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It is shown that above-threshold ionization peaks disappear when the kinetic energy associated with the nondipole radiation-pressure-induced photoelectron momentum in the laser propagation direction becomes comparable to the photon energy, and how peaks can be made reappear if knowledge of the length and direction of the photoelectron momentum is at hand and an emission-direction-dependent momentum shift is accounted for. The reported findings should be observable with intense mid-infrared laser pulses., 5 pages, 2 figures

Using the one-dimensional Hubbard model, which is commonly used for describing, e.g., high-$T_c$ superconducting cuprates, we study high-harmonic generation (HHG) from doped, correlated materials. Doping is modeled by changing the number of electrons in the lattice from the conventional half-filling case. For relatively small Hubbard $U$, i.e., small electron-electron correlation, we find little to no effect of doping on the dynamics and the HHG spectra. For increasing $U$ the degree of doping has a marked effect on the dynamics and spectra. We explain these findings through the quasiparticle-based doublon-holon picture. The dynamics are separated into two types, firstly doublon and holon movement, and, secondly, doublon-holon pair creation and annihilation. Doping results in all configurations containing doublons or holons. Those quasiparticles can move at no extra cost in energy regardless of the correlation level. This motion at no energy cost increases the high-harmonic gain for low and medium harmonic orders. We discuss that in the high-$U$ limit, antiferromagnetic ordering becomes increasingly unlikely with increasing doping rates and explain an associated drop in the high-order harmonics relative to the case of half filling., 11 pages incl. bibliography and 7 figures

The interaction between atoms and molecules and intense laser pulses is typically described in either the velocity gauge, length gauge or in the Kramers-Henneberger frame. Here, certain relations between these forms are discussed. In particular expansions of the solution to the time-dependent Schrodinger equation in adiabatic states are considered. Such expansions in adiabatic states could be expected to be attractive for intense long-wavelength infrared laser pulses, where the external field changes on a slow timescale compared with the electron motion. It is shown that an expansion in velocity gauge adiabatic states gives the equations of motions that follow from an expansion in field-free states and by expressing the interaction operator in the length gauge. Likewise, it is shown that an expansion in the Kramers-Henneberger frame adiabatic states gives the equations of motion following an expansion in field-free states and using the velocity gauge interaction operator. Finally, an alternative form for the laser-matter interaction operator is considered, which is similar to the velocity gauge, but shifts the canonical momentum operator by the impulse associated with the Kramers-Henneberger interaction operator, rather than that of the electric field as is the case in the velocity gauge.

The limit of decreasing laser frequency can not be considered independently from nondipole effects due to increase in the laser-induced continuum electron speed in this limit. Therefore, in this work, tunneling ionization in the adiabatic limit is considered for an effective field that includes effects beyond the electric-dipole term to first order in $1/c$, with $c$ the speed of light. The nondipole term describes the interaction resulting from the electric dipole-induced velocity of the electron and the magnetic field component of the laser. The impact of this term on the ionization rate, tunnel exit point, momentum at the tunnel exit and electron dynamics is discussed. In the appropriate limit, the results of a nondipole strong-field approximation approach and those of the strict adiabatic limit, where time and field strength are parameters, are consistent. The nondipole strong-field approximation approach is used to identify nonadiabatic modifications of the initial conditions. The results open up an avenue to include nondipole effects in the initial tunneling ionization step in semiclassical models of strong-field and attosecond physics., Comment: 9 pages, 3 figures

We revisit strong-field ionization of aligned O2, CO2, and CS2 molecules in light of recent advances in the field of strong-field physics, in particular the inclusion of multielectron polarization in the numerical solution of the time-dependent Schrödinger equation (TDSE) within the singe-active-electron approximation. Multielectron polarization is modeled by the introduction of a long-range induced dipole term based on the polarizability of the cation, and a field at short distances that counteracts the applied external field and leads to a vanishing time-dependent interaction within a certain cutoff radius. For the probed molecules, the main effect of including multielectron polarization is the reduction of the total ionization yields (TIYs), and for molecules with large polarizability of their cation (CO2 and CS2), the alignment angle of maximum TIY will shift. The photoelectron momentum distributions and above-threshold ionization spectra show little imprint of the multielectron polarization associated with the long-range part of the laser-induced dipole potential. For CO2 and CS2, the inclusion of multielectron polarization and the associated induced dipole potential in the TDSE model gives alignment-resolved distributions of total ionization yields which are in better agreement with the available experimental results.

