the AC Stark shift,the transition matrix element between bound and continuum states,and the competition between resonant and non-resonant ionization channels.The separation of the standard AT splitting is typically given by the product of the Rabi frequency and Planck's constant.However,the separation of dynamical AT splitting in a pump-probe scheme can be much larger than that of the standard AT splitting.Dynamical AT splitting can be treated as a consequence of temporal double-slit interference related to the phase jump of the initial ground state amplitude as it approaches zero.Due to the time dependence of the relative AC Stark shift between the initial state and the final continuum state,the photoelectrons ejected at different times have different energies.At two symmetrical time points-the rising edge and the falling edge of the laser pulse—the photoelectrons carrying the same energy can interfere,resulting in a multi-peak structure in the photoelectron energy spectra.This temporal double-slit interference is referred to as dynamic interference.Observing dynamic interference in ground-state atoms requires specific conditions.On one hand,the phase difference between the two photoelectron wave packets ejected at the rising and falling edges must be large enough to induce destructive interference at certain energies.On the other hand,the depletion of the initial state should not be too large to ensure that the second slit of the double-slit remains open.In particular,atomic stabilization is necessary to observe dynamic interference in ground-state hydrogen.Dynamic interference is also predicted in the double ionization of helium with a grid-like interference structure observed in the joint energy spectrum from the numerical solution of the full-dimensional TDSE.In the double ionization scenario,six ionization paths contribute to the grid-like interference structure.Initially,the multi-peak structure between the AT doublets is also attributed to dynamical interference.However,recent studies suggest that the multi-peak structure between the AT doublet may have other physical origins,as this structure can also appear in calculations using rectangular and half-Gaussian pulses,where the temporal double-slits cannot exist.Conclusions and Prospects Most of the nonlinear effects of strong XUV pulses discussed in this review are still limited to theoretical predictions.While atomic stabilization in Rydberg atoms has been experimentally verified,experimental confirmation of atomic stabilization in ground-state atoms remains elusive.Measuring dynamic interference in atoms exposed to a single XUV pulse presents significant challenges since it requires not only extremely high pulse intensity but also proper pulse duration.Recently,the experimental measurement for the AT splitting in the XUV regime has been achieved.With the rapid advancement of free-electron laser sources globally,these intriguing physical effects predicted by the theory are expected to be verified experimentally,and more novel nonlinear physical effects are expected to be discovered.