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Nondipole terms in the atom–laser interaction arising due to the presence of a magnetic component in an electromagnetic wave and its inhomogeneity lead to the nonseparability of the center-of-mass (CM) and electron variables in the neutral atom and, as a consequence, to its acceleration. We investigate this effect and the accompanying excitation and ionization processes for the hydrogen atom in strong ( 10 12 – 2 × 10 14 W/cm2) linearly polarized short-wavelength (5 eV ≲ ℏ ω ≲ 27 eV) electromagnetic pulses of about 8 fs duration. The study was carried out within the framework of a hybrid quantum-quasiclassical approach in which the coupled time-dependent Schrödinger equation for an electron and the classical Hamilton equations for the CM of an atom were simultaneously integrated. Optimal conditions with respect to the frequency and intensity of the electromagnetic wave for the acceleration of atoms without their noticeable ionization were found in the analyzed region. [ABSTRACT FROM AUTHOR]

The possibility to observe the relativistic tunneling effect in ionization of multiply charged ions of heavy atoms in the field of multipetawatt laser beams is discussed. An approximate analytical formula is obtained that makes it possible to estimate the change in the ionization rate of deep levels under the action of a strong quasi-static electromagnetic field under conditions close to saturation. It is shown that the Zeeman level shift and the relativistic twist of the subbarrier electron trajectory in the magnetic field of a laser wave lead to corrections to the ionization probability that are close in magnitude to each other. These corrections become significant only at laser radiation intensities of at least 2 × 1023 W/cm2, which are expected to be achieved at multipetawatt laser facilities, including the XCELS facility. The principal scheme of the experimental observation of the deviation of the ionization rate from the values predicted by the nonrelativistic theory is discussed. [ABSTRACT FROM AUTHOR]

The probability of simultaneous tunneling of two electrons from multiply charged Li-like ions in a high-intensity laser field is estimated. It is shown that for atoms with a nuclear charge Zm 1, the probability per unit time of collective tunneling of a 2s–1s electron pair can be more than an order of magnitude higher than the probability of detachment of a 1s electron. This provides favorable conditions for searching for the collective tunneling effect in the ionization of heavy multiply charged ions. The relative contributions of the sequential and collective ionization channels can be separated using the two-beam experimental scheme. Taking into account that to observe the effect, intensities exceeding 1021 W/cm2 in one of the beams are required, the proposed scheme of the experiment to search for the collective tunneling effect requires the use of laser pulses of extreme power, which is planned to be reached, in particular, at the XCELS facility. [ABSTRACT FROM AUTHOR]

In the strong-field ionization of atomic and molecular systems, the photoelectron is exposed to the long-range Coulomb force which is neglected in the standard theories based on the strong-field approximation (SFA). We introduce an ansatz which takes into account the Coulomb effects and at the same time is as simple as the standard SFA. Our Coulomb distorted plane wave-Volkov approximation provides analytical expressions for the relevant matrix elements. We also present a generalization of this approximation taking into account first-order term of an expansion in the atomic potential. Similarly as in the standard improved SFA, this generalized approximation describes well the rescattering plateau and the cutoff observed in the photoelectron spectra. Our new approximation is illustrated with numerical examples of strong-field ionization of the hydrogen atom exposed to linearly and circularly polarized laser pulses. The spectra obtained are slightly flatten in comparison with the SFA spectra and this effect is stronger for shorter laser wavelengths. [ABSTRACT FROM AUTHOR]

Radiation produced in high-order harmonic generation (HHG) process can be utilized to generate attosecond pulses. According to the semi-classical theory of HHG, the maximum emitted photon energy is I P + 3. 1 7 U P . The ionization potential ( I P) and ponderomotive energy ( U P) , play important roles in the generated photon energy; therefore, choosing the material and field for HHG is important. Here, investigating the probability of ionization of material in laser and plasmonic field for HHG is important. The ionization probability of noble gases atoms and ions was investigated. Using Ammosov–Delone–Krainov (ADK) model formulas, the parameters for ions and atoms were determined. The results show that helium is the most suitable noble gas atom for HHG in both fields. After ionizing the atoms, the ionization potential increases significantly, and the ionization probability also decreases. Then, ions are better than their corresponding atoms for HHG. Among noble gas atoms and ions, the helium ion is the most suitable for HHG. In addition, the plasmonic field increases the ponderomotive energy and decreases the probability of ionization for all atoms, as shown by the simulation. Then, extreme ultraviolet (XUV) high-energy photon can be efficiently generated using noble gas ions. [ABSTRACT FROM AUTHOR]

