  The Photon-Ion Spectrometer at PETRA III - in short, PIPE - is a permanently
installed user facility at the "Variable Polarization XUV Beamline" P04 of the
synchrotron light source PETRA III operated by DESY in Hamburg, Germany. The
careful design of the PIPE ion-optics in combination with the record-high
photon flux at P04 has lead to a breakthrough in experimental studies of photon
interactions with ionized small quantum systems. This short review provides an
overview over the published scientific results from photon-ion merged-beams
experiments at PIPE that were obtained since the start of P04 operations in
2013. The topics covered comprise photoionization of ions of astrophysical
relevance, quantitative studies of multi-electron processes upon inner-shell
photoexcitation and photoionization of negative and positive atomic ions,
precision spectroscopy of photoionization resonances, photoionization and
photofragmentation of molecular ions and of endohedral fullerene ions.
