We theoretically study the double ionization of helium atom induced by an ultrashort extreme ultraviolet (XUV) pulse with the photon energy above the double-ionization threshold in the presence of an infrared (IR) laser pulse. By numerically solving the reduced-dimensional time-dependent Schr & ouml;dinger equation (TDSE), we obtain the joint energy spectra of the two photoelectrons, in which peak splittings are observed both in the main band where one XUV photon is absorbed and in the sidebands where additional IR photons are either absorbed or emitted. A simple laser-assisted dynamic interference (LADI) model, based on the strong-field approximation (SFA) and neglecting the electron-electron correlation, accurately reproduces the numerical TDSE results regarding the peak splittings in the total energy spectra. Additionally, peak-splitting structures arising from the dynamic multiphoton interference (DMPI) mechanism are identified in the joint energy spectra. The DMPI is revealed by excluding the key LADI phase term in the SFA formula. We find that the DMPI is sensitive to the energy sharing between the two photoelectrons, and its contribution to peak splitting is likely smeared out in the total energy spectrum.

Quantum tunneling, a quintessential quantum phenomenon, challenges classical intuitions by allowing particles to pass through seemingly insurmountable barriers. This study delves into the intricate dynamics of electron tunneling, focusing on the critical role of sub-barrier interactions and complex time processes. Through a combination of experimental measurements and simulations based on Feynman’s path integral of complex time trajectories, we reveal that the sub-barrier potential interaction introduces an asymmetric electron wave packet probability distribution along the real time axis and a marked delay at the tunneling exit. The energy-dependent behavior of this delay in electronic tunneling agrees with that observed in the previous atomic tunneling studies, suggesting a universal mechanism underlying quantum tunneling across different scales. Our findings underscore the necessity of incorporating both the wave and particle aspects of quantum entities to fully comprehend tunneling dynamics, which holds promise for an intuitive interpretation of complex-number quantum processes.

Quantum tomography (QT) has found broad applications across multiple disciplines, such as atomic and molecular physics, quantum optics, quantum computing, and quantum information. Here, we provide a comprehensive overview of QT applied to atoms and molecules, which enables the full reconstruction of quantum states when combined with state-of-the-art ultrafast imaging techniques, such as velocity map imaging, photoelectron spectroscopic imaging, and ultrafast electron diffraction imaging. QT reconstructs the quantum states for a variety of ultrafast molecular dynamics processes, including photoionization, dissociation, rotational wave packet alignment, and electronic state internal conversion. By providing fully quantum-mechanical descriptions, the tomographically reconstructed quantum states unveil a range of nonclassical phenomena, such as the spatial interference of wave packets, Dyson orbitals of photoionized molecules, and entanglement between parent ions and emitted electrons in multichannel photoionization.

Attosecond quantum light, formed by the superposition of high-order harmonics driven by intense quantum light, opens new routes to probe quantum-mechanical correlations in matter. In this study, we have investigated the macroscopic propagation effects of quantum high-order harmonics generated by the combination of strong coherent and weak bright squeezed vacuum (BSV) lasers interacting with atomic gas. Our results reveal that the pressure-dependent intensity of harmonics arising from absorbing or emitting BSV photons differs from that of harmonics generated using only strong coherent pulses. Macroscopic propagation simulations indicate that the action phase of harmonics is perturbed by the weak BSV pulses. This perturbation modulates the phase mismatch of sub-cycle attosecond bursts and affects their quantum properties when the gas pressure varies. The ability to generate bright quantum high-order harmonics lays a foundation for the establishment and application of attosecond quantum spectroscopy.

The ionization and fragmentation of water are fundamental processes across numerous scientific and technological fields, yet the relaxation pathways of water dications-particularly the rare D+ + O+ + D channel-remain poorly understood. Here, we report enhanced fragmentation pathways within this channel induced by electron-impact. Using multi-particle coincidence momentum spectroscopy supported by electron-capture-mediated molecular dissociation calculations, we identify a Rydberg state-controlled fragmentation mechanism and resolve its ultrafast relaxation dynamics. Furthermore, we demonstrate that this mechanism also occurs in ammonia and strong-field ionization experiments. In the laser-induced processes, electron-recollision with the cation leads to additional ionization plus excitation into dicationic Rydberg states on a sub-cycle (~2 fs) timescale, effectively freezing nuclear motion. Our findings reveal a general molecular fragmentation pathway governed by high Rydberg states, providing a molecular clock to probe electron-nuclear coupling and offering new insights into water radiolysis.