Applying the semiclassical model, we identify two connected effects in intraband high-order harmonic generation (HHG): (1) propagation time from the point of emission at the source to the point of detection, and (2) beyond-electric-dipole corrections to the light-matter interaction. These effects inherit information regarding the dispersion and cause specific features in the spectra including even-ordered harmonics in systems with space- and time-inversion symmetry. They can for certain experimental geometries be measured free of the dipole background.

We present theoretical results for intensity-dependent above-threshold ionization (ATI) spectra from oriented OCS molecules probed by intense femtosecond laser pulses with wavelengths of 800 and 400 nm. The calculations were performed using the time-dependent Schroodinger equation within the single-active-electron approximation and including multielectron polarization effects. The results are in qualitative agreement with experimental data [Yu et al., J. Phys. B: At. Mol. Opt. Phys. 50, 235602 (2017)]. In particular, characteristic features in the ATI spectra which correspond to resonant multiphoton ionization via highly-excited Rydberg states are captured by the theory., 3 figures

Using the Hubbard model we study how the process of high-order harmonic generation (HHG) is modified by beyond mean-field electron-electron correlation for both finite and bulk systems. A finite-size enhancement of the HHG signal is found and attributed to electrons backscattering off the lattice edges. Additionally, with increasing strength of the electron-electron correlation an enhancement of the high-frequency regime of the HHG spectrum is found. This is attributed to the on-site Coulomb repulsion between electrons giving rise to a localized quiver motion of the electrons. The finite-lattice enhancement dominates the HHG spectra from a few harmonic orders until a threshold from which the correlational enhancement dominates. This threshold is determined by the degree of correlation and decreases into the low-frequency regime for increasing electron-electron correlation. This infers that as the Mott insulator limit of high electron-electron correlation is approached, the finite-size effect on the electron dynamics becomes negligible., 9 pages, 6 figures

To extend application of ω-2ω intense field reaction control to highly symmetric molecules, CF4 was studied by three-dimensional momentum imaging. The C-F bond directed along larger side of the laser fields tends to be broken.

We explore the significance of laser-induced correlation effects for high-order harmonic generation (HHG) in a linear chain model of a generic band gap material for both single-IR-pulse and VUV-pump-IR-probe HHG schemes. We examine various pump-pulse excitations, and categorize signatures of laser-induced correlation effects into two classes related to edge-state and bulklike effects. The relative importance of the edge-state ef- fects decreases with increasing system size, while the bulklike effects remain. We identify regimes where these effects may alter the harmonic yield by an order of magnitude, but also regions where they may be neglected. We characterize the underlying laser-induced dynamics of the correlational mechanisms, using both an electronic population description in a static field-free basis and a comparison to a band-structure picture with laser-induced time-dependent energy shifts. From this we provide a guideline on when to account for laser-induced electron correlations and give an estimate of the timescale in which the laser-induced electron-electron interactions can cause correlational changes in the harmonic spectra. We find this timescale to be a few tens of femtoseconds.