The nondirect product discrete variable representation (npDVR) is developed for the time-dependent Schrödinger equation with nonseparable angular variables and is applied to a hydrogen atom in elliptically polarized strong laser fields. The 2D npDVR is constructed on spherical harmonics orthogonalized on the 2D angular grids of the Popov and Lebedev 2D cubatures for the unit sphere. With this approach we have investigated the dynamics of a hydrogen atom initially in its ground state in elliptically polarized laser fields with the intensity up to I = 10 14 W cm−2 and a wavelength of λ = 800 nm. For these parameters of the laser field and the entire range of ellipticity variation, we have calculated the total excitation and ionization yields of the atom. The performed analysis of the method convergence shows that the achieved accuracy of our calculations significantly exceeds the accuracy of recent works of other authors relevant to the problem, due to the high efficiency of the 2D npDVR in approximating the angular part of the 3D time-dependent Schrödinger equation. We also propose a new simple procedure for infinite summation of the transition probabilities to the bound states of the hydrogen atom when calculating the total excitation yield and prove its accuracy by comparison with conventional methods. The obtained results show the potential prospects of the 2D npDVR for investigating atomic dynamics in stronger laser fields. [ABSTRACT FROM AUTHOR]

Nondipole terms in the atom–laser interaction arising due to the presence of a magnetic component in an electromagnetic wave and its inhomogeneity lead to the nonseparability of the center-of-mass (CM) and electron variables in the neutral atom and, as a consequence, to its acceleration. We investigate this effect and the accompanying excitation and ionization processes for the hydrogen atom in strong (1012 – 2×1014 W/cm2) linearly polarized short-wavelength (5 eV ≲ℏω≲ 27 eV) electromagnetic pulses of about 8 fs duration. The study was carried out within the framework of a hybrid quantum-quasiclassical approach in which the coupled time-dependent Schrödinger equation for an electron and the classical Hamilton equations for the CM of an atom were simultaneously integrated. Optimal conditions with respect to the frequency and intensity of the electromagnetic wave for the acceleration of atoms without their noticeable ionization were found in the analyzed region.

Strong-field induced valence electron excitation is a common process in strong field interaction with atoms and molecules. In the case of polyatomic molecules, the effects of ionization from low-lying molecular orbitals and nuclear dynamics during the interaction can play critical roles for electron excitation. In this work, we investigate the involved molecular orbitals in the electron excitation of singly ionized acetylene in a strong laser field using alignment dependence and laser intensity dependence. Additionally, the involved nuclear dynamics during the electron excitation are identified from the difference in the kinetic energy release and the angular distribution of laser-induced dissociation with different pulse durations and intensities. The laser intensity dependence clearly shows the relative strength change of two excitation pathways in the measured momentum and angle-resolved distributions.

We report the formulation of a new, cost-effective approximation method in the time-dependent optimized coupled-cluster (TD-OCC) framework [T. Sato et al., J. Chem. Phys. 148, 051101 (2018)] for first-principles simulations of multielectron dynamics in an intense laser field. The method, designated as TD-OCCD(T), is a time-dependent, orbital-optimized extension of the “gold-standard” CCSD(T) method in the ground-state electronic structure theory. The equations of motion for the orbital functions and the coupled-cluster amplitudes are derived based on the real-valued time-dependent variational principle using the fourth-order Lagrangian. The TD-OCCD(T) is size extensive and gauge invariant, and scales as O(N7) with respect to the number of active orbitals N. The pilot application of the TD-OCCD(T) method to the strong-field ionization and high-order harmonic generation from a Kr atom is reported in comparison with the results of the previously developed methods, such as the time-dependent complete-active-space self-consistent field (TD-CASSCF), TD-OCC with double and triple excitations (TD-OCCDT), TD-OCC with double excitations (TD-OCCD), and the time-dependent Hartree-Fock (TDHF) methods.

