Photoionization Microscopy on Magnesium Atom and Comparison with Hydrogenic Theory

The experimental technique of photoionization microscopy (PM) allows for the measurement of the slow (meV) electron current probability density in the case of atomic photoionization in the presence of a static electric field. The spatial structure of the electron flux (differential cross section), as imaged on a plane detector perpendicular to the field, exhibits a radial and an angular dependence. Here attention is paid to the radial distribution, obtained after angularly integrating the electron flux. We discuss the energy evolution of this quantity in the form of a two-dimensional contour map with the abscissa being the outgoing electron energy and the ordinate being the radius on the detector. As a working example we employ the experimental map recorded during a PM study of the Mg atom. Near-threshold the map reveals quite complex quantum interference and beating patterns. Many of these patterns are also observed in a theoretical map concerning the hydrogen atom and calculated by employing a non-perturbative quantum mechanical treatment. Similarities between the maps which are of "universal" nature are identified, while the observed differences are attributed to the different target atoms, initial states and excitation schemes. Finally, possible applications and directions of further work are also briefly discussed.