We consider atomic strong-field ionization in the long-pulse limit for linearly polarized infrared laser fields. We show how nondipole effects in the plane formed by the propagation and polarization directions lead to (i) a shift of the origin of the rings describing the energetically allowed final electron momenta for individual above-threshold ionization channels and (ii) a redistribution in the continuum population on each ring., 6 pages, 4 figures

The strong-field approximation for high-harmonic generation in near-infrared and infrared laser pulses is formulated in the accelerated Kramers-Henneberger frame. The accompanying physical picture is discussed and the nature of the leading-order term is contrasted with that of the three-step model following the strong-field-approximation formulation in the length or velocity gauges. The theory is illustrated by high-harmonic generation spectra for atomic hydrogen., 10 pages, 5 figures

For a finite-sized generic nanostructured band gap material, it is shown that significant laser-induced correlation effects can be introduced by a two-color pump-probe scheme where edge states are resonantly pre-excited by the pump before the probe arrives and drives the generation of high-order harmonics. Compared with the response of a bulk sample, we demonstrate an increase in the efficiency of the generated high-harmonic signal over a wide range of frequencies by harnessing the power of these ultrafast many-electron dynamics.

Clusters and nanodroplets hold the promise of enhancing high-order nonlinear optical effects due to their high local density. However, only moderate enhancement has been demonstrated to date. Here, we report the observation of energetic electrons generated by above-threshold ionization (ATI) of helium (He) nanodroplets which are resonantly excited by ultrashort extreme ultraviolet (XUV) free-electron laser pulses and subsequently ionized by near-infrared (NIR) or near-ultraviolet (UV) pulses. The electron emission due to high-order ATI is enhanced by several orders of magnitude compared with He atoms. The crucial dependence of the ATI intensities with the number of excitations in the droplets suggests a local collective enhancement effect.

We describe numerically the ionization process induced by linearly and circularly polarized XUV attosecond laser pulses on an aligned atomic target, specifically, the excited state Ne$^*(1s^22s^22p^5[{}^2\text{P}^\text{o}_{1/2}]3s[^1\text{P}^o])$. We compute the excited atomic state by applying the time-dependent restricted-active-space self-consistent field (TD-RASSCF) method to fully account for the electronic correlation. We find that correlation-assisted ionization channels can dominate over channels accessible without correlation. We also observe that the rotation of the photoelectron momentum distribution by circularly polarized laser pulses compared to the case of linear polarization can be explained in terms of differences in accessible ionization channels. This study shows that it is essential to include electron correlation effects to obtain an accurate description of the photoelectron emission dynamics from aligned excited states., Comment: 10 figures, 11 pages (minor changes with respect to previous version)

We present theoretical calculations of orientation-dependent total ionization yields from the highest occupied molecular orbitals of the oriented OCS molecule by solving the time-dependent Schr\"odinger equation in three dimensions. The calculations were performed within the single-active-electron approximation including multielectron polarization. The multielectron polarization term was represented by an induced dipole term which contains the polarizability of the ${\mathrm{OCS}}^{+}$ cation parallel to the laser polarization. Upon accounting for multielectron polarization, the calculated total ionization yields and their orientation dependence are in good agreement with experimental data.

Carbon disulfide dimers ${({\mathrm{CS}}_{2})}_{2}$ are doubly ionized by an intense 40-fs laser pulse leading to fragmentation into a pair of ${{\mathrm{CS}}_{2}}^{+}$ cations. Unlike previous studies on atomic dimers, their kinetic energy, gained by mutual electrostatic repulsion, deviates strongly from the value determined by applying the Coulomb interaction approximation to the ${{({\mathrm{CS}}_{2})}_{2}}^{2+}$ ion. An ab initio calculation shows that inclusion of non-Coulombic interactions in ${{({\mathrm{CS}}_{2})}_{2}}^{2+}$ is crucial to account for the observed kinetic energy. These interactions result in 16 ${{({\mathrm{CS}}_{2})}_{2}}^{2+}$ electronic microstates, which obstructs accurate determination of intermolecular bond distances in ${({\mathrm{CS}}_{2})}_{2}$ from the fragments' kinetic energy. The situation will be similar for most other weakly bonded atomic and molecular complexes.