Photoionization time delays have been investigated in many streaking experiments in which an extreme ultraviolet (XUV) attosecond pulse is used to ionize the target in the presence of a dressing infrared laser field. The discrepancies between the photoionization time delays thus experiment measured and those from many sophisticated theoretical simulations have generated a great deal of controversy in recent years. The difficulty of achieving an accuracy of the retrieved time delays comes from two facts: a so-called wavepacket approximation is introduced to construct the photoionization electron wavepacket, this approximation is invalid if atto-chirp of XUV is non-zero; the other one is that the lower sensitivity of the streaking spectra to the phase of the photoionization transition dipole. Here we present a time delay retrieval method born from our recently proposed ‘phase retrieval of broadband pulses auto correlation’ (PROBP-AC) technology to overcome above limitations. We carefully exam the validity of our method and make a few compare with some other common used retrieval codes, and the simulations demonstrate that more accurate results can be retrieved using PROBP AC. Based on the present method, the angular dependent photoionization time delays can also be retrieved. Our investigation casts doubts on the measured group time delays in previous streaking experiments. We also mention here that a single photoionization group time delay at the XUV peak energy is not enough to represent a complete photoemission process; instead, a fully characterization of the photoionization group time delay over the whole bandwidth of the wave packet is required.

We report computations for the quantum dynamics and spectra of the iodine atom in the ground state () subjected to a strong laser field at wavenumbers in the range of CO laser emission. The computations include the hyperfine structure of the magnetic dipole transitions between the fine structure levels (and) of the iodine atom near cm−1 important for the iodine atom laser and for the kinetic spectroscopy of iodine atoms generated in infrared multiphoton excitation and dissociation of organic iodides, where simultaneous high time and frequency resolution including hyperfine structure can be achieved, that is only limited by the Heisenberg uncertainty principle. Accurate numerical results from calculations with the URIMIR package are compared with estimates from perturbation theory and they agree well. Possibilities for experimental tests of the theoretical predictions are outlined and the URIMIR package is included in an appendix of supplementary material. [ABSTRACT FROM AUTHOR]

What are the final size and velocity limits of electronic data processing and magnetic information storage areas, and how can we approach these boundaries questions and the understanding and control of microscopic electron motion along with many scientific studies have led to the birth and progress of Attosecond Science. The progress of ultrafast laser technology in recent years, the interaction process between the matter and the intense field play a key role in understanding how energy and charge are carried in atoms as well as in more complex solid and molecular systems. This review study will focus on the basic concepts and experimental tools that allow to observe and control the atomic scale motion of electrons in real time, understand the theoretical models that are critical for combining experimental observability with microscopic variables, and the expected technological effects. For this purpose, the role of nano and attosecond technologies in atomic and molecular physics, and condensed matter physics will be investigated and local literature sources will be presented about several important and recent current application areas of attosecond pulses. [ABSTRACT FROM AUTHOR]

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By numerically solving the two-dimensional time-dependent Schrödinger equation, we investigate the strong-field photoelectron holography (SFPH) for different atomic and molecular targets. The results show that the photoelectron hologram is capable of providing structural information-the phase of scattering amplitude. In our calculations, the holographic pattern shifts with the target, indicating the phase difference between various targets. However, for the near-forward scattering, the phases of scattering amplitude for different targets are similar and thus no structural information has been retrieved in the previous works on near-forward scattering hologram. We suggest to use the backward scattering photoelectron hologram to extract this type of structural information. This paves a significant step towards time-resolved imaging of molecular structure and dynamics with the concept of SFPH. [ABSTRACT FROM AUTHOR]

Ionization of molecules in strong femtosecond laser fields is a key process for understanding a variety of strong-field molecular physical phenomena. In this study, we performed an experimental study on strong-field ionization and dissociation of three linear triatomic molecules, CO 2 , CS 2 , and OCS, using time-of-flight mass spectrometer. We measured the yield of different parent ions and various fragment ions as a function of laser intensity in the range of 2.0 × 10 13 W/cm 2 to 8.0 × 10 14 W/cm 2 , in both 800-nm and 400-nm strong laser fields. The mechanism of strong-field single and double ionization, as well as fragmentation, was discussed based on the experimental results. In addition, the angular distributions of fragment ions were obtained. The results show the anisotropic distributions in the formation of highly charged atomic fragment ions, which may be attributed to the alignment of the molecules in strong laser fields before dissociation. [ABSTRACT FROM AUTHOR]

