Correlated electron emission in laser-induced nonsequence double ionization of helium

Abstract

In this paper, we have investigated the correlated electron emission of the nonsequence double ionization (NSDI) in an intense linearly polarized field. The theoretical model we employed is the semiclassical rescattering model, the model atom we used is the helium. We find a significant correlation between magnitude and direction of the momentum of two emission electrons, and give a good explanation for this striking phenomenon by observing the classical collisional trajectories. We argue that this correlation phenomenon is universal in NSDI process, as revealed by the recent experiment on the argon.PACS numbers: 32.80. Rm, 42.50.Hz, The excessive double ionization observed in Helium experiments [1-3] draws much attention to the multipleelectron dynamics in the laser-atom interaction. In these experiments the single ionization of He in a linearly polarized field is accurately predicted by the single active electron (SAE) approximation [2], well described by the Ammosov-Delone-Krainov (ADK) tunnelling theory [4]. However, the case of double ionization is more complicated. In the regime of very high intensities (I > 10 16 W/cm 2 ) where strong double ionization occurs, the double ionization keeps in good agreement with the sequential SAE models as that in the lower intensities regime(I < 10 14 W/cm 2 ). The double ionization deviates seriously from the sequential SAE model and shows a great enhancement in a "knee" regime [(0.8-3.0) × 10 15 W/cm 2 ]. This surprising large yields of the double ionization obviously indicates that the sequential ionization is no longer the dominating process in this regime and the electron-electron correlation has to be taken into account. Intense efforts to model the twoelectron process of the double ionization in a laser field have reproduced the main feature of the knee structure in the double ionization yield as a function of laser peak intensity and, moreover, yielded quantitative agreement with the experiments in some cases [5,6].The physical mechanism behind this nonsequential process is, however, still debatable. Both the "shake-off" * Email: fu libin@mail.iapcm.ac.cn model and the "recollision" model are suggested to describe the electron's correlation [1,3,7,8]. However, none of the two nonsequence double ionization (NSDI) mechanisms can completely explain the experimental observations. For the "shake-off" model, it can not give the reason for the decrease in the double ionization yields as the polarization of the laser field departs from linear [9-11]. In the "recollision" model, the returning electrons are known to have a maximum classical kinetic energy of ∼ 3.2U p (U p = e 2 F 2 /4m e ω 2 ), so one can determine a minimum intensity required for the rescattering electron to have enough energy to excite the inner electron. But the double ionization yields observed in experiments has no such an intensity threshold. In fact, the double ionization process is rather complicated and subtle, both of the two NSDI processes and the sequential ionization have contributions ...

Strongly directed electron emission in non-sequential double ionization of Ne by intense laser pulses

“…Only recent semi-classical calculations [12] were able to predict a similar pattern as observed. There, the first electron is set free with a velocity To further elucidate electron emission in NS double ionization of Ne, electron energy distributions are shown in Fig.2 In comparison, the electron energy distribution in coincidence with double ionization is significantly less steep, extending to much higher energies.…”

“…Only recently, a new generation of differential measurements [3,4] along with various quantum [5][6][7][8][9][10] as well as classical calculations [11,12] provided convincing evidence that the so-called "rescattering" or recollision scenarioone of the three NS double ionization pathways (beyond collective tunneling, and shake-off) which have been proposed in the past -is the dominating mechanism. Here, one electron is ionized via tunneling, accelerated in the oscillating laser field and, under certain conditions, driven back to its parent ion knocking out another electron.…”

“…In contrast, if a more realistic Coulomb form factor is used a quite asymmetric electron momentum sharing is predicted. Applying a three-dimensional semi-classical model Fu et al [12] have obtained a quite symmetric momentum sharing for 1 PW/cm 2 pulses interacting with He atoms with both electrons having most likely similar or even equal momenta along the polarization direction. Fully quantummechanical theoretical treatments solving the three-dimensional Schrödinger equation [9] are only feasible at higher photon frequencies with present computer capabilities.…”

Rescattering of Ultralow-Energy Electrons for Single Ionization of Ne in the Tunneling Regime

“…[9][10][11][12][13][14][15][16]). Surprisingly, however, at PW/cm 2 intensities, where recollision is now being generally accepted [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] to dominate non-sequential double ionization, only few differential electron emission studies can be found in the literature for single ionization by femtosecond pulses in the optical frequency regime [17][18][19][20][21][22]. Here, at typical Keldysh [23] parameters γ = (I P /2U P )½ << 1 (I P : ionization potential; U P = I/(2ω) 2 : ponderomotive potential with I the laser intensity and ω its frequency; atomic units are used throughout), the photoelectron spectra become increasingly smooth and unstructured, whereas they display a confusingly rich structure at lower intensities for γ ≥ 1.…”

“…Fueled by results of novel many-particle imaging techniques [1,4] recent interest in intense laser-atom interaction has increasingly focused to the investigation of many-electron transitions like double and multiple ionization, experimentally [2][3][4][5][6][7][8][9] as well as theoretically (see e.g. [9][10][11][12][13][14][15][16]). Surprisingly, however, at PW/cm 2 intensities, where recollision is now being generally accepted [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] to dominate non-sequential double ionization, only few differential electron emission studies can be found in the literature for single ionization by femtosecond pulses in the optical frequency regime [17][18][19][20][21][22].…”