We calculate the high-harmonic generation (HHG) spectra, strong-field ionization, and time-dependent dipole-moment of Ne using explicitly time-dependent optimized second-order many-body perturbation method (TD-OMP2) where both orbitals and amplitudes are time-dependent. We consider near-infrared (800 nm) and mid-infrared (1200 nm) laser pulses with very high intensities ($5\times10^{14}$, $8\times10^{14}$ , and $1\times10^{15}$ W/cm$^2$), required for strong-field experiments with the high-ionization potential (21.6 eV) atom. We compare the result of the TD-OMP2 method with the time-dependent complete-active-space self-consistent field method and the time-dependent Hartree-Fock method. Further, we report the implementation of the TD-CC2 method within the chosen active space, which is also a second-order approximation to the TD-CCSD method, and present results of time-dependent dipole-moment and HHG spectra with an intensity of $5\times10^{13}$ W/cm$^2$ at a wavelength of 800 nm. It is found that the TD-CC2 method is not stable in the case with a higher laser intensity, and it does not provide a gauge-invariant description of the physical properties, which makes TD-OMP2 a superior choice to reach out to larger chemical systems, especially for the study of strong-field dynamics. The obtained results indicate that the TD-OMP2 method shows moderate performance, overestimating the response of Ne, while TDHF underestimates it. Nevertheless, it is remarkable that stable computation of such highly nonlinear nonperturbative phenomena is possible within the framework of time-dependent perturbation method, by virtue of the nonperturbative inclusion of the laser-electron interaction and time-dependent optimization of orbitals.
Time-dependent coupled-cluster method with time-varying orbital functions, called time-dependent optimized coupled- cluster (TD-OCC) method, is formulated for multielectron dynamics in an intense laser field. We have successfully derived equations of motion for CC amplitudes and orthonormal orbital functions based on the real action functional, and implemented the method including double excitations (TD-OCCD) and double and triple excitations (TD-OCCDT) within the optimized active orbitals. The present method is size extensive and gauge invariant, a polynomial cost-scaling alternative to the t…
We present a novel approach to calculating strong field ionization dynamics of multielectron molecular targets. Adopting a multielectron wavefunction ansatz based on field-free ab initio neutral and ionic multielectron states, a set of coupled time-dependent single-particle Schroedinger equations describing the neutral amplitude and continuum electron are constructed. These equations, amenable to direct numerical solution or further analytical treatment, allow one to study multielectron effects during strong field ionization, recollision, and high harmonic generation. We apply the method to st…
Strong field ionization of benzene, fluorobenzene, benzonitrile and 1-chloro-2-fluorobenzene, is studied within the framework of real-space and real-time time-dependent density functional theory. Analysis of the ionization rates as function of the molecule orientation, reveal significant multielectron contribution from inner Kohn-Sham orbitals that depends on the electronic structure and on the orbital symmetries of the molecule, as well as on the polarization and intensity of the external laser field. Calculated photoelectron angular distributions at different molecule orientations and in res…
We solve the hydrodynamic-ballistic equations of motion for a one-dimensional time-dependent Thomas-Fermi model of Cl_2 exposed to an intense subpicosecond laser field, and observe simultaneous multielectron ionization and molecular dissociation. The fragment kinetic energy defect with respect to the simple Coulomb explosion picture is found to originate in ejected-electron screening of the escaping fragments; its magnitude agrees with that observed in recent experiments.
