Nonadiabaticity of electron-tunneling-ionization processes in elliptical strong laser fields

We theoretically investigate the electron-tunneling process for a helium atom irradiated by an elliptical strong laser field. The momentum distribution for an electron ionized during the cycle when the laser intensity reaches its maximum is captured, such that we can ignore the interference between the wave packets ionized in different laser cycles and precisely determine the center of the momentum distribution. The quantum mechanical prediction of the center position is further compared to the semiclassical single-trajectory simulation as well as the experimental data. We find that the electron momentums along the minor axis of the laser polarization show good agreement with the nonadiabatic semiclassical calculation for a wide range of laser intensities, indicating the existence of a nonzero lateral momentum when the electron exits the barrier. On the other hand, the offset angles obtained by our quantum mechanical approach for different laser intensities are larger than the nonadiabatic semiclassical results, indicating the importance of the quantum effects during the electron's under-the-barrier dynamics.

Figure 1

(a) Illustration of the electric vector of the laser field. (b)–(d) The photoelectron momentum distribution in the $x-y$ polarization plane with $p_z=0$. In (b), the momentum distribution refers to an electron ionized during the whole laser pulse. In (c) and (d), the momentum distributions are for the electron ionized near the laser peak, with the centers of the time window equal to $0.25T$ and $0.5T$, respectively.

Figure 2

(a) The final momentum along the minor axis of the laser field and (b) the offset angle. The blue cycles are the TDSE results obtained from the PMD for an electron ionized during the peak laser cycle. The black and the red solid lines are the analytical predictions of the adiabatic and the nonadiabatic models, respectively, and the dashed lines are the corresponding SCST results. In (b), the experimental results with the adiabatic (green squares) or nonadiabatic (magenta squares) laser-intensity calibration are from Ref. [13].