Nonsequential Double Ionization Of Atom By Strong-field Laser Pulse

Nonsequential double ionization (NSDI) of atoms in a strong laser field is one of the fundamental and important nonlinear optical phenomena in the interaction between laser pulses and matter because it provides a most simple and efficient manner to study the electron-electron correlation, which is responsible for the structure and the evolution of large parts of our macroscopic world. In addition, the study of NSDI makes it possible to observed the movement of electrons in attosecond time scale, which provides an unprecedented condition for the development and improvement of the quantum theory. We notice that up to now not much attention has been paid to the process of NSDI with longer wavelengths, especially in the mid-IR regime. NSDI with near-IR wavelengths has been known to provide an excellent, probably also the simplest, platform for the study of electron correlation effects. The questions that we were curious about and trying to answer in this paper include:What do recollision and NSDI processes look like with mid-IR laser fields? Is there any substantially new physics? From the semiclassical recollision scenario, it is obvious to see that recollision and NSDI depend critically on the polarization of the external laser field. If the laser field is elliptically or circularly polarized, then the probability of recollision is greatly reduced or even completely eliminated because the additional field along the transverse direction steers away the first-emitted electron, reducing or eliminating its chance of returning to the parent ion core. This point was indeed supported by some early experiments with elliptical polarization on rare gas targets. However, and interestingly, later experiments reported characteristic NSDI events with atomic and with some molecular even with circularly polarized laser fields, reinitiating interest to this once-believed-settled topic. The contents and the achievements are as following:(1) We present a theoretical study of atomic NSDI with mid-infrared laser fields, and compare with results from near-infrared laser fields using a classical ensemble method. Unlike single-electron strong-field processes, double ionization shows complex and unexpected interplays between the returning electron and its parent ion core. As a result of these interplays, NSDI for mid-IR fields is dominated by second-returning electron trajectories, instead of first-returning trajectories for near-IR fields. Some complex NSDI channels commonly happen with near-IR fields, such as the recollision-excitation-with-subsequent-ionization (RESI) channel, are virtually shut down by mid-IR fields. Besides, the final energies of the two electrons can be extremely unequal, leading to novel e-e momentum correlation spectra that can be measured experimentally.(2) We reported that the asymmetry energy sharing (AES) between two electron at recollision is a universal phenomenon in NSDI of atom by mid-infrared laser pulse. We also find that AES strongly depend on the laser intensity and as a result, the correlated electron momentum spectrum along the laser polarization plane strongly depend on the laser intensity. At the relatively low intensity, the momentum spectrum shows an obscure V-like shape due to a relatively weak AES. As the intensity increases, the momentum spectrum displays an obviously V-like structure due to the fact that the AES is relatively strong. When the intensity further increases, the momentum spectrum exhibits a cross-like structure becouse of much strong AES. This phenomenon indicates that AES can be controled through changing of the laser intensity, which is not realized if the pulse is near-infrared. More important, this result can be confirmed by measure of the 3-D moerantum spectrum with cold target recoil ion momentum spectrum experimentally.(3) We propose two schemes which can be used to identify the contributions of short trajectory and long trajectory for the future experimence. First scheme, the contributions of short and long trajectories can be identified according to the difference of the carrier-envelope-phase (CEP) dependences of NSDI with few-cycle linearly and elliptically laser pulses. This difference is due to the different contribution of short and long trajectories by the two laser pulses, so this method can be used to identify the short trajectory and long trajectory in NSDI. The second scheme, one can measure the CEP dependence of correlated electron momentum spectrum using the few-cycle elliptically laser pulses and with different time durations. Through numerical calculation, we find when the time duration is relatively short, the correlated electron momentum spectrum is mainly distributed in the third quadrant. When the time durantion is relatively long, the correlated electron momentum spectrum is mainly distributed in the first quadrant. It means that this method can be also used to identify the contribution of short and long trajectories for the future experimence. More than that, the two schemes that we proposed here is relatively easy to realize in the experiment and as a result, it provides important way to study the microscopic dynamics processes of correlated electron for people.(4) The NSDI of atom is revisited by elliptically polarized few-cycle laser pulse with the classical ensemble method. We focus on the events that both electrons emit into the same direction along the long and short axis of the laser polarization plane, and how do the correlated electron momentum spectra of these two events depends on the CEP. We first exhibit that the double-ionization probability has a negligible dependence on CEP. Back analysis shows that the ionization dynamics of the second electron are strongly depend on the CEP, which is significantly responsible for the CEP-dependent correlated electron momentum spectra. Besides, the correlated electron momentum spectrum along the long axis of the laser polarization plane reproduces the so-called V-like structure observed in experiments. We sort the V-like shape into two regions and find that the different regions exhibit significantly different dynamics behaviors. Simultaneously, we demonstrate that the electron pairs emitted into the same direction along the short axis of the laser polarization plane is a result of the nuclear-electron attraction, and both the nuclear-electron attraction and e-e repulsion significantly contribute to the V-like structure.