Source: http://aoot.osa.org/josab/abstract.cfm?uri=josab-36-4-1076
Timestamp: 2019-04-21 16:07:56+00:00

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Using the semiclassical ensemble model, we investigated the correlated electron dynamics in the nonsequential double ionization (NSDI) of Ar driven by a few-cycle laser pulse at a fixed ponderomotive energy Up=0.2 a.u. and over a wide range of wavelength. When the laser wavelength increases from near-infrared to mid-infrared, the kinetic energy of the retuning electrons and the energy transfer efficiency of recollision both decrease and more NSDI events occur in the second half optical cycle after recollision, which leads to the switching of the electron momentum correlation along the laser polarization direction from side-by-side to back-to-back dominant emission. Our results also show that a significant part of NSDI events undergo a “hard recollision” through which the struck electron acquires more energy and ionizes earlier than the returning electron. This type of NSDI event is responsible for the high momentum part, especially the part with momentum higher than 2Up in the correlated electron momentum spectra along the laser polarization direction.
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Fig. 1. CEP-averaged correlated electron momentum spectra along the laser polarization direction for the NSDI of Ar driven by (a) 800 nm, (b) 1400 nm, and (c) 2000 nm five-cycle laser pulses. The ponderomotive potential of each laser pulse is fixed at Up=0.2 a.u.
Fig. 2. Normalized time delay (Δτ) distribution between double ionization and recollision for the NSDI events in Fig. 1.
Fig. 3. Time evolution of the energy of the returning (red solid line) and struck (blue dotted line) electrons for three illustrative classical trajectories of the NSDI at 800 nm laser wavelength. Δτ1,Δτ2, and Δτ3 denote the time delay between double ionization and recollision for the three trajectories. The brown dashed line expresses the electric field of the laser pulse.
Fig. 4. Separated correlated electron momentum spectra along the laser polarization direction for the NSDI in Fig. 1. The time delay between the double ionization and recollision is in the range of Δτ<0.5T (upper row) and Δτ>0.5T (bottom row) and the wavelengths of the laser fields from the left column to the right column are 800 nm, 1400 nm, and 2000 nm, respectively.
Fig. 5. (a) Kinetic energy distributions of the returning electron at 0.03T before recollision for the NSDI events in Fig. 1(a) (red dots), Fig. 1(b) (blue circles), and Fig. 1(c) (green squares). The data in each curve have been normalized so that the maximal value is 1. (b) Time evolution of the kinetic energy of a tunneling electron form Ar in a linearly polarized monochromatic light wave field. The blue solid and the yellow dashed lines correspond to the tunneling electron only driven by 800 and 2000 nm light field, while the red solid (800 nm) and purple dashed (2000 nm) lines include the Coulomb attraction from the parent ion. Here, only the movement along the polarization direction is considered, and the electrons all are released at 0.2972T, i.e., 17° after the first peak of the light wave field. The ponderomotive energies of the 800 and 2000 nm light wave fields both are 0.2 a.u. and the black circles denote the moment 0.03T before the recollision.
Fig. 6. Correlated electron momentum spectra along the laser polarization direction for NSDI of Ar without considering the identity of the two ionized electrons. Here, Px1 and Px2 denote the momenta along the laser polarization direction of the returning and struck electrons, and the wavelengths of the laser fields are (a) 800 nm, (b) 1400 nm, and (c) 2000 nm, respectively. Other parameters are the same as Fig. 1.
Fig. 7. Energy distributions of the returning and struck electrons at 0.03T after recollision for all NSDI events (a) and the NSDI events with one electron where momentum along the laser polarization direction is more than 2Up (b) in Fig. 1(c).
Fig. 8. (a), (b), and (c) Distributions of the correlated electron ionization time delay between recollision and final ionizations of the returning and struck electrons for the NSDI events with one electron where momentum along the laser polarization direction is more than 2Up in Figs. 1(a), 1(b), and 1(c), respectively. Here, Δτ1 and Δτ2 denote the ionization time delays of the returning and struck electrons, respectively.

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