  The photon-ion merged-beams technique for the photoionization of mass/charge
selected ionized atoms, molecules and clusters by x-rays from synchrotron
radiation sources is introduced. Examples for photoionization of atomic ions
are discussed by going from outer-shell ionization of simple few-electron
systems to inner-shell ionization of complex many-electron ions. Fundamental
ionization mechanisms are elucidated and the importance of the results for
applications in astrophysics and plasma physics is pointed out. Finally, the
unique capabilities of the photon-ion merged-beams technique for the study of
photoabsorption by nanoparticles are demonstrated by the example of endohedral
fullerene ions.
