Siegert-state expansion in the Kramers-Henneberger frame: Interference substructure of above-threshold ionization peaks in the stabilization regime

The Siegert-state expansion approach is applied to the solution of the time-dependent Schrodinger equation describing a model one-dimensional laser-atom interaction problem in the Kramers-Henneberger frame. Our method is mathematically rigorous and numerically exact even though a very restricted spatial box is considered, since the use of Siegert states as a basis in the expansion eliminates unphysical reflections from the boundary of the box and the Kramers-Henneberger frame enables us to fully take the interaction with the laser field into account. The method is demonstrated by calculations of above-threshold ionization spectra generated by strong high-frequency laser pulses. We found an oscillating substructure of multiphoton peaks caused by an interference of photoelectron wave packets produced at different times during the pulse which becomes especially pronounced in the stabilization regime. An interpretation of this effect in terms of the high-frequency Floquet theory is given.