Spin-flip processes and nondipole effects in above-threshold ionization of hydrogen in ultrastrong laser fields

Abstract

We consider above-threshold ionization of hydrogen atoms in ultrastrong laser fields. We use a time-dependent Dirac equation as a calculational tool. This framework allows one to include relativistic effects such as nondipole effects, effects of the relativistic kinematics, and electron spin effects. Inclusion of the spin effects allows one to consider the spin-flip process accompanying above-threshold ionization. We present and discuss electron momenta distributions for ionization processes with and without spin flips. Electron momenta distributions for the ionization process without a spin flip show a gradual increase of the role of the nondipole effects with increasing electric field. Electron spectra for the spin-flip ionization exhibit cusplike singularities, and the absence of features (such as the presence of dips) in the case of the ionization without a spin flip. We explain these features by invoking perturbation theory arguments.

Distribution of absorbed photons in the tunneling ionization process

“…A strong spin asymmetry is present in the spectra of the above threshold ionization process (ATI) 16 , 29 . The free-electron lasers (FEL) 12 – 14 offer a possibility of the experimental study of the relativistic effects in the domain of high photon energies, where the non-dipole effects due to finite photon momentum can be expected to be particularly important 30 , 31 .…”

“…Alternatively, for not very high field strengths, one can use perturbative approach, by adding the terms describing relativistic interactions to the non-relativistic Hamiltonian 23 , 24 and solving the resulting TDSE numerically. Treatment of higher field strengths, where electron acquires relativistic velocities, but electromagnetic fields can can still be considered classically, is possible with the use of the approaches based on the numerical solution of the time-dependent Dirac equation (TDDE) 31 – 34 .…”

Relativistic effects in photoionizing a circular Rydberg state in the optical regime

“…Numerical solutions of the time-dependent Dirac equation in full dimensionality beyond the dipole approximation have been reported for atoms exposed to fields in the x-ray and extreme ultraviolet regions (see, e.g., Refs. [3,9,10]) and in the optical and infrared regions (see, e.g., Refs. [11][12][13]).…”