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We report on kinematically complete measurements and ab initio nonperturbative calculations of double ionization of He and H 2 by a single 800 eV circularly polarized photon. We confirm the quasifree mechanism of photoionization for H 2 and show how it originates from the two-electron cusp in the ground state of a two-electron target. Our approach establishes a method for mapping electrons relative to each other and provides valuable insight into photoionization beyond the electric-dipole approximation.

Wigner time delay in photodetachment from the 3p3/2 and 3p1/2 subshells of Cl− has been studied in the vicinity of the 2p3/2 and 2p1/2 thresholds, using the relativistic random phase approximation (RRPA). The results show time delay spectra dominated by many-body correlations along with very complicated dependence on the energy over a broad spectral range. In addition, the time delay spectra of the two spin-orbit split 3p subshells differ significantly from one another, thereby revealing the importance of relativistic effects even in the case of a low-Z system.

Attosecond angular streaking (or “attoclock”) is an insightful technique for probing the ultrafast electron dynamics in strong laser fields. Up until recently, this technique relied solely on an accurate measurement of the photoelectron momentum distribution and has remained restricted to atomic targets. Here, we propose a novel attosecond angular streaking scheme applicable to molecules, for which the ionic fragments of dissociative ionization are detected in the polarization plane of a close-to-circular polarized laser light. Our ionic attoclock measurements are consistent with theoretical results from a numerical solution of the time-dependent Schrödinger equation and an upper bound of 10 as on the tunneling time from the attoclock readings in the H2 molecule has been given, which is significantly smaller than any definitions of tunneling time available in the literatures.

Numerical attoclock is a theoretical model of attosecond angular streaking driven by a very short, nearly a single oscillation, circularly polarized laser pulse. The reading of such an attoclock is readily obtained from a numerical solution of the time-dependent Schrödinger equation as well as a semiclassical trajectory simulation. By comparing the two approaches, we highlight the essential physics behind the attoclock measurements. In addition, we analyze the predictions of the Keldysh-Rutherford model of the attoclock [A. W. Bray et al., Phys. Rev. Lett. 121, 123201 (2018)]. In molecular hydrogen, we highlight a strong dependence of the width of the attoclock angular peak on the molecular orientation and attribute it to the two-center electron interference. This effect is further exemplified in the weakly bound neon dimer.

We uncover dramatic variations of the Wigner photoemission time delay with energy and angle in the vicinity of a Fano resonance with the time delay taking opposite signs at different angles at the same energy as well as at the opposite sides of the resonance at the same angle. These variations are illustrated by choosing the Ne 2s→3p autoionizing state as a case study. Moreover, we demonstrate the existence of strikingly significant changes in time delay due to relativistic effects despite Ne being a low-Z atom. This finding shows the possibility for utilizing time delay chronoscopy as a route towards experimental probing of relativistic interactions and the phases of individual transition matrix elements upon atomic photoionization of low- Z atoms. Finally, we develop a practical parametrization to model and explain the angle and energy variation of the autoionizing resonance time delay in the nonrelativistic limit.

Using Cl− as a test case, Wigner time delay in the photodetachment process has been investigated theoretically, along with the photoionization of the isoelectronic Ar atom, for the outer 3p shell using the relativistic-random-phase approximation (RRPA). Time delay was probed in these systems from threshold to 80 eV, to investigate threshold effects, the centrifugal barrier shape resonance, and the Cooper minimum region. This study focuses on Cl − because, for negative ions, the phase of the photoemission process is not dominated by the Coulomb phase as it is in photoionization. The results show significant differences, both qualitative and quantitative, between the time delays for Cl − and Ar photoemission at low photoelectron energy, but they are rather similar in the Cooper minimum region, where the Coulomb phase is small. In particular, the Wigner time delay in Cl − exhibits a dramatic energy dependence just above threshold, and a rapidly increasing time delay in the vicinity of the shape resonance. A strong angular dependence of time delay has also been found near the threshold region for the Cl− case, and is absent in the case of the photoionization of Ar. The origin of these phenomenologies is explained and a prospectus for future work is presented.

We demonstrate a clear similarity between attoclock offset angles and Rutherford scattering angles taking the Keldysh tunneling width as the impact parameter and the vector potential of the driving pulse as the asymptotic velocity. This simple model is tested against the solution of the time-dependent Schrodinger equation using hydrogenic and screened (Yukawa) potentials of equal binding energy. We observe a smooth transition from a hydrogenic to "hard-zero" intensity dependence of the offset angle with variation of the Yukawa screening parameter. Additionally, we make a comparison with the attoclock offset angles for various noble gases obtained with the classical-trajectory Monte Carlo method. In all cases we find a close correspondence between the model predictions and numerical calculations. This suggests a largely Coulombic origin of the attoclock offset angle and casts further doubt on its interpretation in terms of a finite tunneling time.