We report successful implementation of the time-dependent second-order many-body perturbation theory using optimized orthonormal orbital functions called time-dependent optimized second-order many-body perturbation theory [TD-OMP2] to reach out to relatively larger chemical systems for the study of intense-laser-driven multielectron dynamics. We apply this method to strong-field ionization and high-order harmonic generation (HHG) of Ar. The calculation results are benchmarked against ab initio time-dependent complete-active-space self-consistent field (TD-CASSCF), time-dependent optimized coup…
We present a cost-effective treatment of the triple excitation amplitudes in the time-dependent optimized coupled-cluster (TD-OCC) framework called TD-OCCDT(4) for studying intense laser-driven multielectron dynamics. It considers triple excitation amplitudes correct up to fourth-order in many-body perturbation theory and achieves a computational scaling of O(N7), with N being the number of active orbital functions. This method is applied to the electron dynamics in Ne and Ar atoms exposed to an intense near-infrared laser pulse with various intensities. We benchmark our results against the ti…
We review time-dependent multiconfiguration self-consistent-field (TD-MCSCF) method and time-dependent optimized coupled-cluster (TD-OCC) method for first-principles simulations of high-field phenomena such as tunneling ionization and high-order harmonic generation in atoms and molecules irradiated by a strong laser field. These methods provide a flexible and systematically improvable description of the multielectron dynamics by expressing the all-electron wavefunction by configuration interaction expansion or coupled-cluster expansion, using time-dependent one-electron orbital functions. The …
We report the implementation of a cost-effective approximation method within the framework of time-dependent optimized coupled-cluster (TD-OCC) method [J. Chem. Phys. 148, 051101 (2018)] for real-time simulations of intense laser-driven multielectron dynamics. The method, designated as TD-OCEPA0, is a time-dependent extension of the simplest version of the coupled-electron pair approximation with optimized orbitals [J. Chem. Phys. 139, 054104 (2013)]. It is size extensive, gauge invariant, and computationally much more efficient than the TD-OCC with double excitations (TD-OCCD). We employed th…
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 …
The time-dependent complete-active-space self-consistent-field (TD-CASSCF) method for the description of multielectron dynamics in intense laser fields is presented, and a comprehensive description of the method is given. It introduces the concept of frozen-core (to model tightly bound electrons with no response to the field), dynamical-core (to model electrons tightly bound but responding to the field), and active (fully correlated to describe ionizing electrons) orbital subspaces, allowing compact yet accurate representation of ionization dynamics in many-electron systems. The classification…
The time-dependent multiconfiguration self-consistent-field method based on the occupation-restricted multiple active space model is proposed (TD-ORMAS) for multielectron dynamics in intense laser fields. Extending the previously proposed time-dependent complete-active-space self-consistent-field method [TD-CASSCF; Phys. Rev. A, {\bf 88}, 023402 (2013)], which divides the occupied orbitals into core and active orbitals, the TD-ORMAS method {\it further} subdivides the active orbitals into an arbitrary number of subgroups, and poses the {\it occupation restriction} by giving the minimum and max…
Visualizing intense laser field driven electron dynamics in a multielectron molecule: dynamic electron localization, bonding properties and multiple ionization bursts
A mixed quantum-classical approach is introduced which allows the dynamical response of molecules driven far from equilibrium to be modeled. This method is applied to the interaction of molecules with intense, short-duration laser pulses. The electronic response of the molecule is described using time-dependent density functional theory (TDDFT) and the resulting Kohn-Sham equations are solved numerically using finite difference techniques in conjunction with local and global adaptations of an underlying grid in curvilinear coordinates. Using this approach, simulations can be carried out for a …
From the famous classical "simple man" model to the recent multichannel model for polyatomic molecules, this tutorial will guide you through the several landmarks in our understanding of high harmonic generation and high harmonic spectroscopy. Our goal was to provide recipes and insights for modelling the harmonic response in various regimes, from the single active electron regime typical for noble gas atoms to the laser-driven attosecond hole dynamics in polyatomics. We have tried to pay particular attention to both simple recipes and their limitations, including the connection of real-valued…
We report the observation of above-threshold dissociation of a multielectron molecular system, Na$_2^+$. The process is due to continuum-continuum transitions in a field-dressed molecular continuum. Three well resolved fragmentation channels with different kinetic energy release patterns have been detected, of which, two are associated with ordinary photodissociation via light-induced potentials into Na(3s) + Na$^+$ and Na(3p) + Na$^+$, and one with continuum-continuum transitions and above-threshold dissociation into Na(3p) + Na$^+$. We show that when we dress this molecule with a field whose…
We present the fully general time-dependent multiconfiguration self-consistent-field method to describe the dynamics of a system consisting of arbitrary different kinds and numbers of interacting fermions and bosons. The total wave function is expressed as a superposition of different configurations constructed from time-dependent spin-orbitals prepared for each particle kind. We derive equations of motion followed by configuration-interaction (CI) coefficients and spin-orbitals for general, not restricted to full-CI, configuration spaces. The present method provides a flexible framework for t…
The structure of approximate two electron wavefunction is deeply investigated, both theoretically and numerically, in the strong-field driven ionization dynamics. Theoretical analyses clarify that for two electron singlet systems, the previously proposed time-dependent extended Hartree-Fock (TD-EHF) method [Phys. Rev. A 51, 3999 (1995)] is equivalent to the multiconfiguration time-dependent Hartree-Fock method with two occupied orbitals. The latter wavefunction is further transformed into the natural expansion form, enabling the direct propagation of the natural orbitals (NOs). These methods, …
We discuss the production of ultracold molecules in their electronic ground state by photoassociation employing electronically excited states with ion-pair character and strong spin-orbit interaction. A short photoassociation laser pulse drives a non-resonant three-photon transition for alkali atoms colliding in their lowest triplet state. The excited state wave packet is transferred to the ground electronic state by a second laser pulse, driving a resonant two-photon transition. After analyzing the transition matrix elements governing the stabilization step, we discuss the efficiency of popul…
