Multiple recollisions in nonsequential double ionization by counter-rotating two-color circularly polarized laser fields

The Coulomb effect in nonsequential double ionization by counter-rotating two-color elliptical polarization fields

Using a three-dimensional classical ensemble model, nonsequential double ionization (NSDI) of Ar atoms by counter-rotating two-color elliptical polarization (TCEP) fields is investigated. The major axes of the two elliptical fields are aligned in different directions. The relative alignment of the two elliptical fields strongly affects the waveform of the combined electric field and the ultrafast dynamics of NSDI in TCEP fields. Numerical results show that the correlated electron momentum distributions in the x direction evolve from a V-shaped structure near the axis to a distribution concentrated on the diagonal with the angle between the two elliptical major axes increasing. The asymmetry of the energy sharing between the two electrons during recollision results in the V-shaped structure in the correlated momentum spectrum. Back analysis indicates that the recollision times of a part of the trajectories move from the peak to the valley of the combined electric field with the angle between the two elliptical major axes increasing. Therefore, for the case of a larger angle between the two elliptical major axes, the electrons experience a longer time to escape away from the vicinity of the parent ion and thus the stronger Coulomb effect from the parent ion makes the momentum difference between two electrons small, which results in a distribution concentrated on the diagonal. This provides an effective avenue to control the electron ultrafast dynamics in NSDI.

Electron correlation and recollision dynamics in nonsequential double ionization by counter-rotating two-color elliptically polarized laser fields

Nonsequential double ionization (NSDI) of Argon atoms by counter-rotating two-color elliptically polarized (TCEP) fields is investigated with a three-dimensional classical ensemble model. Different from two-color circularly polarized fields, the combined electric field in TCEP pulses has no symmetry and the ionized electron mainly returns to the parent ion from one direction. Thus the electron momentum distributions show strong asymmetry. Numerical results show with the increase of the relative phase between the two elliptical fields, the return angle of the travelling electron, i.e., the angle between the return direction of the electron and the +x direction, gradually decreases. Moreover, the dominant behavior of electron pairs evolves from anti-correlation to correlation with the relative phase increasing. This provides an avenue to control the return angle and electron correlation behavior by the relative phase between the two elliptical fields.

Photoelectron interference of He atoms in the attosecond ionization gating

We theoretically investigate the photoelectron momentum distribution of He atoms by numerically solving the time-dependent Schro¨dinger equation (TDSE) in few-cycle ionization gating, which is synthesized by two linearly polarized laser pulses. When applying the TDSE, we can clearly see the spider-like structures in the photoelectron momentum spectra. We also find that the spider-like structures can be isolated by changing the relative phase. The directionality of the spider-like structure is changed from right-side to left-side and the ring-like interference structure gradually appears in the photoelectron momentum spectra when increasing the relative phase. The interference patterns observed in TDSE are recaptured well by the quantum-trajectory Monte Carlo (QTMC) model. We separate the ionization time window of the tunneling electron by analyzing the ionization rate. With the help of QTMC simulation, we illustrate the change of the interference structure and its directionality in the photoelectron momentum spectra. By changing the relative phase, the forward-backward asymmetry of the momentum distribution of the emitted electrons can also be controlled. Moreover, we find that the relative contribution of the nonrescattering and the rescattering trajectories can be controlled. These properties are beneficial for the application of photoelecron holography in probing atomic and molecular structures and dynamics.

Electron angular correlation in nonsequential double ionization of molecules by counter-rotating two-color circularly polarized fields

Electron correlation in nonsequential double ionization (NSDI) of molecules by counter-rotating two-color circularly polarized (TCCP) fields is investigated with a three-dimensional classical ensemble model. Numerical results indicate that the two electrons from NSDI of molecules in counter-rotating TCCP fields show strong angular correlation and the angular correlation behavior sensitively depends on the internuclear distance. With the internuclear distance increasing, the dominant behavior of electron pairs evolves from correlation to anti-correlation. It leaves a clear imprint on the ion momentum distributions, which exhibit an inverted Y-shape distribution at a small internuclear distance and a triangle-shape distribution at a large internuclear distance. Back analysis indicates that the asymmetric electron energy sharing by soft recollision and longer time delay of double ionization are responsible for more anti-correlated emissions at large internuclear distances.

