The first attempts to use photoionization microscopy were performed on atoms of Xenon by a team of Dutch researchers in 2001. The differences between direct and indirect ionization create different trajectories for the outbound electron. Direct ionization corresponds to electrons ejected down-field towards the bottleneck in the Coulomb + dc electric field potential, whereas indirect ionization corresponds to electrons ejected away from the bottleneck in the Coulomb + dc electric field and only ionize upon further Coulomb interactions. These trajectories produce a distinct pattern that can be detected by a two-dimensional flux detector and subsequently imaged. The images exhibit an outer ring that correspond to the indirect ionization process and an inner ring, which correspond to the direct ionization process. This oscillatory pattern can be interpreted as interference among the trajectories of the electrons moving from the atom to the detector.