Classical motion of a photoelectron interacting with its ionic core: Slow photoelectron imaging

Photoelectron imaging spectroscopy relies on the properties of the ballistic trajectories of photoelectrons moving in a homogeneous electric field. In this paper we show that the simple picture of parabolic trajectories is no longer valid when slow photoelectrons produced in the vicinity of the zero-field ionization threshold, and especially just above the saddle-point energy, are concerned. The discussion presented here is based on the exact classical simulation of the trajectories of photoelectrons in the combined Coulomb and Stark field. It is shown that under rather common experimental conditions the influence of the Coulomb interaction on the projection pattern of photoelectrons at large distance is dramatic. In standard photoelectron imaging spectroscopy a single feature corresponds to a given ionization channel, while we show here that the observed features are much more complex for slow photoelectrons. We then discuss the relevance of our classical simulations as compared with wave-packet calculations. Finally, the application of this effect to the precise characterization of near-zero-energy resonance in atoms in external fields and to the dynamics of zero-kinetic-energy electrons is presented.