Numerical simulation of high-order above-threshold-ionization enhancement in argon

Resonant enhancement of high-order peaks in the photoelectron spectrum of argon at 800 nm is studied by numerical precision integration of the time-dependent Schrödinger equation in the single-electron approximation. It is shown that electrons in the backscattering region of the spectrum are almost exclusively due to resonances. Wave-function analysis shows that there are two types of resonant states: high-angular-momentum states that stay away from the nucleus and mainly decay by emission of low-energy electrons, and states that are located near the polarization axis. These latter states predominantly decay through violent collision with the ionic core, and are responsible for the enhancement of high-energy photoelectrons.