We report on a kinematically complete measurement of double ionization of helium by a single 1100 eV circularly polarized photon. By exploiting dipole selection rules in the two-electron continuum state, we observed the angular emission pattern of electrons originating from a pure quadrupole transition. Our fully differential experimental data and companion ab initio nonperturbative theory show the separation of dipole and quadrupole contributions to photo-double-ionization and provide new insight into the nature of the quasifree mechanism.

We consider photoemission from the 4 d shell of the free Xe and encapsulated Xe@ C 60 atoms by ionizing XUV and probing IR fields typical for a RABBITT (reconstruction of attosecond beating by interference of two-photon transitions) measurement.

We predict an observable Wigner time delay in outer atomic shell photoionization near inner shell thresholds. The near-threshold increase of time delay is caused by intershell correlation and serves as a sensitive probe of this effect. The time delay increase is present even when the inner and outer shell thresholds are hundreds of electron volts apart. We illustrate this observation by several prototypical examples in noble gas atoms from Ne to Kr. In our study, we employ the random phase approximation with exchange and its relativistic generalization. We also support our findings by a simplified, yet quite insightful, treatment within the lowest-order perturbation theory.

We report on the first streaking measurement of water-window attosecond pulses generated via high-harmonic generation, driven by sub-2-cycle, carrier-to-envelope-phase-stable, 1850-nm laser pulses. Both the central photon energy and the energy bandwidth far exceed what has been demonstrated thus far, warranting the investigation of the attosecond streaking technique for the soft-x-ray regime and the limits of the FROGCRAB retrieval algorithm under such conditions. We also discuss the problem of attochirp compensation and issues regarding much lower photoionization cross sections compared with the extreme ultraviolet in addition to the fact that several shells of target gases are accessed simultaneously. Based on our investigation, we caution that the vastly different conditions in the soft-x-ray regime warrant a diligent examination of the fidelity of the measurement and the retrieval procedure.

The convergent close-coupling method for atomic photoionization is modified by treating the singularity in the Green's function analytically. The resultant close-coupling equations are then free of any singularities, and can be solved at all incident energies including at the exact thresholds. The utility and superiority of the approach is demonstrated by considering single photoionization of the ground state of helium at photoelectron energies from the ionic n=1,2,3 thresholds through to 1 keV. For completeness, the double photoionization cross section is also presented.

We develop a technique for modeling of atomic and molecular ionization in superposition of XUV and IR fields with characteristics typical for attosecond streaking and RABBITT (reconstruction of attosecond beating by interference of two-photon transitions) experiments. The method is based on solving the time-dependent Schrödinger equation in the coordinate frame expanding along with the photoelectron wave packet. The efficiency of the method is demonstrated by calculating angular anisotropy of photoemission time delay of the H2+ ion in a field configuration of recent RABBITT experiments.

Time delays of electrons emitted from an isotropic initial state with the absorption of a single photon and leaving behind an isotropic ion are angle independent. Using an interferometric method involving XUV attosecond pulse trains and an IR-probe field in combination with a detection scheme, which allows for full three-dimensional momentum resolution, we show that measured time delays between electrons liberated from the 1s2 spherically symmetric ground state of helium depend on the emission direction of the electrons relative to the common linear polarization axis of the ionizing XUV light and the IR-probing field. Such time delay anisotropy, for which we measure values as large as 60 as, is caused by the interplay between final quantum states with different symmetry and arises naturally whenever the photoionization process involves the exchange of more than one photon. With the support of accurate theoretical models, the angular dependence of the time delay is attributed to small phase differences that are induced in the laser-driven continuum transitions to the final states. Since most measurement techniques tracing attosecond electron dynamics involve the exchange of at least two photons, this is a general and significant effect that must be taken into account in all measurements of time delays involving photoionization processes.

Characteristic features of Wigner photoemission time delay from endohedral anions A@C60q along with their dependence on the anion charge q are unraveled. Specifically, significant enhancement of the time delay in the innermost dipole photoionization channels near threshold is found, owing to the presence of the Coulomb confined resonances (CRs). Moreover, it is shown that interchannel coupling of the inner-shell Coulomb CRs with outer-shell photoionization channels results in resonantly enhanced time delay in the release of the outer-shell photoelectron well above, several hundreds eV, the outer-shell thresholds. It is also demonstrated that, and explained why, photoionization cross sections of the innermost subshells as well as outer subshells (near the inner-subshell threshold) depends only very weakly on the anion charge q, but the dependence of the corresponding time delays on q can be significant. Furthermore, Coulomb CRs are found to emerge in the innermost quadrupole photoionization channels as well, thereby causing considerable time delay in the quadrupole photoemission. These findings are illustrated in calculations of the photoionization of inner and outer subshells of the endohedral anions Ne@C60−1 and Ne@C60−5 that were chosen as case studies

