Using sequential strong-field double ionization in a pump–probe scheme we show through calculations how electronic dynamics can be prepared and imaged. Electronic dynamics may arise whenever multiple states of the ion are accessed in the ionization step. The dynamics in the cation influence the rate of the second ionization step and the momentum distribution of the ejected electron, allowing their detailed characterization. We show how the probe step is controlled through spatial propensities of the ionizing orbitals and the energy level structure of the dication. Both the final electronic state of the dication and the spin state of the ejected electron pair can be controlled through the time delay between the two ionizing pulses. We discuss how our results will extend to the preparation and measurement of attosecond electron dynamics.
Using sequential strong-field double ionization in a pump–probe scheme we show through calculations how electronic dynamics can be prepared and imaged. Electronic dynamics may arise whenever multiple states of the ion are accessed in the ionization step. The dynamics in the cation influence the rate of the second ionization step and the momentum distribution of the ejected electron, allowing their detailed characterization. We show how the probe step is controlled through spatial propensities of the ionizing orbitals and the energy level structure of the dication. Both the final electronic state of the dication and the spin state of the ejected electron pair can be controlled through the time delay between the two ionizing pulses. We discuss how our results will extend to the preparation and measurement of attosecond electron dynamics.
The past thirty years witnessed rapid developing of strong field physics that leads to the first generation of attosecond (10−18 s) pulses. The duration of available pulses has recently been reduced to less than 100 as, approaching the Kepler period of a classical electron revolving around the nucleus. For the first time, electron motions inside atoms, molecules and solids can be traced, probed and even steered in real time. It pushed the study on ultrafast dynamic processes into a new frontier, giving birth to attosecond physics. The development of attosecond pulses provides unprecedented coherent ultrashort XUV (extreme ultra-violet) of soft X-ray sources that enable imaging the atomic structure and resolving the related quantum dynamics for matter science. In particular, it allows the probing and monitoring of electron dynamics in the natural time scale of attoseconds with atomic spatial resolution. The generation and applications of attosecond pulses are directly related to the control and manipulation of the sub-cycle electron dynamics. Recent studies on the correlated multielectron dynamics have provided insight into the optical and dynamical properties of matter in the perspectives of time, phase and entanglement. It deepens our understanding on some of the fundamental questions, e.g., how the single or multiple photons are absorbed and how short are the quantum processes such as photoionization, tunneling ionization and charge migration. Although currently attosecond pulses are limited by the flux and pulse duration, the combination of attosecond pulses with intense infrared laser pulses paves new ways of controlling and probing electron dynamics in the domains of energy, time and space that lays the foundation for petahertz optoelectronics and attosecond pulse based transient spectroscopy. In this review, we will first briefly summarize the advancement of strong field and ultrafast physics including the relevant experimental findings, the theoretical explorations, the related technologies and the possible applications. Then we focus on the sub-cycle electron or multielectron dynamics and their manifestation on the ionization and coherent radiation properties. We start with the time behavior of Fano resonance and how it can be probed in time domain. In section 2, the correlated electron dynamics is discussed in term of anti-screening effect and Pauli effect that influence strong field ionization and high harmonic generation processes. In section 3, the coherent emission with frequency from terahertz to soft XUV and their joint measurement are discussed by emphasizing the generation mechanism of terahertz wave in two-color laser pulses. The developed high-harmonic and terahertz wave spectroscopy (HATS) is shown capable of imaging the molecular structure and dynamics. In section 4, the time-resolving of electron and hole dynamics and their coherence is proposed based on attosecond transient absorption technique. The coherence of ionization and the correlated electron dynamics is hinted crucial for electron-hole interaction. The topics are very limited and mainly selected from the perspectives of our group.
Experiments are presented on real-time probing of coherent electron dynamics in xenon initiated by strong-field double ionization. Attosecond transient absorption measurements allow for characterization of electronic coherences as well as relative ionization timings in multiple electronic states of Xe^{+} and Xe^{2+}. A high degree of coherence g=0.4 is observed between ^{3}P_{2}^{0}-^{3}P_{0}^{0} of Xe^{2+}, whereas for other possible pairs of states the coherences are below the detection limits of the experiments. A comparison of the experimental results with numerical simulations based on an uncorrelated electron-emission model shows that the coherences produced by strong-field double ionization are more selective than predicted. Surprisingly short ionization time delays, 0.85 fs, 0.64 fs, and 0.75 fs relative to Xe^{+} formation, are also measured for the ^{3}P_{2}, ^{3}P_{0}, and ^{3}P_{1} states of Xe^{2+}, respectively. Both the unpredicted selectivity in the formation of coherence and the subfemtosecond time delays of specific states provide new insight into correlated electron dynamics in strong-field double ionization.
