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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 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.

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

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

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)

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.

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.

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 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 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 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

We present an ab initio study of strong-field ionization of H and by two-colour bicircular counter-rotating 1ω–2ω laser pulses containing two and ten optical cycles at the fundamental frequency corresponding to a wavelength of 800 nm. We address effects of laser intensity, pulse duration and in the case of of alignment on photoelectron momentum distributions (PMDs). In the long pulse limit, as expected from a simple classical picture, the PMDs of H and of aligned perpendicular to the polarization plane show a three-fold symmetry imposed by the bicircular laser field. Compared with the expectation for the positions of the peaks in this three-fold structure, we observe off-set angles that depend on the photoelectron energy as well as the laser intensity. For , the symmetry in the PMDs is broken by the anisotropic molecular potential at molecular alignment not perpendicular to the plane of laser polarization. In the short pulse limit, the PMDs are sensitive to the carrier envelope phase of the laser field.

Recent research interests have been raised in uncovering and controlling ultrafast dynamics in excited neutral molecules. In this work we generalize the Monte Carlo wave packet (MCWP) approach to XUV-pump--IR-probe schemes to simulate the process of dissociative double ionization of ${\mathrm{H}}_{2}$ where singly excited states in ${\mathrm{H}}_{2}$ are involved. The XUV pulse is chosen to resonantly excite the initial ground state of ${\mathrm{H}}_{2}$ to the lowest excited electronic state of ${}^{1}{{\mathrm{\ensuremath{\Sigma}}}_{u}}^{+}$ symmetry in ${\mathrm{H}}_{2}$ within the Franck-Condon region. The delayed intense IR pulse couples the excited states of ${}^{1}{{\mathrm{\ensuremath{\Sigma}}}_{u}}^{+}$ symmetry with the nearby excited states of ${}^{1}{{\mathrm{\ensuremath{\Sigma}}}_{g}}^{+}$ symmetry. It also induces the first ionization from ${\mathrm{H}}_{2}$ to ${{\mathrm{H}}_{2}}^{+}$ and the second ionization from ${{\mathrm{H}}_{2}}^{+}$ to ${\mathrm{H}}^{+}+{\text{H}}^{+}$. To reduce the computational costs in the MCWP approach, a sampling method is proposed to determine in time the dominant ionization events from ${\mathrm{H}}_{2}$ to ${{\mathrm{H}}_{2}}^{+}$. By conducting a trajectory analysis, which is a unique possibility within the MCWP approach, the origins of the characteristic features in the nuclear kinetic energy release spectra are identified for delays ranging from 0 to 140 fs and the nuclear dynamics in the singly excited states in ${\mathrm{H}}_{2}$ is mapped out.

We investigate high-order above-threshold ionization of model helium in the long-wavelength regime up to 2400 nm by solving the two-electron time-dependent Schr\"odinger equation in one dimension. To bypass the difficulty of solving the multielectron time-dependent Schr\"odinger equation with the long-wavelength laser interaction, we revisit and examine two typically used theoretical methods: the single-active-electron approximation and the strong-field approximation. For the description of the high-energy rescattered electrons in the ground-state ionic channel, the single-active-electron approximation performs better with increasing ponderomotive energy. Single ionization in the excited-state ionic channels, in general, has much weaker spectral intensity than that in the ground-state ionic channel. The above-threshold-ionization cutoffs in the excited-state ionic channels are clear signatures of two-electron dynamics, which cannot be explained within the single-active-electron approximation. By applying the two-electron strong-field approximation including rescattering and a saddle-point method analysis, we explain the channel-resolved cutoffs, and relate them to elastic and inelastic rescattering processes.

We apply a three-dimensional (3D) implementation of the time-dependent restricted-active-space self-consistent-field (TD-RASSCF) method to investigate effects of electron correlation in the ground state of Be as well as in its photoionization dynamics by short XUV pulses, including time-delay in photoionization. First, we obtain the ground state by propagation in imaginary time. We show that the flexibility of the TD-RASSCF on the choice of the active orbital space makes it possible to consider only relevant active space orbitals, facilitating the convergence to the ground state compared to the multiconfigurational time-dependent Hartree-Fock method, used as a benchmark to show the accuracy and efficiency of TD-RASSCF. Second, we solve the equations of motion to compute photoelectron spectra of Be after interacting with a short linearly polarized XUV laser pulse. We compare the spectra for different RAS schemes, and in this way we identify the orbital spaces that are relevant for an accurate description of the photoelectron spectra. Finally, we investigate the effects of electron correlation on the magnitude of the relative time-delay in the photoionization process into two different ionic channels. One channel, the ground state channel in the ion, is accessible without electron correlation. The other channel is only accessible when including electron correlation. The time-delay is highly sensitivity to the choice of the active space, and hence to the account of electron-electron correlation., 16 pages and 9 figures. Updated Section III.C and small corrections