We study the time delay in the primary photoemission channel near the opening of an additional channel and compare it with the Wigner time delay in elastic scattering of the photoelectron near the corresponding inelastic threshold. The photoemission time delay near threshold is significantly enhanced, to a measurable 40 as, in comparison to the corresponding elastic scattering delay. The enhancement is due to the different lowest order of interelectron interaction coupling the primary and additional photoemission channels. We illustrate these findings by considering photodetachment from the H− negative ion, and compare it with electron scattering on the hydrogen atom near the first excitation threshold. Other threshold processes of atomic photoionization and molecular photofragmentation, where photoemission time delay is enhanced, are identified. This opens the possibility of studying threshold behavior utilizing attosecond chronoscopy.

Angular dependence of photoemission time delay for the valence np3/2 and np1/2 subshells of Ar, Kr, and Xe is studied in the dipole relativistic random phase approximation. Strong angular anisotropy of the time delay is reproduced near respective Cooper minima while the spin-orbit splitting affects the time delay near threshold.

Initial insight into time delay in Mn photoionization in the region of the 3p →, 3d giant autoionization resonance is gained in the framework of the "spin-polarized" random-phase approximation with exchange. The dramatic effect of the giant autoionization resonance on the time delay of photoemission from the 3d and 4s valence subshells of the Mn atom is unraveled. Strong sensitivity of the time delay of the 4s photoemission to the final-state term of the ion remainder [Mn+(4s1,5S) vs Mn+(4s1,7S)] is discovered. It is shown that photoionization time delay in the autoionizing resonance region is explicitly associated with the resonance lifetime, which can thus be directly measured in attosecond time-delay experiments. Similar features are expected to emerge in photoionization time delays of other transition-metal and rare-earth atoms with half-filled subshells that possess giant autoionization resonances as well.

We conduct a systematic study of the dipole phase and the photoelectron group delay (Wigner time delay) in inner shell photoionization of noble gas atoms from Ne to Xe. Our study encompasses the tender x-ray spectral range and extends to 1 keV photoelectron energy. We employ both the relativistic and the nonrelativistic versions of the random-phase approximation with exchange. We identify the long-range Coulomb and short-range Hartree-Fock contributions to the dipole phase which governs the Wigner time delay variation from the threshold to the whole considered range of photoelectron energies.

The photo-double-ionization (PDI) of Mg to the Mg2+(3s-2) state has been studied by photoelectron-photoelectron-coincidence experiments at a photon energy corresponding to the excitation of the 2p→3d resonance. The equal energy sharing (E1 = E2 = 16.4 e

We analyze a transfer ionization (TI) reaction in the fast proton-helium collision H++He→H0+He2++ e- by solving a time-dependent Schrödinger equation (TDSE) under the classical projectile motion approximation in one-dimensional kinematics. In addition

Time delay of photoemission from valence ns, np3/2, and np1/2 subshells of noble-gas atoms is theoretically scrutinized within the framework of the dipole relativistic random phase approximation. The focus is on the variation of time delay in the vicinity of the Cooper minima in photoionization of the outer subshells of neon, argon, krypton, and xenon, where the corresponding dipole matrix element changes its sign while passing through a node. It is revealed that the presence of the Cooper minimum in one photoionization channel has a strong effect on time delay in other channels. This is shown to be due to interchannel coupling.

We study strong-field atomic ionization driven by an XUV pulse with a nonzero displacement, the quantity defined as the time integral of the pulse vector potential taken over the pulse duration. The use of such pulses may lead to an extreme sensitivity of the ionization process to subtle changes of a driving XUV pulse, in particular, the ramp-on and off profile and the carrier envelope phase. We illustrate this sensitivity for atomic hydrogen and lithium driven by few-femtosecond XUV pulses with intensity in the 1014W/cm2 range. The observed effect is general and should modify strong-field ionization of any atom or molecule, provided the ionization rate is sufficiently high.

The effects of confinement resonances on photoelectron group delay (Wigner time delay) following ionization of an atom encapsulated inside a C60 cage have been studied theoretically using both relativistic and nonrelativistic random phase approximations.

The photo-double-ionization (PDI) of Mg to the Mg2+(3s−2) state has been studied by photoelectron-photoelectron-coincidence experiments at a photon energy corresponding to the excitation of the 2p→3d resonance. The equal energy sharing (E1 = E2 = 16.4 eV) as well as the complementary unequal energy (E1↔E2 = 10.4↔22.4 eV) sharing kinematics have been investigated. From the experimental results without any approximation the symmetrized gerade and ungerade amplitudes have been obtained. The experimental angular correlation patterns as well as the amplitudes are compared to CCC calculations in which the resonant process has been incorporated. The results confirm that the amplitudes of the photo-double-ionization carry the signature of the target radial wave function. The investigation of the triple differential cross sections has been then extended by simulations to a kinematics with the fixed detector at 90°, which can not be studied experimentally by the present setup. These simulations shed light on results of previous measurements on alkaline-earth-metal atoms in this kinematics, which were not consistent with the common understanding of photo-double-ionization derived from He experiments.