We study the crystal-momentum-resolved contributions to the high-order harmonic generation (HHG) in band-gap materials, and identify the relevant initial crystal momenta for the first and higher plateaus of the HHG spectra. We do so by using a time-dependent density-functional theory model of one-dimensional linear chains. We introduce a self-consistent periodic treatment for the infinitely extended limit of the linear chain model, which provides a convenient way to simulate and discuss the HHG from a perfect crystal beyond the single-active-electron approximation. The multi-plateau spectral feature is elucidated by a semiclassical k-space trajectory analysis with multiple conduction bands taken into account. In the considered laser-interaction regime, the multiple plateaus beyond the first cutoff are found to stem mainly from electrons with initial crystal momenta away from the Gamma point (k = 0), while electrons with initial crystal momenta located around the Gamma point are responsible for the harmonics in the first plateau. We also show that similar findings can be obtained from calculations using a sufficiently large finite model, which proves to mimic the corresponding infinite periodic limit in terms of the band structures and the HHG spectra., 13 pages, 7 figures, REVTeX 4.1

We analyze the Hamiltonian of a charged particle in a strong external infrared laser pulse including nondipole effects to first order in 1/c. We consider the corresponding classical equations of motions in the high-intensity, long-wavelength limit and identify for linearly polarized fields an accurate nondipole strong-field-approximation Hamiltonian. This Hamiltonian is expressed in a mixed gauge with separate light-matter interaction terms depending either on coordinate or momentum operators. It can be transformed into a nondipole velocity gauge and nondipole length gauge version. The associated beyond dipole Volkov states are given. Implications for laser-assisted scattering, strong-field ionization, laser-assisted photoeletric effect, and attosecond streaking are outlined.

Nuclear and electronic dynamics in a wavepacket comprising bound Rydberg and valence electronic states of nitrogen from 12 to 15 eV are investigated using attosecond transient absorption. Vibrational quantum beats with a fundamental period of 50 femtoseconds persist for a picosecond in the b′ 1Σ+u valence state. Multi-state calculations show that these coherences result primarily from near infrared-induced coupling between the inner and outer regions of the b′ 1Σ+u state potential and the dark a″ 1Σ+g state. The excellent spectral and temporal resolution of this technique allows measurement of the anharmonicity of the b′ 1Σ+u potential directly from the observed quantum beats.

We discuss the mechanism by which the inclusion of multielectron polarization improves the solution of the time-dependent Schr\"{o}dinger equation (TDSE) for the oriented CO molecule in a strong external laser pulse within the single-active-electron (SAE) approximation. A challenging problem of using the SAE approximation is that the active electron, instead of undergoing ionization, may be driven by the external field to lower-lying orbitals. For the oriented CO molecule, dipole coupling to the lower-lying bound states of the potential depends on the orientation angle, thereby affecting the orientation-dependent ionization dynamics. By including multielectron polarization, the external field is turned off within the molecular radius, thereby minimizing dipole coupling of the highest occupied molecular orbital to the lower-lying states of the potential. We discuss how turning off the external field within an appropriate molecular radius without accounting for the induced dipole term in the effective potential beyond this radial cut-off distance, constitutes an effective and accurate approach to describe strong-field ionization of CO and to minimize dipole coupling to lower-lying bound states., Comment: 6 figures

In this Colloquium, the wavefunction-based Multiconfigurational Time-Dependent Hartree approaches to the dynamics of indistinguishable particles (MCTDH-F for Fermions and MCTDH-B for Bosons) are reviewed. MCTDH-B and MCTDH-F or, together, MCTDH-X are methods for describing correlated quantum systems of identical particles by solving the time-dependent Schrödinger equation from first principles. MCTDH-X is used to accurately model the dynamics of real-world quantum many-body systems in atomic, molecular, and optical physics. The key feature of these approaches is the time-dependence and optimization of the single-particle states employed for the construction of a many-body basis set, which yields nonlinear working equations. We briefly describe the historical developments that have lead to the formulation of the MCTDH-X methods and motivate the necessity for wavefunction-based approaches. We sketch the derivation of the unified MCTDH-F and MCTDH-B equations of motion for complete and also specific restricted configuration spaces. The strengths and limitations of the MCTDH-X approach are assessed via benchmarks against an exactly solvable model and via convergence checks. We highlight some applications to instructive and experimentally-realized quantum many-body systems: the dynamics of atoms in Bose-Einstein condensates in magneto-optical and optical traps and of electrons in atoms and molecules. We discuss the current development and frontiers in the field of MCTDH-X: theories and numerical methods for indistinguishable particles, for mixtures of multiple species of indistinguishable particles, the inclusion of nuclear motion for the nonadia-batic dynamics of atomic and molecular systems, the so-called multilayer generalizations to the MCTDH-F and MCTDH-B methods, and the time-dependent orbital-adaptive coupled cluster theory are discussed.