We analyze theoretically the interplay between the torsional and the rotational motion of an aligned biphenyl-like molecule. To do so, we consider a transition between two electronic states with different internal torsional potentials, induced by means of a resonant laser pulse. The change in the internal torsional potential provokes the motion of the torsional wavepacket in the excited electronic state, modifying the structure of the molecule, and hence, its inertia tensor. We find that this process has a strong impact on the rotational wave function, displaying different behavior depending on the electronic states involved and their associated torsional potentials. We describe the dynamics of the system by considering the degree of alignment and the expectations values of the angular momentum operators for the overall rotation of the molecule., 10 pages, 6 figures

We consider the interaction of gapped graphene in the two-band approximation using an explicit time-dependent approach. In addition to the full high-order harmonic generation (HHG) spectrum, we also obtain the perturbative harmonic response using the time-dependent method at photon energies covering all the significant features in the responses. The transition from the perturbative to the fully non-perturbative regime of HHG at these photon energies is studied in detail., Comment: 11 pages, 9 figures

We apply the Monte Carlo Wave Packet (MCWP) approach to investigate the kinetic energy release (KER) spectra of the protons following double ionization in H$_{2}$ when interacting with laser pulses with central wavelengths ranging from the near-IR (800 nm) to the mid-IR (6400 nm) regions and with durations of 3-21 laser cycles. We uncover the physical origins of the peaks in the nuclear KER spectra and ascribe them to mechanisms such as ionization following a resonant dipole transition, charge-resonance-enhanced ionization (CREI) and ionization in the dissociative limit of large internuclear distances. For relatively large pulse durations, i.e., for 15 or more laser cycles at 3200 nm and 10 or more at 6400 nm, it is possible for the nuclear wave packet in H$_{2}^{+}$ to reach very large separations. Ionization of this part of the wave packet results in peaks in the KER spectra with very low energies. These peaks give direct information about the dissociative energy in the $2p\sigma_{u}$ potential energy curve of H$_{2}^{+}$ at the one- and three-photon resonances between the $2p\sigma_{u}$ and $1s\sigma_{g}$ curves in H$_{2}^{+}$. With the MCWP approach, we perform a trajectory analysis of the contributions to the KER peaks and identify the dominant ionization pathways. Finally, we consider a pump-probe scheme by applying two delayed pulses to track the nuclear dynamics in a time-resolved setting. Low-energy peaks appear for large delays and these are used to obtain the $2p\sigma_{u}$ dissociative energy values at the one-photon resonance between the $2p\sigma_{u}$ and $1s\sigma_{g}$ curves in H$_{2}^{+}$ for different wavelengths.

Double ionization of a model helium system by intense extreme ultraviolet laser pulses is investigated by solving the time-dependent Schr\"odinger equation and extracting the photoelectron spectra. Considering a frequency regime where the nonsequential two-photon double ionization is expected to dominate the double ionization process according to the perturbation theory, we identify the sequential and nonsequential mechanisms from the joint energy spectra of two electrons in the strong-field regime. In the nonperturbative regime, we find that sequential multiphoton double ionization shows remarkable peaks in the joint energy spectra and these peaks spread their widths with increasing laser field strength, which is caused by the ac Stark effect. Thus in the strong-field regime, the ac Stark shifts of different ionic states can be observed from the sequential multiphoton features in the joint energy spectra and the signs of the ac Stark shifts can be controlled by the laser frequency. We also find that the nonsequential mechanism dominates double ionization only at low field strengths while the sequential multiphoton mechanism contributes substantially at high field strengths, in accordance with estimates from rate equations.