We investigate the single-photon double ionization of helium at photon energies of 440 and 800 eV. We observe doubly charged ions with close to zero momentum corresponding to electrons emitted back to back with equal energy. These slow ions are the unique

We use the soft-photon approximation to extract the Wigner time delay from atomic two-color photoionization experiments. Unlike the strong-field approximation, the present method does not require introduction of the Coulomb-laser coupling corrections and

Accurate knowledge of the intensity of focused ultrashort laser pulses is crucial to the correct interpretation of experimental results in strong-field physics. We have developed a technique to measure laser intensities approaching 1015W/cm2 with an accur

We use the nonrelativistic random-phase approximation with exchange to perform calculations of valence-shell photoionization of Ne, Ar, Kr, and Xe from their respective thresholds to photon energy of 200 eV. The energy derivative of the complex phase of t

We study time delay in atomic photoionization by circularly polarized light. By considering the Li atom in an excited 2p state, we demonstrate a strong time-delay asymmetry between the photoemission of the target electrons that are co- and counter-rotating with the electromagnetic field in the polarization plane. In addition, we observe the time-delay sensitivity to the polar angle of the photoelectron emission in the polarization plane. This modulation depends on the shape and duration of the electromagnetic pulse.

The double photoionization of Mg has been studied experimentally and theoretically in a kinematic where the two photoelectrons equally share the excess energy. The observation of a symmetrized gerade amplitude, which strongly deviates from the Gaussian ansatz, is explained by a two-electron interference predicted theoretically, but never before observed experimentally. Similar to the Cooper minima in the single photoionization cross section, the effect finds its origin in the radial extent and oscillation of the target wave function.

We perform time-dependent calculations of triply differential cross sections (TDCS) of two-photon double-electron ionization of the aligned H 2 molecule. Our TDCS results for equal energy sharing between photoelectrons agree quite well with a recent time-dependent calculation by Guan who employed a slightly different numerical technique. We supplement these studies by calculating TDCS at an unequal energy sharing and by generating symmetrized ionization amplitudes.

We perform time-dependent calculations of single-photon two-electron ionization of the aligned H2 molecule by an xuv pulse. Solution of the time-dependent Schrödinger equation is sought in spherical coordinates on a radial grid by time propagation using

We apply the concept of photoemission time delay to the process of single-photon one-electron ionization of the H2 molecule. We demonstrate that, by resolving the photoelectron detection in time on the attosecond scale, one can extract differential photoionization cross sections for particular field and molecule orientations from the measurement on a randomly oriented molecule

We present experimental measurements and theoretical calculations of photoionization time delays from the 3s and 3p shells in Ar in the photon energy range of 32-42 eV. The experimental measurements are performed by interferometry using attosecond pulse trains and the infrared laser used for their generation. The theoretical approach includes intershell correlation effects between the 3s and 3p shells within the framework of the random-phase approximation with exchange. The connection between single-photon ionization and the two-color two-photon ionization process used in the measurement is established using the recently developed asymptotic approximation for the complex transition amplitudes of laser-assisted photoionization. We compare and discuss the theoretical and experimental results, especially in the region where strong intershell correlations in the 3s→kp channel lead to an induced "Cooper" minimum in the 3s ionization cross section.

We perform a time-delay analysis of the strong field ionization of atomic hydrogen in the tunneling regime. We obtain values for the time delay by solving the time-dependent Schrödinger equation, and use these values as parameters to define the correspon

We present experimental measurements and theoretical calculations of photoionization time delays from the 3s and 3p shells in Ar in the photon energy range of 32-42 eV. The experimental measurements are performed by interferometry using attosecond pulse trains and the infrared laser used for their generation. The theoretical approach includes intershell correlation effects between the 3s and 3p shells within the framework of the random-phase approximation with exchange. The connection between single-photon ionization and the two-color two-photon ionization process used in the measurement is established using the recently developed asymptotic approximation for the complex transition amplitudes of laser-assisted photoionization. We compare and discuss the theoretical and experimental results, especially in the region where strong intershell correlations in the 3s -> kp channel lead to an induced "Cooper" minimum in the 3s ionization cross section.

We analyze correlated photoelectron spectra of single-photon two-electron ionization [double photoionization (DPI)] of helium to reconstruct the phase of the spectral amplitude of this process. The phase can be reconstructed reliably in a wide range of photoelectron momenta, thus allowing one to retrieve information about the wave function of the DPI process and its temporal evolution. Our simulation indicates that the proposed phase reconstruction technique can be applied in experiment to trace dynamics of the DPI process with attosecond precision.