Systems In Intense Laser Fields Research Articles

Multiphoton single ionization of two-electron systems in intense laser fields

Multiphoton single-ionization of a two-electron system (He and He-like ions) in intense circularly polarized laser field is reported in a relativistic field theoretic method. Coulomb-correction factor is introduced to estimate the effect of the Coulomb field of the residual ion on the rate. Antisymmetric wave function is considered both in the initial and in the final state. The spin-specific currents are calculated. Angular asymmetry in current generation is noted with the change in the spin direction of the ionized electron. Coulomb-corrected relativistic-result for total rate is compared with the Coulomb-corrected nonrelativistic KFR rate. At the high-intensity regime nonrelativistic rate overestimates the relativistic rate. Formation of orthopositronium and parapositronium from positronium negative ion by multiphoton ionization is discussed.

Electron–nuclear correlations in mixed quantum–classical calculations of laser-induced ionization and dissociation

Abstract Mixed quantum–classical methods, in which the electronic system is described quantum mechanically and the nuclear system classically, allow an efficient theoretical description of molecular systems in intense laser fields. In standard mean field approaches, the missing correlation between the nuclear and the electronic system may lead to a source of error, however. Here, we present an approximate treatment of this correlation for strongly perturbed single electron systems. The performance of the methodology is demonstrated by the qualitative reproduction of full quantum results for the ionization as well as the dissociation probability of the H+2 molecule interacting with an intense laser field.

Detailed instantaneous ionization rate ofH2+in an intense laser field

Component instantaneous ionization rate (IIR) is introduced and the approach of its calculation is formulated. The component IIR's and the overall (time-averaged) component ionization rates are calculated for H$_2^+$ at different values of inter-nuclear distance in a linearly polarized laser field with $1.0 \times10^{14}$W cm$^{-2}$ intensity and $\lambda \sim 1064 $nm wavelength by direct numerical solution of the fixed-nuclei full dimensional time-dependent Schr \"odinger equation. The exact overall component ionization rates calculated by time-averaging of the component IIR are compared with those calculated approximately via the virtual detector method (VD). Details of the time dependent behavior of the outgoing and incoming electron wavepackets of the H$_2^+$ system in intense laser field at sub-femtosecond time scale are studied based on the calculated component IIR. It is shown clearly that the positive (outgoing electron wavepacket) signals of the IIR and its z component are strong and sharp but the negative (returning electron wavepacket) signals of the IIR are smooth and weak. The structure of the $\rho$ component of the IIR has smooth structure. Relation between the R-dependent ionization rate and duration of the ramp of the laser pulse is studied and it is explicitly shown that for internuclear distance R<5.6, when the laser pulse is turned on without a ramp, the first peak of R-dependent ionization rates moves towards the peak of the lower time dependent Floquet quasi-energy state (QES).

Effect of electron correlation on high-order-harmonic generation of helium atoms in intense laser fields: Time-dependent generalized pseudospectral approach in hyperspherical coordinates

We present a time-dependent generalized pseudospectral (TDGPS) approach in hyperspherical coordinates for fully ab initio nonperturbative treatment of multiphoton dynamics of atomic systems in intense laser fields. The procedure is applied to the investigation of high-order-harmonic generation (HHG) of helium atoms in ultrashort laser pulses at a KrF wavelength of $248.6\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The six-dimensional coupled hyperspherical-adiabatic-channel equations are discretized and solved efficiently and accurately by means of the TDGPS method. The effects of electron correlation and doubly excited states on HHG are explored in detail. A HHG peak with Fano line profile is identified which can be attributed to a broad resonance of doubly excited states. Comparison of the HHG spectra of the ab initio two-electron and the single-active-electron model calculations is also presented.

Generalized Floquet Formulation of Time‐Dependent Density Functional Theory for Multiphoton Processes in Intense Laser Fields

AbstractWe present a short review article of recent new developments of Floquet formulations of time‐dependent density functional theory (TDDFT) for ab initio nonperturbative treatment of multiphoton and high‐order nonlinear optical processes of many‐electron quantum systems in intense laser fields. In the presence of monochromatic or polychromatic (multi‐color) time‐dependent fields, it is shown that the time‐dependent Kohn‐Sham equation can be exactly reformulated as a time‐independent generalized Floquet matrix eigenvalue problem. The theory has been extended to include the time‐dependent current density functional theory (TDCDFT) as well. In the case of bound‐free transition processes such as multiphoton ionization or dissociation, we show that the complex scaling transformation can be implemented into the Floquet formulation of TDDFT and TDCDFT, and the field‐induced complex quasienergy (dressed) states of many‐electron quantum systems can be determined by means of the solution of a time‐independent non‐Hermitian Floquet matrix eigenvalue problem. To facilitate the exploration of the constraints satisfied by the universal but unknown time‐dependent exchange‐correlation energy functionals, we develop several exact relations and theorems for the quasienergy functionals. Finally we present some initial applications of the Floquet‐TDDFT formalism for the study of multiphoton ionization of neutral atoms and electron angular distribution in multiphoton detachment of negative ions in intense laser fields. The Floquet‐TDDFT/TDCDFT formalism provides a powerful new theoretical foundation and feasible procedures for the exploration of many‐electron atomic and molecular physics in strong fields in the future.