Strong-field ionization of carbonyl sulphide (OCS) molecules, induced by a linearly polarized mid-infrared (mid-IR) probe laser pulse is investigated experimentally and theoretically. We focus on the dependence of the single-ionization yield on the alignment of the molecular axis with respect to the probe pulse polarization axis. In the experiment, the OCS molecules are 1-dimensionally adiabatically aligned and ionized by a 12-femtosecond pulse centered at 1850 nm. The alignment-dependent ionization yields are compared to the theory based on the two-step model for strong-field ionization. Overall the measurements are consistent with the theoretical results.

We present a complete theory of bicircular high-order-harmonic emission from N-fold rotationally symmetric molecules. Using a rotating frame of reference we predict the complete structure of the high-order-harmonic spectra for arbitrary driving frequency ratios and show how molecular symmetries can be directly identified from the harmonic signal. Our findings reveal that a characteristic fingerprint of rotational molecular symmetries can be universally observed in the ultrafast response of molecules to strong bicircular fields., 7 pages

We theoretically study dissociative ionization of H$_2^+$ exposed to strong linearly polarized few-cycle visible, near-infrared and midinfrared laser pulses. We find rich energy-sharing structures in the combined electron and nuclear kinetic energy spectra with features that are a priori at odds with simple energy conservation arguments. We explain the structures as interferences between wave packets released during different optical cycles, and during the same optical cycle, respectively. Both inter- and intracycle interference structures are clearly visible in the joint energy spectra. The shapes of the interference structures depend on the dynamics leading to the double continuum, and carry sub-femtosecond information., Comment: 6 pages, 4 figures

Nonsequential double-recombination (NSDR) high-order-harmonic generation (HHG) is studied in a molecular model system. We observe a unique molecular two-electron effect with a characteristic cutoff in the HHG spectrum at higher energies than what was previously seen for NSDR HHG in atoms. The effect is corroborated with a classical model where it is found that the effect is sensitive to the molecular potential and originates from same-period emission and recombination (SPEAR) of two electrons. The effect persists for intermediate nuclear distances of $R\ensuremath{\gtrsim}8.0$ a.u.

We present a theoretical study of the dissociative tunneling ionization process. Analytic expressions for the nuclear kinetic energy distribution of the ionization rates are derived. A particularly simple expression for the spectrum is found by using the Born-Oppenheimer (BO) approximation in conjunction with the reflection principle. These spectra are compared to exact non-BO ab initio spectra obtained through model calculations with a quantum mechanical treatment of both the electronic and nuclear degrees of freedom. In the regime where the BO approximation is applicable, imaging of the BO nuclear wave function is demonstrated to be possible through reverse use of the reflection principle, when accounting appropriately for the electronic ionization rate. A qualitative difference between the exact and BO wave functions in the asymptotic region of large electronic distances is shown. Additionally, the behavior of the wave function across the turning line is seen to be reminiscent of light refraction. For weak fields, where the BO approximation does not apply, the weak-field asymptotic theory describes the spectrum accurately.

Science, 350 (6262), ISSN:0036-8075, ISSN:1095-9203

Single ionization of He by two oppositely circularly polarized, time-delayed attosecond pulses is shown to produce photoelectron momentum distributions in the polarization plane having helical vortex structures sensitive to the time delay between the pulses, their relative phase, and their handedness. Results are obtained by both ab initio numerical solution of the two-electron time-dependent Schrödinger equation and by a lowest-order perturbation theory analysis. The energy, bandwidth, and temporal duration of attosecond pulses are ideal for observing these vortex patterns.

All attosecond time-resolved measurements have so far relied on the use of intense near-infrared laser pulses. In particular, attosecond streaking, laser-induced electron diffraction and high-harmonic generation all make use of non-perturbative light–matter interactions. Remarkably, the effect of the strong laser field on the studied sample has often been neglected in previous studies. Here we use high-harmonic spectroscopy to measure laser-induced modifications of the electronic structure of molecules. We study high-harmonic spectra of spatially oriented CH3F and CH3Br as generic examples of polar polyatomic molecules. We accurately measure intensity ratios of even and odd-harmonic orders, and of the emission from aligned and unaligned molecules. We show that these robust observables reveal a substantial modification of the molecular electronic structure by the external laser field. Our insights offer new challenges and opportunities for a range of emerging strong-field attosecond spectroscopies., Although expected to have an influence, the effect of strong laser fields on molecules in high-harmonic generation in gases is rarely explored. Kraus et al. show that the laser modifies the electronic structure of large polyatomic molecules and present a new theoretical framework to model this.