The attosecond waveform in high-harmonic generation (HHG) exhibits intricate spectral-temporal structures owing to the abundant coupling effect during laser-matter interaction. In this article, we present a flexible method for coherent attosecond waveform control leveraging propagation-induced spectral structure in high harmonics. By engineering these spectral features, we achieve precise temporal modulation and characterization of attosecond pulse trains (APTs). The intrinsic association between this unique spectral-temporal structure and macroscopic propagation effects is verified under three-dimensional simulation analysis. It establishes an intuitive connection between spectral and temporal manifestations of attosecond pulses, while pioneering a straightforward approach for coherently manipulating the waveform of the tabletop attosecond light sources. (c) 2026 Chinese Laser Press

The capability to control molecular rotation for field-free orientation, which arranges molecules in specific spatial directions without external fields, is crucial in physics, chemistry, and quantum information science. However, conventional methods typically lead to transient orientations characterized by periodic directional reversals and necessitate the generation of coherent superpositions across a broad spectrum of rotational states of ultracold molecules. In this work, we develop a theoretical framework for achieving unidirectional field-free orientation by selectively manipulating two specific rotational states of symmetric top molecules. By leveraging the interplay between coherent superpositions and the precise selection of initial states, we demonstrate that both the maximum achievable orientation and its direction can be effectively controlled. To attain the desired two-state orientation, we present a quantum control strategy that utilizes a single control pulse, significantly simplifying the complexities of conventional multistate or multipulse schemes. Numerical simulations validate the effectiveness and feasibility of this approach for methyl iodide (CH_3I) molecules, even when accounting for molecular centrifugal distortion.The results highlight the critical roles of initial-state selection and quantum coherence in achieving long-lasting, high unidirectional molecular orientation, opening new directions in stereochemistry, precision spectroscopy, and quantum computing.

Attosecond ionization dynamics,a central topic in ultrafast science,largely depends on advances in experimental techniques and theoretical modeling to reveal the fundamental processes that control the evolution of matter on an ultrafast timescale.Among the cutting-edge approaches in this field,the strong-field multiphoton transition interferometry(SFMPTI)method stands out due to its ability to detect multiphoton ionization dynamics with attosecond time resolution via quantum path interference.This technique has been widely applied to the attosecond-scale measurements and characterizations of ionization time delays with quantum-state specificity,ranging from atomic systems to complex molecules.It provides a novel time-domain perspective in the study of strong-field physics.This article focuses on the application of the SFMPTI in probing strong-field multiphoton ionization time delays in atoms and molecules.We systematically present the quantum interference mechanisms behind the method:electrons undergo multi-photon above-threshold ionization(ATI)driven by a 400 nm laser pulse,while an additional 800 nm laser pulse induces the sideband signals through two-color interference.The relative phases encoding of these sidebands provides precise timing information about the ionization process.Furthermore,we summarize the recent advances in attosecond-resolved investigations of ATI dynamics and resonance-state-mediated time delays.For instance,the significant influence of resonance-enhanced multiphoton ionization processes involving different intermediate states in Ar atoms on ionization time delays is elucidated,highlighting the important influences of Freeman resonances on photoelectron emission dynamics in strong laser fields.Additionally,nuclear vibrations in NO molecules change ionization trajectories via nonadiabatic coupling of potential energy surfaces,leading to variations in time delay.Notably,the substantial influence of internuclear distance on ionization delay highlights the high sensitivity of electron-nuclear co-evolution to ultrafast phenomena.Finally,we discuss the potential applications and remaining challenges of this emerging technique,which will continue to open up new avenues for exploring attosecond electron dynamics in complex systems.

We investigated the ionization and dissociation processes of ammonia clusters ranging from dimer to pentamer in-duced by 800-nm femtosecond laser fields.Time-of-flight(TOF)mass spectra of the ammonia clusters were recorded over a range of laser intensities from 2.1 × 1012 W/cm2 to 5.6× 1012 W/cm2.The protonated ion signals dominate the spectra,which is consistent with the stability of the geometric structures.The ionization and dissociation channels of ammonia clusters are discussed.The competition and switching among observed dissociation channels are revealed by analyzing the variations in the relative ionic yields of specific protonated and unprotonated clusters under different laser intensities.These results indicate that the ionization of the neutral multiple-ammonia units,produced through the dissociation of cluster ions,may start to contribute,as well as the additional processes to consume protonated ions and/or produce unprotonated ions induced by the femtosecond laser fields when the laser intensity is above～4× 1012 W/cm2.These findings provide deeper insights into the ionization and dissociation dynamics in multi-photon ionization experiments involving ammonia clusters.