The advent of the ultrafast laser pulse provides the powerful and efficient tool for probing the ultrafast electron dynamics in atoms and molecules. The various nonlinear process induced by the laser-matter interaction allows one to obtain the electron motion information on the sub-femtosecond time scale. A series of the ultrafast spectroscopic technique, such as attosecond streak camera, attosecond transient absorption spectrum, and etc., have been successfully applied to the probe of electron dynamics in atoms, molecules, and solids. Using two-color field is one of the significant methods to achieve the coherent control and exploring of the electron motion. This paper summarizes recent research activities in the field of the atomic and molecular ultrafast process investigated in State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, including the detection of the electron dynamics of the multi-bound states, measurement of the carrier envelope phase (CEP) and the phase of the attosecond pulse, and the ultrafast electron control with the THz/UV and MIR/IR field. To measure the dynamics of the multi-bound states, a broadband attosecond pulse can be used to ionize the electrons after it is excited by the pump laser. By changing the delay between the pump laser and the attosecond pulse, the measured electrons ionized by the broadband xuv attosecond pulse can present the multi-bound states dynamics simultaneously. The XUV/IR scheme is popularly used in attosecond dynamics measurement. But usually, the IR field is not very strong. We find that, if the IR field is strong enough to induce the above threshold ionization (ATI), the interference between the ATI electron and the electron from XUV pulse can be used to measure the CEP of the attosecond XUV pulse. Besides, if the electron ionized by attosecond pulse can be pushed back to the nuclei, the emission from the recombination can be used to determine the spectral phase of the attosecond pulse, which is an all-optical measurement. We also investigate the two color scheme of THz/UV and MIR/IR fields. With THz/UV two color scheme, very high electron localization can be achieved duration molecular dissociation when we use the UV pulse to excite the electron and the THz pulse to control the following electron movement. When we use the MIR/IR field to control the electron motion during the high harmonic generation, the recollision can be greatly decreased and the single attosecond pulse can be produced with multi-cycle MIR laser field.
A universal mechanism of ultrafast 2-electron orbital swap is discovered through 2-photon sequential double ionization of Li. After a 1s electron in Li is ionized by absorbing an extreme ultraviolet photon, the other 2 bound electrons located on 2 different shells have either parallel or antiparallel spin orientations. In the latter case, these 2 electrons are in the superposition of the singlet and triplet states with different energies, forming a quantum beat and giving rise to the 2-electron orbital swap with a period of several hundred attoseconds. The orbital swap mechanism can be used to manipulate the spin polarization of photoelectron pairs by conceiving the attosecond-pump attosecond-probe strategy and thus serves as a knob to control spin-resolved multielectron ultrafast dynamics.
Summary form only given. Angstrom and attosecond control of free electron wave packets is one of the pinnacles of attosecond science. Orthogonally polarized two-color (OTC) laser fields allow to control the motion of field-ionizing electronic wave packets both in time and space[1]. In OTC pulses time and space are connected and thus an attosecond time scale is established in the polarization plane for both the emitted and the re-colliding wave packets[2,3].In this submission, we report on experiments that use OTC pulses for studying atomic single and double ionization. The three-dimensional momentum vector of electrons and ions created by single and double ionization of neon atoms was measured with the COLTRIMS technique as a function of the sub-cycle shape of the OTC pulses. We introduce a concept similar to attosecond streaking that allows reading sub-cycle timing information from the momentum vector of emitted electrons. The concept is then applied to gaining information about the influence of the long-range Coulomb potential on the trajectories of tunneling electrons.Fig.1 (a) shows the measured momentum distributions of electrons correlated with singly ionized neon in the polarization plane of the OTC field with relative phase Δφ between the two color components. The spectra show that the electron emission direction is sensitive to Δφ. Furthermore, the spectra feature a prominent x-shaped central structure and weaker fine-scale modulations due to wave packet interferences. To compare with simulated spectra with strong field approximation (SFA) and solving two-dimensional time-dependent Schrödinger equation (TDSE) with single active electron approximation, we conclude that Coulomb effect plays important role on the spatial distribution of released electron wave packets. In the conference, we will furthermore demonstrate control over the correlation between the two electrons emitted during double ionization upon electron recollision by tuning the shape of the electric field of the OTC pulses on the sub-cycle scale. The results of these experiments are in agreement with theoretical predictions [4].
Ultrafast spectroscopy with attosecond resolution has enabled the real time observation of ultrafast electron dynamics in atoms, molecules and solids. These experiments employ attosecond pulses or pulse trains and explore dynamical processes in a pump-probe scheme that is selectively sensitive to electronic state of matter via photoelectron or XUV absorption spectroscopy or that includes changes of the ionic state detected via photo-ion mass spectrometry. Here, we demonstrate how the implementation of combined photo-ion and absorption spectroscopy with attosecond resolution enables tracking the complex multidimensional excitation and decay cascade of an Auger auto-ionization process of a few femtoseconds in highly excited krypton. In tandem with theory, our study reveals the role of intermediate electronic states in the formation of multiply charged ions. Amplitude tuning of a dressing laser field addresses different groups of decay channels and allows exerting temporal and quantitative control over the ionization dynamics in rare gas atoms.