Atoms In Intense Laser Fields Research Articles

Quantum correlated atoms in intense laser fields

Quantum correlated atoms in intense laser fields

Effects of the noninteger-cycle pulse and the initial phase on the Magnus approximation to study Rydberg atoms in intense laser fields

The Magnus expansion method is popular for the study of Rydberg atoms in intense laser fields. In such studies, the pulse length is usually set as an integer multiple of wavelength, at the same time, the initial phase of the laser pulse is usually set as zero, i.e., without considering the effect of the initial phase in the calculation. However, this will result in some items multiplied by zero. Then, contributions from these items will be lost. Here, we investigated the effects of the noninteger-cycle pulse and initial phase of the laser on the calculated transition probability of Rydberg atoms in intense laser fields. Our calculation results show that these effects are trivial when the nondipole approximation is adopted. In dipole approximation i.e., when the wavelength of the laser field exceeds the characteristic size of atomic or molecular orbitals, these effects are trivial only in the case of a long pulse duration and a large principal quantum number, however for ultrashort laser pulses (pulse duration that lasts less than five light cycles), these effects are evident.

Effects of the Non-Integer-Cycle Pulse and the Initial Phase on the Magnus Approximation to Study Rydberg Atoms in Intense Laser Fields

Effects of the Non-Integer-Cycle Pulse and the Initial Phase on the Magnus Approximation to Study Rydberg Atoms in Intense Laser Fields

High-order correction calculation for survival of Rydberg atoms in intense laser fields

Using a method based on the Magnus expansion, the survival of Rydberg atoms in intense laser fields is studied. Usually, for atoms in intense laser fields, effects beyond the electric dipole approximation need to be considered. In the past few years, researchers have primarily paid attention to the nondipole effects induced by the electric quadrupole term of the optical field. The quadrupole term plays a more important role than the electric dipole term in the calculation of the survival probability of Rydberg atoms in intense laser fields even though the magnitude of the former is smaller than that of the latter in terms of the multipole expansion of the optical vector potential. Meanwhile, the effects of higher-order corrections beyond the quadrupole term remain ambiguous. In this paper, we confirm that corrections containing higher-order terms, except for the quadrupole term, can be neglected in the calculation of the transitions of Rydberg atoms in external fields. We then use the second-order time evolution operator containing the quadrupole term to study the scaling law of the transition probability of Rydberg He in laser fields with different parameters.

Resonance-enhanced two-photon ionization of hydrogen atom in intense laser field investigated by Bohmian-mechanics**Project supported by Jilin Province Science and Technology Development Plan Project–Excellent Youth Talents Fund Project, China (Grant No. 20180520174JH) and the National Natural Science Foundation of China (Grant Nos. 11704145 11904050, 11774129, 11747007, and 11534004).

Resonance enhanced two-photon ionization process of hydrogen atom via the resonant laser pulse is studied by Bohmian mechanics (BM) method. By analyzing the trajectories and energies of Bohmian particles (BPs), we find that under the action of high frequency and low intensity multi-circle resonant laser pulses, the ionized BPs first absorb one photon completing the excitation, and then absorb another photon, completing the ionization after staying in the first excited state for a period of time. The analysis of work done by the forces shows that the electric field force and quantum force play a major role in the whole ionization process. At the excitation moment and in the excitation-ionization process, the effect of the quantum force is greater than that of the electric field force. Finally, we discuss the principle of work and energy for BPs, and find that the electric field force and quantum force are non-conservative forces whose work is equal to the increment of mechanical energy of the system. In addition, it is proved that the quantum potential energy actually comes from the kinetic energy of the system and the increment of kinetic energy is equal to that of the kinetic energy of the system.

Semi-integer harmonic generation from an argon atom by bichromatic counter-rotating circularly polarized laser field**Project supported by the National Natural Science Foundation of China (Grant Nos. 61575077 and 11904122), the Natural Science Foundation of Jilin Province of China (Grant No. 20180101225JC), and the China Postdoctoral Science Foundation (Grant Nos. 2018M641766 and 2019T120232).

We theoretically investigated the properties of the high-order harmonic generation from an argon atom by bichromatic counter-rotating circularly polarized (BCCP) laser field. The harmonic emission processes have been illustrated by numerically solving the two-dimensional time-dependent Schrödinger equation of an atom in intense laser fields. It is found that with the decrease of the right-circularly polarized laser wavelength, the harmonic spectra are gradually splitting and the harmonic orders move towards the higher frequency. Meanwhile, the integer and semi-integer harmonic emission will be generated when the frequency ratios of right- and left-circularly polarized lasers are semi-integer. The emission mechanism of the semi-integer-order harmonics has been investigated by using the rules of photon absorption and emission.