A novel projective measurement technique utilizing conventional photoelectron velocity-map imaging to probe the spatial part of the wave function by projecting it into momentum space is demonstrated. Oscillations between the states |m_{l}=0,m_{s}=±1/2⟩ and |m_{l}=±1,m_{s}=∓1/2⟩ are observed with no detectable decoherence over 170 cycles with a speed of around 1.7 THz driven purely by spin-orbit interaction. Our measurements achieve a quantum operation fidelity of approximately 90%, limited by the photoionization detection scheme. This Letter establishes a new platform for high-fidelity quantum operations driven by the intrinsic spin-orbit coupling.

Spontaneous symmetry breaking, driven by nonadiabatic electron-nuclear coupling, can lead to geometric complexity in molecules and solids. While structural distortion from symmetry breaking occurs in femtoseconds, the timescale to lift electronic state degeneracy has remained elusive. We use the vibrationally resolved attosecond chronoscope to capture the electronic symmetry breaking induced by the Renner-Teller effect in bent CO2 molecules after photoionization by an extreme ultraviolet photon by measuring attosecond ionization delays. Relative photoionization delays between the four cation states are observed, with vibrational state-dependent delays, we analyze the evolution of the degenerate [Formula: see text] state to the nondegenerate A' and A″ states due to molecular bending. With the help of theoretical analysis, we show that the relative photoionization delays of up to 72 as between the vibrational levels originate from the symmetry breaking-induced shape resonance. This study offers fundamental insights by resolving the coupled electron and structural dynamics simultaneously.

In recent years,the attosecond extreme ultraviolet(XUV)pulse generation and advanced spectroscopic techniques have provided powerful tools for investigating electron dynamics.Researches on an attosecond timescale can realize real-time tracking of electronic motion in atoms and molecules,enabling the measurement of electron wave packet evolution and quantum characteristics,which are crucial for revealing complex dynamical processes within atomic and molecular systems.High-resolution photoelectron interferometers based on attosecond XUV pulse trains have played an important role in a wide range of applications due to their unique combination of high energy and temporal resolution.These applications include the characterization of attosecond pulse trains,the measurement of photoionization time delays in atoms and molecules,quantum state reconstruction of photoelectrons,and laser-induced electronic interference phenomena.By integrating attosecond temporal resolution with millielectronvolt level energy resolution,high-resolution photoelectron interferometric spectroscopy has emerged as a key technique for probing ultrafast dynamics and quantum state characterization.This review systematically summarizes recent advances in high-resolution attosecond photoelectron interferometry,with a focus on the experimental approaches and spectroscopic techniques required to access electron dynamics on an attosecond scale.These include the generation of narrowband attosecond XUV pulse trains,attosecond-stable Mach-Zehnder interferometers,high-energy resolution time-of-flight electron spectrometers,and quantum interference-based measurement schemes such as RABBIT and KRAKEN.This review discusses in detail the reconstruction of attosecond pulse sequences,shell-resolved photoionization time delay measurements in atoms,spectral phase evolution in Fano resonances,tomographic reconstruction of photoelectron density matrices on an attosecond timescale,and control experiments of laser-induced electronic dynamic interference effects.Through the analysis of recent studies,we demonstrate the powerful potential of attosecond high-energy resolution photoelectron interferometry in tracking ultrafast electron dynamics.Finally,the prospects of attosecond photoelectron spectroscopy in ultrafast dynamics and coherent manipulation of quantum systems are discussed.

Proton transfer underpins number of chemical and biochemical processes, yet its sub-100 fs dynamics have rarely been captured in real time. Here, we report direct and time-resolved observation of ionizing radiation-induced proton transfer in a heteroaromatic hydrate: the pyrrole-water complex. Both the electron-impact and strong-field laser experiments create a locally and doubly charged pyrrole unit (C4H5N2+), which immediately (within 60 fs) donates a proton to the adjacent H2O, generating deprotonated C4H4N+ and hydronium H3O+ cations that subsequently undergo Coulomb explosion. The electron-impact experiments directly revealed initial states and provided dynamical insights through fragment ions and electron coincidence momentum imaging. The strong-field femtosecond laser experiments tracked the ultrafast dynamics of proton transfer; complementary ab initio calculations unraveled the dynamical details. The 50-60 fs proton transfer qualifies as one of the fastest acid-base reactions observed to date. This study offers a novel perspective on radiation-induced proton transfer in hydrated biomolecules.

A two-photon resonance in the sodium atom 3s-4s transition is investigated in the strong coupling regime with photoelectron momentum imaging techniques. The kinetic energies of the ionized electron map out the eigenenergies of the dressed states and emerge as the Autler-Townes (AT) splitting, which is proportional to the laser intensity due to the two-photon nature. The partial wave analysis of the angular distribution of the photoelectrons reveals the relative phase shift from dressed states which is related to the attosecond ionization time delays. The phase shift between the two electron wave packets produced from AT splitting are attributed to the combination of the Coulomb phase and the quantum defect phase.

