Frequency-domain theory of nonsequential double ionization in intense laser fields based on nonperturbative QED

Abstract

We study nonsequential double ionization (NSDI) processes of an atom by applying the frequency-domain theory based on the nonperturbative quantum electrodynamics. We obtain the transition formulas that describe the NSDI processes caused by the collision ionization (CI) and the collision-excitation ionization (CEI) mechanisms. By analyzing the NSDI results of each above-threshold ionization (ATI) channel, we investigate the contributions to the NSDI from the backward and forward collisions. In particular, for the CI process, the backward collision makes a major contribution to the NSDI probability, whereas for the CEI process, it depends on the characteristics of the laser-atom system: if the energy that the recolliding electron needs to excite a bound electron is much larger than the laser photon energy, such as for the case of helium in this work, the backward collision dominates the contribution; otherwise, the forward collision dominates the contribution. We also discuss the source of interference fringes in the NSDI momentum spectra due to the CI mechanism and find that the fringes can be predicted by using a simple cosine function. This work can be regarded as a development of the frequency-domain theory, which may shed light on the study of multiparticle dynamics in intense laser fields.

Quantum Effects in Double Ionization of Argon below the Threshold Intensity

“…A simplified semiclassical model, which includes the decay of the excited state, is able to reproduce the experimental observations [16]. In addition, a rigorous semianalytic study of the RESI process has also been performed based on the strong-field approximation, which shows a strong dependence of the electron momentum distributions on the bound state involved [17,18]. However, all distributions were found to be equally spread over the four quadrants of the momentum-momentum correlation, which is inconsistent with the experimental observations.…”

Observation of Electron Energy Discretization in Strong Field Double Ionization

“…A similar ATI structure is predicted for nonsequential double ionization. For double ionization, it is, however, the sum energy of both electrons which is expected to show the discretization [2][3][4][5][6][7].…”

“…All quantum models of strong field ionization, on the contrary, predict a discretization of energy in the final state continuum (see, e.g., Refs. [2][3][4][5][6][7]). In a time dependent picture, energy discretization arises from the periodicity of the ionization events in time [6,17].…”

Above-threshold ionization spectra asymmetrically broadened in the extreme-ultraviolet pulse train and infrared laser fields

“…Traditionally the theoretical method by solving the time-dependent Schrödinger equation is considered as timedomain theory, where the temporal evolution of the electron wavepacket is obtained by solving the interaction between an electron and a time-dependent electric field. However, in the frequency-domain theory, the laser field is treated as a quantized field, and the ionization process of an atom in a laser field is obtained by calculating the time-independent transition matrix between two states of the atom-laser system [23,[27][28][29]. When an atom is exposed to a two-color linearly polarized laser field, the Hamiltonian of this atom-laser system can be expressed as H H 0 Ur V, where H 0 −i∇ 2 ∕2 ω 1 N 1 ω 2 N 2 is the energy operator for a free electron-photon laser system; N 1 and N 2 are the photon number operators of the two fields with frequency ω 1 and ω 1 , respectively; Ur is the atomic binding potential; and V is the electron photon interaction operator.…”