We study strong-field ionization of H and selected rare-gas atoms by solving the three-dimensional time-dependent Schr\"odinger equation in the single-active-electron approximation with particular attention to the effect of pulse length on the total ionization yields (TIYs) produced by counter-rotating bicircular (CRBC) lasers compared to co-rotating bicircular (COBC) lasers. To this end, TIYs, photoelectron momentum distributions, and above-threshold ionization spectra are presented for various pulse durations and peak intensities. For pulses containing five or more cycles of the fundamental near-infrared laser field, the CRBC/COBC ratio of TIYs is affected due to ionization via excited states and multiphoton ionization channels. For pulses containing less than five cycles, the CRBC/COBC ratio of TIYs depends on both the frequency combination and number of optical cycles: With the $1\ensuremath{\omega}\text{\ensuremath{-}}2\ensuremath{\omega}$ frequency combination, the ratio of TIYs is enhanced (reduced) for pulses with an odd (even) number of optical cycles. The $1\ensuremath{\omega}\text{\ensuremath{-}}3\ensuremath{\omega}$ frequency combination was considered for pulses containing two or three optical cycles, and for both pulse lengths the CRBC/COBC ratio of TIYs is less than unity.

Dichroism in double photoionization of H_{2} molecules by elliptically polarized extreme ultraviolet pulses is formulated analytically as a sum of atomiclike dichroism (AD) and molecular symmetry-mixed dichroism (MSMD) terms. The MSMD originates from an interplay of ^{1}Σ_{u}^{+} and ^{1}Π_{u}^{+} continuum molecular ionization amplitudes. For detection geometries in which the AD vanishes, numerical results for the sixfold differential probabilities for opposite pulse helicities show that the MSMD is significant in the electron momentum and angular distributions and is controllable by the ellipticity.

High-order harmonic generation (HHG) in imperfect crystals, where the disorder is modeled by random shifts of the ionic positions, is studied using time-dependent density-functional theory. When irradiated by midinfrared laser pulses, the disorder-free system produces HHG spectra with two plateaus. Compared with the disorder-free system, disordered systems are found to emit suppressed harmonics in the first plateau region and enhanced harmonics in the second plateau region. The suppression of harmonics in the first plateau becomes less pronounced when decreasing the displacement of the nuclei, while the enhancement in the second plateau region is insensitive to the range of the ionic displacement. We have confirmed these findings for many different disordered sample systems and for different laser field strengths. The increase of the HHG signals in the second plateau region is proposed to stem from a change of the dynamics in the system, evidenced by the transition matrix elements between the field-free Kohn-Sham orbitals. In addition, a time-frequency profile of HHG spectra shows that the emission of harmonics is less regular in the time domain for a disordered system than for the disorder-free system.