Through the numerical solution of the 3D time-dependent Schrödinger equation, the two-dimensional photoelectron energy spectra (PES) of argon have been calculated, showing good accordance with the experimental measurements. Further analysis on the TDSE calculated data and comparison of PES under linearly and circularly polarized laser pulses demonstrated that the forward rescattering of the recolliding electron is mainly responsible for the appearance of the low energy structure (LES) in the PES. It is further revealed that the LES is correlated with the high angular momenta ( l = 8 – 13 ) of the photoelectron. [ABSTRACT FROM AUTHOR]

Highlights: [•] We present a 3D quantum approach for studying the atom–laser interaction. [•] Sine-DVR and split-operator propagator are used for economizing computational time. [•] Our calculated electron spectrum has good agreement with experimental result. [•] The ATI spectra have studied in linearly and circularly polarized lights. [Copyright &y& Elsevier]

Based on spin-unrestricted hartree fock theory, we present the spin unrestricted multi-configuration time dependent hartree fock theory (UMCTDHF) to describe the electron correlation dynamics of systems interacting with laser field. The positive spin orbitals and the negative spin orbitals are propagated in their own subspace respectively. The spin orbital in the spin-down subspace acts with that in the spin-up subspace by the reduced density matrix and mean field operator. The ground energy is acquired by propagating the trial wave function in the imaginary time by using spin-restricted MCTDHF (RMCTDHF) and UMCTDHF respectively. Then the ionization probabilities and the electrons energies are calculated by using RMCTDHF and UMCTDHF when the laser field is present. The ionization probability calculated with UMCTDHF agrees with the previous theoretical reports very well. The UMCTDHF method is accurate and applicable for open shell system beyond the capability of the RMCTDHF method. [ABSTRACT FROM AUTHOR]

The probabilities of predissociation and vibronic transitions between the states of the oxygen molecule in the Schumann-Runge band in the presence of a strong laser field are examined. The interaction of the molecule with the laser field is described using the rotating wave approximation. The predissociation probabilities for the avoided crossing of two adiabatic molecular terms are calculated within the framework of the Landau-Zener model. The energies of the vibrational states in the laser field are determined by diagonalization of the adiabatic Hamiltonian in the harmonic oscillator basis set. The predissociation thresholds are determined and the Franck-Condon factors are calculated as functions of the frequency and intensity of the external electromagnetic field. [ABSTRACT FROM AUTHOR]

The nonlinear scattering of a laser pulse off spherical nanoclusters with free electrons and with a diffuse surface is examined in the collisionless hydrodynamics approximation in the framework of perturbation theory with respect to the laser pulse intensity, as well as of the steady-state approximation. In a previous publication [S.V. Fomichev and W. Becker, Phys. Rev. A 81, 063201 (2010)] we reported the full nonlinear hydrodynamic model of forced collective electron motion confined to a cluster with diffuse surface and introduced two different perturbation theories corresponding to different laser intensity regimes. In the current paper, in the framework of this hydrodynamic model we focus on the properties of plasmon resonance-enhanced third-harmonic generation in a spherical cluster and its dependence on the shape of its diffuse surface whose role increases for nonlinear processes. At the same time, the quadrupole second-harmonic generation in a spherical cluster is also inspected as a necessary intermediate step. Both cold metal clusters in vacuum or in a dielectric surrounding and hot laser-heated and laser-ionized clusters are considered within the same approach for a wide range of the fundamental laser frequency. Nonlinear laser excitation of the dipole plasmon Mie resonance in spherical clusters, as well as of other respective multipole plasmon resonances is investigated analytically and numerically in detail (position, width, and strength) versus the cluster-surface diffuseness, the outer ionization degree in charged clusters, the electron-density diffuseness, and their interplay. Under certain conditions, depending on the various cluster parameters, different secondary nonlinear resonances are found. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

We propose a physical mechanism based on breakdown of the Born-Oppenheimer approximation to rationalize the ultrafast hydrogen migration in strong laser driven isomerization reactions. A three nuclei (proton, donor, and acceptor) model is employed to develop a three step solution scheme. The proton-donor Coulomb repulsion is shown to be responsible for the high proton mobility. We identify a proton tunneling process and use the Keldysh-Faisal-Reiss theory to calculate the tunneling probability. The effect of laser parameters (intensity, frequency, polarization, and pulse duration) has been studied and found to be consistent with recent experiments. [ABSTRACT FROM AUTHOR]

