Photoionization microscopy (PM) is a photoelectron imaging method based on the measurement of the electron current probability density in the case of photoionization of an atom in an external uniform static electric field. The resolution of PM is high enough for observing the quantum oscillatory spatial structure of the outgoing electron flux. We examine the Stark states of the magnesium atom and focus on PM features which are specific to the energy range just above the saddle-point energy. Particularly, the existence of $m$-dependent saddle-point thresholds is clearly demonstrated by following the evolution of the recorded angular distributions with energy. Furthermore, the outer inflection point radii of the radial distributions of the images are generally found to increase with energy monotonically but occasionally abruptly, signaling parabolic channel openings (above which these channels are converted to continua). This observation is in accord with recently reported quantum mechanical PM calculations for hydrogenic systems. More importantly, there are a number of nonmonotonic variations (local maxima) of the outer inflection point. The latter are attributed to the local emergence of an intensity halo at the outer part of the images, a feature relevant to the excitation of quasibound Stark states (resonances). The emergence of the halo for a given resonance is found to depend on the static electric field strength. These observations along with the difficulties in observing resonant effects on the recorded images of nonhydrogenic atoms, such as the medium-size magnesium atom, are discussed in detail.

• Received 20 December 2018

DOI:https://doi.org/10.1103/PhysRevA.99.023428