We consider the autocorrelation function technique for obtaining excitation spectra for indistinguishable particles. The interacting particles are described by coherent superpositions of configurations built from time-dependent spin-orbitals. The fermionic or bosonic character of the particles is taken into account by considering Slater determinants or permanents, respectively. The approach involves the calculation of overlaps between nonorthonormal Slater determinants for fermions and permanents for bosons. Efficient methods already exist for fermions. In the case of bosons, the evaluation of permanents generally scales exponentially with system size. We present an efficient approach for bosons for calculating the excitation spectrum, which circumvents this scaling. The approach is illustrated and validated by comparison with an analytical model for interacting bosons, for a system with a number of bosons so large that the autocorrelation technique could not be applied without the present development.We consider the autocorrelation function technique for obtaining excitation spectra for indistinguishable particles. The interacting particles are described by coherent superpositions of configurations built from time-dependent spin-orbitals. The fermionic or bosonic character of the particles is taken into account by considering Slater determinants or permanents, respectively. The approach involves the calculation of overlaps between nonorthonormal Slater determinants for fermions and permanents for bosons. Efficient methods already exist for fermions. In the case of bosons, the evaluation of permanents generally scales exponentially with system size. We present an efficient approach for bosons for calculating the excitation spectrum, which circumvents this scaling. The approach is illustrated and validated by comparison with an analytical model for interacting bosons, for a system with a number of bosons so large that the autocorrelation technique could not be applied without the present development.

We report the theory for time-resolved spectator resonant interparticle Coulombic decay (ICD) processes. Following excitation by a short extreme ultraviolet pulse, the spectrum of the resonant ICD electron develops. Strong-field ionization is imagined to quench the decay at different time delays and to initiate regular ICD. In this latter process, the ICD electron signal can be measured without interference effects. The typical lifetimes of ICD processes allow for the observation of oscillations of the time- and energy-differential ionization probability. We propose to utilize this oscillation to measure lifetimes of electronic decay processes.

We explore the effects of correlation on the ground-state energies and on photoionization dynamics in atomic Be and Ne. We apply the time-dependent restricted-active-space self-consistent-field method for several excitation schemes and active orbital spaces with and without a dynamic core to address the effects systematically at different levels of approximation. For the ground-state many-electron wave functions, we compare the correlation energies with entropic measures of entanglement. A larger magnitude of the correlation energy does not always correspond to a larger value of the considered entanglement measures. To evaluate the impact of correlation in a process involving continua, we consider photoionization by attosecond pulses. The photoelecton spectra may be significantly affected by including a dynamical core., 8 figures

Laser-assisted electron scattering is often described using the electric dipole approximation for the interaction between the electron and the assisting electromagnetic field. Within the dipole approximation, theory predicts a vanishing scattering cross section for processes involving exchange of photons in critical geometries, where the electron momentum transfer vector is perpendicular to the linear polarisation of the external field. In a nonrelativistic spin-free treatment, we consider modifications of the scattering cross section by nondipole contributions to the first order in 1/c in the interaction between the laser field and the electron using the nondipole strong-field-approximation Hamiltonian (Jensen et al 2020 Phys. Rev. A 101 043408). The approach allows for an analytical solution for the wave function of a free electron in the laser field with a nondipole contribution. This wave function leads to an analytical formula for the laser-assisted scattering cross section including nondipole effects. The nondipole corrections depend on the propagation direction of the field relative to the electron momentum transfer vector. The approach gives a finite cross section in scattering geometries, where the dipole approach predicts a vanishing cross section. The theory is illustrated by application to scattering geometries similar to those considered in experiments where large deviations between experimental data and predictions based on the dipole approximation have been reported. In these scattering geometries, the nondipole strong-field-approximation approach suggests an increase in the cross section by orders of magnitude compared to results obtained within the dipole approximation.

We find that a donor-doped band-gap material can enhance the overall high-order harmonic generation (HHG) efficiency by several orders of magnitude, compared with undoped and acceptor-doped materials. This significant enhancement, predicted by time-dependent density functional theory simulations, originates from the highest-occupied impurity state which has an isolated energy located within the band gap. The impurity-state HHG is rationalized by a three-step model, taking into account that the impurity-state electron tunnels into the conduction band and then moves according to its band structure until recombination. In addition to the improvement of the HHG efficiency, the donor-type doping results in a harmonic cutoff different from that in the undoped and acceptor-doped cases, explained by semiclassical analysis for the impurity-state HHG., 9 pages, 7 figures, REVTeX 4.1