Frustrated nonsequential double ionization of Ar atoms in counter-rotating two-color circular laser fields

We theoretically investigate the frustrated double ionization (FDI) of Ar atoms with counter-rotating two-color circular (CRTC) laser fields using the three-dimensional (3D) classical ensemble method. Our results show that the FDI probability depends upon the intensity ratio of the CRTC laser fields. The FDI event accompanied with the recollision excitation with subsequent ionization is prevalent and three pathways exist in FDI processes driven by CRTC laser fields. The momentum distribution of a recaptured electron at the ionization time after recollision indicates that the momentum being close to the vector potential is a necessary condition for FDI events to occur. In addition, the recaptured electron most probably transitions to a Rydberg state of which the quantum number is ten in the CRTC fields.

Relative phase effect of nonsequential double ionization of molecules by counter-rotating two-color circularly polarized fields

Relative phase effect of nonsequential double ionization (NSDI) of aligned molecules by counter-rotating two-color circularly polarized (TCCP) fields is investigated with a three-dimensional classical ensemble model. Numerical results show that NSDI yield in counter-rotating TCCP fields sensitively depends on the relative phase of the two components, which exhibits a sin-like behavior with the period of π/2. NSDI yield achieves its maximum at the relative phase π/8 and minimum at 3π/8. Back analysis indicates the recollision time and the return angle of the electron strongly depend on the relative phase of the two components, which results in the dominant emission direction of the electrons, is different for different relative phases. This indicates that the recollision process can be steered by changing the relative phase of the two components in counter-rotating TCCP laser fields. Meantime, it provides an avenue to obtain information about the recollision time and the return angle in the recollision process from the electron momentum distribution.

Enhanced double ionization rate from O2 molecules driven by counter-rotating circularly polarized two-color laser fields

We report that the nonsequential double ionization (NSDI) probability of an O ₂ target can be enhanced greatly in a counter-rotating circularly polarized two-color driving field. The field is composed of a fundamental frequency and its third harmonic, and the combined electric field traces out a four-leaf-clover pattern. The electron ionized by such a field has more chances to collide with the valence electrons in the O ₂ molecule, which significantly enhances the NSDI probability. This effect is more evident in low-intensity fields. We also find that the enhancement appears in a broad range of the field ratio of two colors and that both the NSDI yield and the underlying electronic behavior varies notably with the field ratio.

Nonsequential double ionization by co-rotating two-color circularly polarized laser fields

Nonsequential double ionization (NSDI) of Ar in co-rotating two-color circularly polarized (TCCP) laser fields is investigated with a three-dimensional classical ensemble model. Our numerical results indicate that co-rotating TCCP fields can induce NSDI by recollision process, while the yield is an order of magnitude lower than counter-rotating case. NSDI yield in co-rotating TCCP fields strongly depends on field ratio of the two colors and achieves its maximum at a ratio of 2.4. In co-rotating TCCP fields, the short recollision trajectory with traveling time smaller than one cycle is dominant. Moreover, the recollision time in co-rotating TCCP laser fields depends on the field ratio, which is mapped to the electron momentum distribution. This provides anavenue to obtain information about recollision time and access the subcycle dynamics of the recollision process.

Intensity-dependent two-electron emission dynamics in nonsequential double ionization by counter-rotating two-color circularly polarized laser fields

Nonsequential double ionization of helium in counter-rotating two-color circularly polarized laser fields is investigated with a three-dimensional classical ensemble model. At moderate intensity, the momentum distribution of the two electrons shows a maximum in the middle of each side of the triangle of the negative vector potential. At high intensity, the momentum distribution exhibits a double-triangle structure, which is attributed to the different values of the laser intensity where the two electrons are released after recollision. At low intensity, the momentum distribution shows a shift deviating from the middle of the side of the triangle of the negative vector potential. This is because the first electrons are emitted within a narrow time window after the field maximum. In addition, at low intensity, double-recollision events and NSDI originating from doubly excited states induced by recollision are prevalent.