Multiphoton above-threshold ionization in superintense free-electron x-ray laser fields: Beyond the dipole approximation

Zhou, Zhóngyuan; Chu, Shih‐I

doi:10.1103/physreva.87.023407

Cited by 18 publications

(23 citation statements)

References 22 publications

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“…Development of free-electron lasers (FELs) [12][13][14] opened up the possibility of experimental study of the relativistic effects for multiphoton processes in the domain of high photon energies, where the nondipole effects due to finite photon momentum can be expected to be particularly important. That this is indeed the case was shown in [15], where a nonperturbative study of ATI in superintense and ultrashort wavelength was reported. It was found that inclusion of the nondipole effects may modify ATI spectra appreciably.…”

Section: Introductionmentioning

confidence: 60%

“…The region of the field strengths F 50 a.u. was considered in [15] [with pulse parameters identical to those used in Eq. (6)].…”

Section: Resultsmentioning

confidence: 99%

“…corresponding to the carrier frequency ω. The pulse shape and the carrier frequency we employ coincide with those used in [15], thereby enabling comparison of the results obtained by different methods.…”

Section: A Solution Of the Time-dependent Dirac Equationmentioning

confidence: 99%

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Spin-flip processes and nondipole effects in above-threshold ionization of hydrogen in ultrastrong laser fields

Ivanov

2017

Phys. Rev. A

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We consider above-threshold ionization of hydrogen atoms in ultrastrong laser fields. We use a time-dependent Dirac equation as a calculational tool. This framework allows one to include relativistic effects such as nondipole effects, effects of the relativistic kinematics, and electron spin effects. Inclusion of the spin effects allows one to consider the spin-flip process accompanying above-threshold ionization. We present and discuss electron momenta distributions for ionization processes with and without spin flips. Electron momenta distributions for the ionization process without a spin flip show a gradual increase of the role of the nondipole effects with increasing electric field. Electron spectra for the spin-flip ionization exhibit cusplike singularities, and the absence of features (such as the presence of dips) in the case of the ionization without a spin flip. We explain these features by invoking perturbation theory arguments.

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Section: Introductionmentioning

confidence: 60%

“…The region of the field strengths F 50 a.u. was considered in [15] [with pulse parameters identical to those used in Eq. (6)].…”

Section: Resultsmentioning

confidence: 99%

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Spin-flip processes and nondipole effects in above-threshold ionization of hydrogen in ultrastrong laser fields

Ivanov

2017

Phys. Rev. A

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“…However, under the conditions of short-wavelength x-rays this assumption may no longer be valid. Indeed, Zhou and Chu indicate that nondipole effects significantly change the photoelectron angular distribution and that the nondipole ATI spectra are enhanced in the high photon-energy regime [26].…”

Section: Resultsmentioning

confidence: 99%

Wave-packet propagation based calculation of above-threshold ionization in the x-ray regime

Tilley

Karamatskou

Santra

2015

J. Phys. B: At. Mol. Opt. Phys.

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We investigate the multi-photon process of above-threshold ionization for the light elements hydrogen, carbon, nitrogen and oxygen in the hard x-ray regime. Numerical challenges are discussed and by comparing Hartree-Fock-Slater calculations to configuration-interaction-singles results we justify the mean-field potential approach in this regime. We present a theoretical prediction of two-photon above-threshold-ionization cross sections for the mentioned elements. Moreover, we study how the importance of above-threshold ionization varies with intensity. We find that for carbon, at x-ray intensities around 10 23 Wcm −2 , two-photon above-threshold ionization of the K-shell electrons is as probable as one-photon ionization of the L-shell electrons.

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“…In the dipole description of the interaction, the electrons are emitted along the axis of polarization only, whereas in the beyond-dipole case, electrons are also ejected in the counterpropagation direction. The corresponding structure in the angular distribution, which is known as the nondipole lobe [8,34,35], is explained by a process in which the electron experiences a temporary push in the laser propagation direction due to the radiation pressure caused by the combined effort of the electric and magnetic fields. Then, when the laser pulse ramps off at the end of the interaction, the (displaced) electron falls back in the Coulomb potential of the bare nucleus and scatters off it in the backward (counterpropagation) direction.…”

Section: Resultsmentioning

confidence: 99%

Ionization dynamics beyond the dipole approximation induced by the pulse envelope

et al. 2016

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When atoms and molecules are ionized by laser pulses of finite duration and increasingly high intensities, the validity of the much-used dipole approximation, in which the spatial dependence and magnetic component of the external field are neglected, eventually breaks down. We report that, when going beyond the dipole approximation for the description of atoms exposed to ultraviolet light, the spatial dependence of the pulse shape, the envelope, provides the dominant correction, while the spatial dependence of the carrier is negligible. We present a first-order beyond-dipole correction to the Hamiltonian which accounts exclusively for nondipole effects stemming from the carrier envelope of the pulse. We demonstrate by ab initio calculations for hydrogen that this approximation, which we refer to as the envelope approximation, reproduces the full interaction beyond the dipole approximation for absolute and differential observables and proves to be valid for a broad range of high-frequency fields. This is done both for the Schrödinger and the Dirac equation. Moreover, it is demonstrated that the envelope approximation provides an interaction-term which gives rise to faster numerical convergence in terms of partial waves compared to its exact counterpart.

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Multiphoton above-threshold ionization in superintense free-electron x-ray laser fields: Beyond the dipole approximation

Cited by 18 publications

References 22 publications

Spin-flip processes and nondipole effects in above-threshold ionization of hydrogen in ultrastrong laser fields

Spin-flip processes and nondipole effects in above-threshold ionization of hydrogen in ultrastrong laser fields

Wave-packet propagation based calculation of above-threshold ionization in the x-ray regime

Ionization dynamics beyond the dipole approximation induced by the pulse envelope

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