Light-induced conical intersections (LICIs) present a distinctive mechanism for nonadiabatic coupling, thereby facilitating ultrafast chemical reactions, including the indirect photodissociation of diatomic molecules. In contrast to static conical intersections, LICIs are dynamically tunable, providing a pathway for precise control of molecular dissociation. In this study, we employ the time-dependent quantum wave packet method to investigate the dissociation dynamics of the OH molecule, focusing on its ground state X2 Pi and repulsive state 12 Sigma-. By varying laser field parameters (intensity, full width at half maximum (FWHM), and wavelength), we elucidate how nonadiabatic coupling governs selective dissociation channel control. Our findings reveal that the choice of initial vibrational states and the tailoring of laser conditions significantly influence dissociation pathways, providing theoretical insights into manipulating molecular dynamics via LICIs. These results provide a foundation for future experimental studies and the development of advanced molecular control techniques.

Resonance-enhanced multiphoton ionization (REMPI) in potassium atom is investigated within the strong coupling regime using photoelectron momentum imaging techniques. The kinetic energy distribution of the ionized electrons reveals the eigenenergies of the dressed states, which exhibit Autler-Townes (AT) splitting. This splitting is proportional to the laser field strength. Partial wave analysis of the photoelectrons angular distributions (PAD) uncovers relative phase shifts between the dressed states, shedding light on attosecond ionization time delays. The observed phase shift between the two electron wave packets generated by the AT splitting arises from the combined effects of the Coulomb phase and the quantum defect phase.

Vibronic coupling and coherence are crucial in the charge and energy transfer of photoexcited molecules. Here we investigate the coupled electron-nuclear dynamics of the photoionized benzene molecule using the time-resolved Coulomb-explosion imaging method. A long-period oscillation is experimentally observed in the ion yields of the C 6 H 6 2 + channel, as well as the C+ + C+, and the C+ + C+ + C+ Coulomb explosion channels. Quantum dynamics simulations reveal that this  ~600 fs oscillation, which notably exceeds the period of any vibrational modes, originates from pseudorotation of the benzene cation. This motion arises from quantum beating between two coherent vibronic states of the benzene molecule coupled via the Jahn-Teller effect around the conical intersection. The structural evolution of the benzene cation via pseudorotation is visualized by the time-resolved momentum imaging in the C+ + C+ + C+ three-body Coulomb explosion channel. Our work offers a comprehensive characterization of coherent vibronic dynamics of the benzene cation and demonstrates the power of the time-resolved Coulomb-explosion imaging for unraveling coupled electronic and nuclear motions in aromatic molecules.

Attosecond ionization dynamics, as a key research direction in ultrafast science, relies critically on breakthroughs in both experimental techniques and theoretical models to reveal the fundamental processes underlying ultrafast matter evolution. Among the cutting-edge approaches in this field, the strong-field multiphoton transition interferometry (SFMPTI) method stands out for its ability to achieve attosecond time-resolved probing of multiphoton ionization dynamics via quantum path interference. This technique has been widely applied to attosecond-scale measurements and characterizations of ionization time delays with quantum state resolution, ranging from atomic systems to complex molecules, offering a novel time-resolved perspective for strong-field physics.This article focuses on the application of the SFMPTI method in probing strong-field multiphoton ionization time delays in atoms and molecules. We systematically present the quantum interference mechanisms underlying the method, summarize recent progress in attosecond-resolved studies of above-threshold ionization dynamics and resonance-state-mediated delays, and discuss the prospective applications and challenges that lie ahead for this emerging technique.

The ionization of large-sized ammonia clusters in 800 nm femtosecond laser fields is investigated by using the velocity map imaging method. The photoelectron spectra obtained at laser intensities of similar to 1013 W/cm2 exhibit two distinct regions with structureless, exponentially decaying distributions. By utilizing electron temperature, these distributions are characterized and the different mechanisms are revealed. In the low-energy region (epsilon <= 0.4 eV), the electron temperature increases with rising laser intensity, primarily due to the enhanced probability of frustrated recombination, where quasi-free electrons are temporarily trapped before eventual ionization. In contrast, in the high-energy region (0.4 eV <= epsilon <= 20 eV) the electron temperature decreases with increasing laser intensity, highlighting the crucial role of cluster expansion during electron thermalization. The photoelectron angular distributions reveal the isotropic nature of thermalized electrons, while field-induced electron scattering significantly influences even high-energy electrons (epsilon >= 2Up) beyond thermalization. Our results provide new insights into the underlying mechanisms governing electron thermalization in ammonia clusters under intense laser fields.