We apply the Monte Carlo wave-packet approach to study the interatomic Coulombic decay (ICD) dynamics of neon dimers after removing a $2s$ electron from one of the Ne atoms by one-photon absorption from an XUV pulse. This method reproduces well both the lifetime for the $2s$ inner-valence vacancy in $\mathrm{Ne}{}_{2}{}^{+}(2{s}^{\ensuremath{-}1})$ and the kinetic energy release (KER) spectra for the coincident ${\mathrm{Ne}}^{+}$ and ${\mathrm{Ne}}^{2+}$ fragments following triple ionization, i.e., two photoionizations and one ionization via ICD, of ${\mathrm{Ne}}_{2}$ measured in an XUV-pump--XUV-probe experiment [K. Schnorr et al., Phys. Rev. Lett. 111, 093402 (2013)]. Comparisons between the calculated and measured nuclear KER spectra give physical insights in the considered process. For example, an analysis of the ratios between the low- and high-energy peaks in the nuclear KER spectra for large delays provides an estimate of the photoionization cross sections for removing a $2p$ electron from the excited states in $\mathrm{Ne}{}_{2}{}^{+}(2{s}^{\ensuremath{-}1})$. Such comparisons also allow an estimate for the ICD rates for the $2s$ inner-valence vacancy in the single-site two hole state in $\mathrm{Ne}{{}_{2}}^{2+}(2{s}^{\ensuremath{-}1}2{p}^{\ensuremath{-}1})$. Finally, the influence of photon statistics of the free electron laser pulses on the nuclear KER spectra is considered.

We investigate electron correlation effects in internuclear-distance-dependent enhanced ionization of $\mathrm{H}_2$, $\mathrm{LiH}$, and $\mathrm{HF}$ molecules by intense near-infrared laser pulses using a 3D description of the systems with the time-dependent generalized-active-space configuration-interaction method. This method systematically incorporates electron-electron correlation of the quantum many-electron system under consideration. Our correlated description of diatomic molecules shows that enhanced ionization occurs at certain critical internuclear separations and electron correlation systematically improves the ionization probability in this process until convergence is reached. We demonstrate the failure of the single-active-electron and the configuration-interaction singles approximations to produce the correct internuclear position and probability of the strong-field enhanced-ionization process. We elucidate the role of low-lying electronic excited states in the enhanced ionization process of diatomic molecules. There is clear evidence that an accurate description of low-lying electronically excited states is important to describe the non-perturbative enhanced ionization phenomenon in the ultrashort intense near infrared laser pulses., 11 pages, 11 figures

We study electron momentum vortices in single-photon double ionization of ${\mathrm{H}}_{2}$ by time-delayed, counter-rotating, elliptically polarized attosecond pulses propagating along $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\mathbf{k}}$ either parallel or perpendicular to the molecular axis $\mathbf{R}$. For $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\mathbf{k}}\ensuremath{\parallel}\mathbf{R}$, kinematical vortices occur similar to those found for He. For $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\mathbf{k}}\ensuremath{\perp}\mathbf{R}$, we find dynamical vortex structures originating from an ellipticity-dependent interplay of $^{1}\mathrm{\ensuremath{\Sigma}}_{u}^{+}$ and $^{1}\mathrm{\ensuremath{\Pi}}_{u}^{+}$ continuum amplitudes. We propose a complete experiment to determine the magnitudes and relative phase of these amplitudes by varying pulse ellipticities and time delays.

We present a general methodology for evaluating structure factors defining the orientation dependence of tunneling ionization rates of molecules, which is a key process in strong-field physics. The method is implemented at the Hartree-Fock level of electronic structure theory and is based on an integral-equation approach to the weak-field asymptotic theory of tunneling ionization, which expresses the structure factor in terms of an integral involving the ionizing orbital and a known analytical function. The evaluation of the required integrals is done by three-dimensional quadrature which allows calculations using conventional quantum chemistry software packages. This extends the applications of the weak-field asymptotic theory to polyatomic molecules of almost arbitrary size. The method is tested by comparison with previous results and illustrated by calculating structure factors for the two degenerate highest occupied molecular orbitals (HOMOs) of benzene and for the HOMO and HOMO-1 of naphthalene. Published by AIP Publishing.