Quantum-fluid-dynamics approach for strong-field processes: Application to the study of multiphoton ionization and high-order harmonic generation of He and Ne atoms in intense laser fields

We explore the feasibility of extending the quantum-fluid dynamics (QFD) approach for quantitative investigation of nonlinear optical processes of many-electron quantum systems in intense laser fields. Through the amalgamation of the QFD and density-functional theory (DFT), a single time-dependent hydrodynamical equation of motion can be derived. This equation has the form of a generalized nonlinear Schr\odinger equation (GNLSE) but includes the many-body effects through a local time-dependent exchange-correlation potential. The time-dependent generalized pseudospectral method is extended to the solution of the GNLSE in spherical coordinates, allowing nonuniform spatial discretization and efficient, accurate solution of the hydrodynamical density and wave function in space and time. The procedure is applied to the study of multiphoton ionization (MPI) and high-order harmonic generation (HHG) of He and Ne atoms in intense laser fields. Excellent agreement with other recent self-interaction-free time-dependent DFT calculations is obtained for He, while for Ne, good agreement is achieved. Four different exchange-correlation energy functionals are used in the study with an aim to explore the roles of exchange and correlation on MPI/HHG processes in details. The method offers a conceptually appealing and computationally practical approach for nonperturbative treatment of strong-field processes of many-electron systems beyond the time-dependent Hartree-Fock level.

Time-dependent density-functional theory for molecular processes in strong fields: Study of multiphoton processes and dynamical response of individual valence electrons ofN2in intense laser fields

We present a time-dependent density-functional theory (TDDFT) with proper asymptotic long-range potential for nonperturbative treatment of multiphoton processes of many-electron molecular systems in intense laser fields. A time-dependent generalized pseudospectral method is extended for precision solution of the TDDFT equations for two-center diatomic systems. The procedure allows nonuniform optimal spatial grid discretization of the Hamiltonian in prolate spheroidal coordinates and a split-operator scheme in energy representation is used for the time propagation of the individual molecular spin orbital in space and time. The theory is applied to the first detailed all-electron study of multiphoton ionization and high-order harmonic generation (HHG) processes of ${\mathrm{N}}_{2}$ in intense laser fields. The results reveal unexpected and intriguing nonlinear optical response behaviors of the individual valence spin orbital to strong fields. In particular, it is found that the dominant contribution to the total HHG power spectrum of ${\mathrm{N}}_{2}$ is due to the constructive and destructive interferences of the induced dipoles of the two highest-occupied bonding $(3{\ensuremath{\sigma}}_{g})$ and antibonding $(2{\ensuremath{\sigma}}_{u})$ molecular orbitals in the presence of intense laser fields.

Self-interaction-free time-dependent density-functional theory for molecular processes in strong fields: High-order harmonic generation ofH2in intense laser fields

We present a self-interaction-free time-dependent density-functional theory (TDDFT) for nonperturbative treatment of multiphoton processes of many-electron molecular systems in intense laser fields. The time-dependent exchange-correlation (xc) energy potential with proper short- and long-range potential is constructed by means of the time-dependent optimized effective potential (OEP) method and the incorporation of an explicit self-interaction-correction (SIC) term. The resulting time-dependent OEP/SIC equations are structurally similar to the time-dependent Hartree-Fock equations, but include the many-body effects through an orbital-independent single-particle local time-dependent xc potential. A numerical time-propagation technique is introduced for accurate and efficient solution of the TDDFT/OEP-SIC equations for two-center diatomic molecular systems. This procedure involves the use of a generalized pseudospectral method for nonuniform optimal grid discretization of the Hamiltonian in prolate spheroidal coordinates and a split-operator scheme in the energy representation for the time development of the electron orbital wave functions. High-precision time-dependent wave functions can be obtained by this procedure with the use of only a modest number of spatial grid points. The theory is applied to a detailed study of high-order harmonic generation (HHG) processes of ${\mathrm{H}}_{2}$ molecules in intense pulsed laser fields. Particular attention is paid to the exploration of the spectral and temporal structures of HHG by means of the wavelet time-frequency analysis. The results reveal striking details of the spectral and temporal fine structures of HHG, providing new insights regarding the detailed HHG mechanisms in different energy regimes.

Hydrodynamic description of laser-induced coulomb explosion for small molecules

We have performed extensive numerical studies of Coulomb explosion of small molecules such as CO2 and N2O with the help of a hydrodynamic model, which allows one to deal with many-electron systems in intense laser fields. The predicted kinetic energy releases of the fragmentation channels are in good agreement with the measured energies, using realistic molecular and laser excitation condition parameters.

Enhanced ionization of multi-well quantum systems in intense laser fields

The ionization dynamics of multi-well quantum systems in intense laser fields (~ 1014W/cm) has been numerically investigated through solving the time-dependent Schrodinger equation. The enhanced ionization effect at a critical inter-well distance is found as a general feature of multi-well quantum systems subjected to intense laser fields. It is well rationalized in terms of.the field-induced over-the-barriers ionization mechanism. With the number of wells increasing the whole ionization peak slightly shifts to small inter-well distance, but the enhanced ionization tendency still keeps. The ionization potential of multi-well systems is not the sole important factor of intense field-ionization of system.