Abstract: We have demonstrated the use of the multi-configuration time-dependent Hartree–Fock (MCTDHF) method to compute total ionization probabilities of a 3-dimensional (3D) model helium atom interacting with a strong laser field. The time dependence and the intensity dependence of the total ionization probabilities of helium are investigated by using the MCTDHF method. The MCTDHF results are consistent with the previous theoretical expectations by directly solving the time-dependent Schrödinger equation. [Copyright &y& Elsevier]

We report the formulation of a new, cost-effective approximation method in the time-dependent optimized coupled-cluster (TD-OCC) framework [T. Sato et al. , J. Chem. Phys. 148, 051101 (2018)] for first-principles simulations of multielectron dynamics in an intense laser field. The method, designated as TD-OCCD(T), is a time-dependent, orbital-optimized extension of the "gold-standard" CCSD(T) method in the ground-state electronic structure theory. The equations of motion for the orbital functions and the coupled-cluster amplitudes are derived based on the real-valued time-dependent variational principle using the fourth-order Lagrangian. The TD-OCCD(T) is size extensive and gauge invariant, and scales as O ( N 7 ) with respect to the number of active orbitals N . The pilot application of the TD-OCCD(T) method to the strong-field ionization and high-order harmonic generation from a Kr atom is reported in comparison with the results of the previously developed methods, such as the time-dependent complete-active-space self-consistent field (TD-CASSCF), TD-OCC with double and triple excitations (TD-OCCDT), TD-OCC with double excitations (TD-OCCD), and the time-dependent Hartree-Fock (TDHF) methods., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Pathak, Sato and Ishikawa.)

A theory describing above-threshold ionization of atoms and ions in a strong electromagnetic field is presented. It is based on the widely known strong field approximation and incorporates the Coulomb interaction between the photoelectron and the nucleus using the method of complex classical trajectories. A central result of the theory is the Coulomb-corrected ionization amplitude whose evaluation requires little extra numerical effort. By comparing our predictions with the results of ab initio numerical solutions for two examples we show that the new theory provides a significant improvement of the Coulomb-free strong field approximation. For the case of above-threshold ionization in elliptically polarized fields a comparison with available experimental data is also presented. [ABSTRACT FROM AUTHOR]

We investigate the laser-wavelength dependence of high-order above-threshold ionization and high-order harmonic generation rates. For both processes we have found pronounced enhancements in the spectra for those wavelengths for which the channel-closing condition is satisfied. These enhancements are explained in two complementary ways. One is in terms of the constructive interference of a large number of long quantum orbits, while the second interpretation is based on threshold anomalies. There are two types of these enhancements, which leave very different footprints in the spectra. When the ground-state parity of the atom changes, the two types are interchanged. [ABSTRACT FROM AUTHOR]

Abstract: The elimination of H2 + from alkyl iodides under strong (up to 5×1015 Wcm−2) laser irradiation is studied by means of time-of-flight mass spectrometry. The study has been performed by using 60fs (λ =800nm) and 35ps (λ =1064, 532, 355 and 266nm) laser pulses. It is concluded that the H2 + ions are ejected from ionic states via Coulomb explosion processes. The molecular rearrangement leading to H2 + formation is attributed to a tunneling process through a H transfer barrier. For the case of methyl iodide, about 10% of the doubly charged parent ions undergo molecular rearrangement. From a comparison of the H2 +/H+ ion yield ratio of the studied molecules, it turns out that the H2 + formation from H atoms bonded to a terminal carbon atom is more efficient than that arising from H atoms bonded to central C atoms of the molecular chain. [Copyright &y& Elsevier]

Abstract: We analyze the laser ellipticity dependence of n-photon differential cross sections for electron–atom scattering in an intense elliptically polarized laser field. We show that there exist two plateau-like structures in the dependence of on n for any ellipticity, including for the case of circular polarization. We present numerical predictions for e–H scattering in a CO2-laser field and an analytical description of the plateau features in terms of the rescattering scenario. [Copyright &y& Elsevier]

We report on strong-field ionization of dense water gas in a short infrared laser pulse. By employing a unique combination of photoelectron spectroscopy with a liquid micro-jet technique, we observe how the character of electron emission at high kinetic energies changes with the increase of the medium density. This change is associated with the process of laser-assisted electron scattering (LAES) on neighboring particles, which becomes a dominant mechanism of hot electron emission at higher medium densities. The manifestation of this mechanism is found to require densities that are orders of magnitude lower than those considered for heating the laser-generated plasmas via the LAES process. The experimental results are supported by simulations of the LAES yield with the use of the Kroll–Watson theory.