Product imaging of positive ions formed by REMPI detection is only one of the areas where charged particle imaging has become useful.  Another area was in the detection of electrons.  The first ideas along these lines seem to have an early history. Demkov et al. were perhaps the first to propose a "photoionization microscope".  They realized that trajectories of an electron emitted from an atom in different directions may intersect again at a large distance from the atom and create an interference pattern.  They proposed building an apparatus to observe the predicted rings.  Blondel et al. eventually realized such a "microscope" and used it to study the photodetachment of Br. It was Helm and co-workers, however, who were the first to create an electron imaging apparatus. The instrument is an improvement on previous photoelectron spectrometers in that it provides information on all energies and all angles of the photoelectrons for each shot of the laser.  Helm and his co-workers have now used this technique to investigate  the ionization of Xe, Ne, H, and Ar.  In more recent examples, Suzuki, Hayden, and Stolow have pioneered the use of femtosecond excitation and ionization to follow excited state dynamics in larger molecules.