Time-dependent density-functional theory with optimized effective potential and self-interaction correction: Application to the study of coherent control of multiple high-order harmonic generation of He atoms in mixed laser fields

We present a time-dependent density-functional theory (TDDFT) with optimized effective potential (OEP) and self-interaction correction (SIC) for nonperturbative treatment of multiphoton and nonlinear optical processes of many-electron systems in intense laser fields. The resulting TDOEP/KLI-SIC equations are structurally similar to the time-dependent Hartree-Fock equations but include the many-body effects through an orbital-independent single-particle local time-dependent exchange-correlation (xc) potential. The method is applied to the all-electron study of the coherent control of multiple high-order harmonic generation (HHG) of He atoms in two-color mixed laser fields. A generalized pseudospectral time propagation method is introduced for efficient and accurate solution of the TDOEP/KLI-SIC equations in three spatial dimensions. It is found that the mixing of a strong second-harmonic (527 nm) field with a weak fundamental (1053 nm) field can lead to the production of both odd and even harmonics simultaneously with relatively high yield. Further, the relative efficiency of the HHG production can be controlled by varying the relative polarization of the two laser beams. The theoretical predictions are in accord with recent experimental observations. (C) 1998 John Wiley & Sons, Inc.

Time-dependent density-functional theory for strong-field multiphoton processes: Application to the study of the role of dynamical electron correlation in multiple high-order harmonic generation

We present a self-interaction-free time-dependent density-functional theory (TDDFT) for nonperturbative treatment of multiphoton processes of many-electron atomic systems in intense laser fields. The theory is based on the extension of the time-dependent Kohn-Sham formalism. The time-dependent exchange-correlation potential with proper short-and long-range behavior is constructed by means of the time-dependent optimized effective potential (TDOEP) method and the incorporation of an explicit self-interaction correction (SIC) term. The resulting TDOEP-SIC equations are structurally similar to the time-dependent Wartree-Fock equations, but include the many-body effects through an orbital-independent single-particle local time-dependent exchange-correlation potential. We also introduce a generalized pseudospectral lime-propagation method, allowing optimal spatial grid discretization, for accurate and efficient numerical solution of the TDOEP-SIC equations. The theory is applied to the study of the role of dynamical electron correlation on the multiple high-order harmonic generation (HHG) processes of He atoms in intense laser fields. We also perform a detailed study of the mechanisms responsible for the production of the higher harmonics in He atoms observed in a recent experiment that cannot be explained by the single-active-electron model. We found that both the dynamical electron correlation and the He+ ion are important to the generation of the observed higher harmonics. The present TDDFT is thus capable of providing a unified and self-consistent dynamical picture of the HHG processes. [S1050-2947(98)04601-0].

Introduction to the subject of coherent radiation processes in strong fields

Abstract A fundamental understanding of coherent electromagnetic radiation processes, which is most rigorously based on quantum electrodynamics, is important for investigating the basic radiative interactions of condensed matter systems, relativistic free electrons in strong fields, and atomic systems in intense laser fields. There has been considerable recent interest in the investigation of coherent radiative interactions involving energetic electrons in crystal lattices, in spatially-periodic static-field structures (such as magnetic wigglers and undulators), and in electromagnetic fields (laser beams). There has also been current interest in radiation processes involving multiply-charged many-electron ions in high- temperature plasmas and in intense laser fields. The attention directed at these processes has been motivated in part by the need to assess various mechanisms for the generation of amplified spontaneous emission at short photon wavelengths (approaching the X-ray region). A precise theoretic...

Adiabatic representation for the coherent dynamics of N-level systems in intense laser fields.

The exact adiabatic representation scheme of Nguyen-Dang [J. Chem. Phys. 90, 2657 (1989)] is applied to time evolution of the general laser-driven two- and three-level systems. It leads to exact formal results that do not assume a specific shape of the pulse and do not invoke the rotating-wave approximation (RWA). These results are a generalization of the corresponding RWA results because the former depend on an effective pulse area that varies with the detuning and involves a Rabi frequency modulated by a phase cphi(t). In addition, the exact adiabatic representation scheme is applied to N-level systems exhibiting various dynamic symmetries and, as in the previous cases, leads to results that do not invoke the RWA and depend on an effective pulse area. Time-reversal analysis on two- and three-level systems leads to quantization of the effective pulse area for time-symmetric pulses. This quantization leads to localized asymptotic population distributions.

On nonadiabatic transitions and the Wigner path integral

We continue our study of dynamics in terms of the Wigner path integral. We investigate how nonadiabatic electronic transitions are described in the formalism. We find, using the approximation of Miller and George for the reduced propagator, and the classical path approximation for the nuclear degrees of freedom (i.e., assuming nuclear localization), that the Wigner path integral leads to a Tully–Preston surface hopping model. We discuss the relation of this result to general classically forbidden processes and to the dynamics of molecular systems in intense laser fields.