Above-threshold ionization in few-cycle laser pulses:
Comparison between exact numerical solution and quantum orbit theory

Dieter Bauer, MPI für Kernphysik Heidelberg

D. Bauer, D.B. Milosevic, and W. Becker
As the laser pulse duration approaches the few-cycle regime, angle-resolved photoelectron spectra become strongly dependent on the carrier-envelope phase. Pronounced interference patterns in the rescattering plateau have been attributed to the interference of the relevant electron trajectories that lead to the same final energy. This interpretation in the spirit of Feynman's path integral approach yields an intuitive physical understanding while the actual calculation requires several simplifying assumptions, in particular the neglect of Coulomb effects once the electron is emitted, and a simplified view of the rescattering process.
The numerical solution of the time-dependent Schroedinger equation (TDSE), on the other hand, yields the exact final wave function and thus all observables of interest, often, however, without revealing the dominant physical processes. A window-operator method is proposed that allows to extract energy-resolved wave packet information from the exact wave function, facilitating a sensitive test of quantum orbit theory. The case of atomic hydrogen in linearly and circularly polarized few-cycle pulses is discussed. Photoelectron spectra obtained from the numerical solution of the TDSE are compared with those predicted by the strong field approximation.