The time-dependent restricted-active-space self-consistent-field (TD-RASSCF) theory is a wavefunction-based theory for the TD many-electron problem. The RAS scheme allows an inclusion of specific orbital spaces. Hence, the theory allows an identification of the spaces that are most important for a given process. The RAS reduces the configuration space, i.e., the space describing all the possible arrangements of the electrons in the different orbitals by the imposed restrictions on the excitations between orbital spaces. The use of TD SCF orbitals additionally reduces the number of configurations because the orbitals adjust in an optimal manner during time propagation. The theoretical concepts underlying the TD-RASSCF theory are discussed. A derivation of the equations of motion is presented. It is discussed how to extract observables. Some properties including gauge invariance and numerical performance are discussed. Finally some illustrative applications in high-order harmonic generation and photoionization including time-delay studies are given. The TD-RASSCF theory is a many-electron theory that accounts for correlation between the electrons, and its evaluation is, accordingly, numerically demanding. Therefore, at present, its application to strong-field many-photon processes is restricted to one spatial dimension. For calculations in three spatial dimensions, application is best suited for few-photon processes in attosecond pulses.

Bohmian mechanics is an interpretation of quantum mechanics that describes the motion of quantum particles with an ensemble of deterministic trajectories. Several attempts have been made to utilize Bohmian trajectories as a computational tool to simulate quantum systems consisting of many particles, a very demanding computational task. In this paper, we present a novel ab-initio approach to solve the many-body problem for bosonic systems by evolving a system of one-particle wavefunctions representing pilot waves that guide the Bohmian trajectories of the quantum particles. In this approach, quantum entanglement effects arise due to the interactions between different configurations of Bohmian particles evolving simultaneously. The method is used to study the breathing dynamics and ground state properties in a system of interacting bosons., Comment: 10 pages, 5 figures; to appear in Scientific Reports

We discuss the multispecies time-dependent restricted-active-space self-consistent-field theory, an \textit{ab initio} wavefunction-based theory for mixtures of ultracold atomic and molecular gases. We present the general theory, based on the time-dependent variational principle, and derive the equations of motion. The theory captures in a time-dependent setting, via the specification of the restricted-active-space scheme, different levels of approximation from the mean-field to the full configuration interaction approach. To assess its accuracy and to illustrate its ability to identify correlation effects at successive approximation levels, we apply the theory to compute the ground state energy of a Bose-Bose mixture interacting through a harmonic potential, for which the exact ground state energy is known analytically. We focus on the case of an ideal Bose gas interacting with a few impurities. The intra-species interaction between the impurities is relatively strong compared to the inter-species interaction between the impurities and the ideal noninteracting Bose gas. For this system, we find that an accurate description of the ground state necessitates the possibility of the theory to account for few-particle excitations out of the condensed phase; a situation well accounted for by the present restricted-active-space theory for mixtures; and not within reach for approaches not incorporating orbital-restriction schemes., Comment: 23 pages, 3 figures, 1 table

We extend the semiclassical two-step model [Phys. Rev. A 94, 013415 (2016)] to include a multielectron polarization-induced dipole potential. Using this model we investigate the imprints of multielectron effects in the momentum distributions of photoelectrons ionized by a linearly polarized laser pulse. We predict narrowing of the longitudinal momentum distributions due to electron focusing by the induced dipole potential. We show that the polarization of the core also modifies interference structures in the photoelectron momentum distributions. Specifically, the number of fanlike interference structures in the low-energy part of the electron momentum distribution may be altered. We analyze the mechanisms underlying this interference effect. The account of the multielectron dipole potential seems to improve the agreement between theory and experiment., 25 pages, 7 figures