Third-order S-matrix study of electron-electron correlation in nonsequential double ionization

Correlated electron momentum distribution of nonsequential double ionization of a Ne atom in intense laser fields is calculated using a third-order S-matrix formalism which takes into account the interaction between the Coulomb potential of the parent ion and the photoelectrons. Compared to the second-order S-matrix calculation, the correlation distributions of the two processes which take into account the effect of the ionic Coulomb potential on either of the two ionized electrons after their collision tend to be concentrated off the diagonal of the first and third quadrants, which are closer to the experimental observations. Our further analysis shows that this modification to the second-order calculation is due to the fact that, scattering of the emitted electron upon the parent ion will alter the photoelectron angular distribution from concentration in the laser direction to focus in a direction deviating from the laser field. And this will significantly reduce the momentum of the electron in the direction of the laser field. Our results are in accord with previous studies which reveal that electron-nuclear Coulomb interaction, electron-electron collision and the final-state electron repulsion all play significant roles in the formation of the fingerlike structure.

Effect of electron excitation in nonsequential double ionization by intense laser fields**Project supported by the National Natural Science Foundation of China (Grant Nos. 61475168, 11674231, and 61575124), Natural Science Foundation of Shanghai, China (Grant No. 15ZR1430600), and Shanghai Gaofeng & Gaoyuan Project for University Academic Program Development, China.

We study the double ionization process of atoms in intense laser fields. The momentum distributions of the correlated electrons are calculated. Contrary to the general expectation, we show an increasing proportion of the electrons ionized via excitation with the increasing laser intensity. These electrons generally have small energy thus they concentratedly distribute on the central region of the momentum diagram. Consequently, the central part of the momentum diagram becomes more notable in higher intensity laser fields. Further study suggests that this phenomenon is general in double ionization.

Experimental investigation of strong-field-ionization theories for laser fields from visible to midinfrared frequencies

Ionization of atoms in intense laser fields is experimentally studied to evaluate the validity of the widely used ionization rate formulas from the ADK and PPT models at different laser wavelengths, intensities, polarizations and types of targets. It is found that the PPT model including the Coulomb correction agrees well with all the experimental data in the study, whereas the ADK formula can only be applied in some limited regimes.

Wavelength scaling of atomic nonsequential double ionization in intense laser fields

Experimental and theoretical investigations of the wavelength dependence of the nonsequential double ionization (NSDI) process of the xenon atom in intense laser fields are reported. The observed wavelength dependence of the ratio Xe2+/Xe+ deviates significantly from the prediction of the three-step model of ionization (sometimes called the simple-man model), both in the slope of the overall decrease with increasing wavelength and in the existence of some pronounced humps. A semiclassical model calculation reproduces the changing slope of the overall decrease, which is due to the interplay of wave-packet diffusion, Coulomb focusing, and a closely related Coulomb defocusing effect of the liberated electron. The hump is beyond the scope of the semiclassical model.

High-order-harmonic spectra from atoms in intense laser fields: Exact versus approximate methods

High-order-harmonic spectra from atoms in intense laser fields: Exact versus approximate methods

Spherical-harmonics expansion method for density-matrix simulations of quantum electron dynamics in continuum states

Spherical-harmonics expansion method is proposed to solve the quantum time-evolution equations for density matrices numerically in momentum space. This method facilitates efficient real-time simulations of quantum electron dynamics in continuum states through extension of our previous formalism developed for discrete states. Numerical accuracy and efficiency are demonstrated through two examples: (i) multiphoton ionization of a one-electron atom in intense laser field, and (ii) real-time dynamics of plasma oscillations in electron liquids. In case (i), coupled dynamics of density matrices for bound and continuum states reveals an enhancement of multiphoton ionization over the Keldysh approximation through resonant intermediate states. © 2015 Wiley Periodicals, Inc.

Survival of Rydberg atoms in intense laser fields and the role of nondipole effects

We consider the interaction of Rydberg atoms with strong infrared laser pulses using an approach based on the Magnus expansion of the time evolution operator. First-order corrections beyond the electric dipole approximation are also included in the theory. We illustrate the dynamics of the interaction at the parameters of the experiment [Eichmann et al., Phys. Rev. Lett. 110, 203002 (2013)]. It emerges that the depletion of Rydberg atoms in this regime comes predominantly from the nondipole effects.

Photoelectron angular distributions of excited atoms in intense laser fields

Angular distributions of photoionization differential rates for an atom in arbitrary excited states ionized by intense laser fields with arbitrary polarization are reported. Relativistic effects are incorporated into the Keldysh theory, yielding semi-analytical expressions of ionization rates for hydrogenic initial states in intense linear, circular, and elliptical laser polarizations. Angular distributions are compared for different angular momentum quantum numbers, magnetic quantum numbers, and Keldysh parameters $\ensuremath{\gamma}$. The angular distributions are shown to depend strongly on $\ensuremath{\gamma}$, thus also reflecting the influence of relativistic effects. The sign of the magnetic quantum number, corresponding to different electron rotations, is shown to have a significant effect on photoelectron angular distributions in circularly polarized laser fields.

Attosecond-resolution two-electron harmonic emission

• A two-electron time-dependent Schrödinger equation analysis. • Harmonic emission spectrum from the neutral He atom in intense laser field. • An extended harmonic plateau with many new harmonic cutoffs. • A propose model of double recombination arising from electron correlation effect. • A general characteristic of double recombination in the two-electron system. Two-electron harmonic emission from the He atom has been investigated by solving the two-electron time-dependent Schrödinger equation (TDSE), which exhibits an extended plateau and many new harmonic cutoffs beyond the classical single-electron harmonic cutoff. Theoretical analyses show that these extended new cutoffs are caused by the sequential double recombination and the nonsequential double recombination of the two electrons, which is a general characteristic for the two-electron harmonic emission and which is revealed through the investigation on laser parameter effects. Moreover, from analyzing the time-dependent wave functions, the motions of the two electrons and the single and double ionization time have been described.

Exterior complex scaling method in TDDFT: HHG of Ar atoms in intense laser fields

The exterior complex scaling (ECS) method is applied in the framework of time-dependent density-functional theory (TDDFT) to study high-order harmonic generation (HHG) of multielectron atoms in intense laser fields. With the help of ECS, correct outgoing-wave boundary conditions can be imposed on the wave functions at large distances. In our implementation, ECS is combined with the time-dependent generalized pseudospectral method for accurate and efficient solution of the time-dependent Kohn-Sham equations. We make use of LB94 exchange-correlation potential which appears quite accurate in calculations of unperturbed electronic structure of Ar. Calculations of HHG are performed for the laser fields with the wavelength of 800 nm and several peak intensities. The HHG spectrum exhibits an intensity-independent minimum corresponding to the photon energy of about 51 eV which is closely related to the Cooper minimum in the photoionization cross section of Ar. We found that HHG spectra calculated with the frozen-core potential (not including dynamic response of the electron density) differ significantly from those obtained by TDDFT.

Exterior complex scaling method in TDDFT: HHG of Ar atoms in intense laser fields

Exterior complex scaling (ECS) method is applied in the framework of the time-dependent density functional theory to study high-order harmonic generation (HHG) of multielectron atoms in intense laser fields. ECS allows to impose correct (outgoing-wave) boundary conditions on the wave functions at large distances. In the present work, ECS is combined with the time-dependent generalized pseudospectral method for accurate and efficient solution of the time-dependent Kohn-Sham equations. Results for HHG of Ar atoms are presented.

Transferring the attoclock technique to velocity map imaging

Attosecond angular streaking measurements have revealed deep insights into the timing of tunnel ionization processes of atoms in intense laser fields. So far experiments of this type have been performed only with a cold-target recoil-ion momentum spectrometer (COLTRIMS). Here, we present a way to apply attosecond angular streaking experiments to a velocity map imaging spectrometer (VMIS) with few-cycle pulses at a repetition rate of 10 kHz and a high ionization yield per pulse. Three-dimensional photoelectron momentum distributions from strong-field ionization of helium with an elliptically polarized, sub-10-fs pulse were retrieved by tomographic reconstruction from the momentum space electron images and used for the analysis in the polarization plane.

Time-dependent density-functional theory with optimized effective potential and self-interaction correction and derivative discontinuity for the treatment of double ionization of He and Be atoms in intense laser fields

We present a self-interaction-free time-dependent density-functional theory (TDDFT) for the treatment of double-ionization processes of many-electron systems. The method is based on the extension of the Krieger-Li-Iafrate (KLI) treatment of the optimized effective potential (OEP) theory and the incorporation of an explicit self-interaction correction (SIC) term. In the framework of the time-dependent density functional theory, we have performed three-dimensional (3D) calculations of double ionization of He and Be atoms by intense near-infrared laser fields. We make use of the exchange-correlation potential with the integer discontinuity which improves the description of the double-ionization process. We found that a proper description of the double ionization requires the TDDFT exchange-correlation potential with the discontinuity with respect to the variation of the total particle number (TPN). The results for the intensity-dependent rates of double ionization of He and Be atoms are presented.

Exterior complex scaling method in time-dependent density-functional theory: Multiphoton ionization and high-order-harmonic generation of Ar atoms

The exterior complex scaling (ECS) method is applied in the framework of time-dependent density-functional theory (TDDFT) to study multiphoton ionization (MPI) and high-order-harmonic generation (HHG) of multielectron atoms in intense laser fields. ECS allows one to impose correct (outgoing-wave) boundary conditions on the wave functions at large distances. In our implementation, ECS is combined with the time-dependent generalized pseudospectral method for accurate and efficient solution of the time-dependent Kohn-Sham equations. We make use of LB94 exchange-correlation potential which proved accurate in calculations of unperturbed electronic structure of Ar. Calculations of MPI and HHG are performed for the laser pulses with the wavelength of 800 nm and several peak intensities. The HHG spectrum exhibits an intensity-independent minimum corresponding to the photon energy of about 51 eV which is closely related to the Cooper minimum observed in the photoionization cross section of Ar. We found that MPI probabilities and HHG spectra calculated with the frozen-core potential (that is, not including dynamic response of the electron density) differ significantly from those obtained by TDDFT.