text: A breakthrough has been brought about in the last decades by the development of large scale synchrotron radiation facilities. Here, bunches of relativistic electrons kept in orbit inside a storage ring are accelerated through bending magnets or insertion devices like wigglers and undulators to produce a high brilliance and high flux photon beam. The beam is  orders of magnitude more intense and better collimated than typically produced by anode-based sources. Synchrotron radiation is also tunable over a wide wavelength range, and can be made polarized in several distinct ways. This way, photon can be selected yielding optimum photoionization cross-sections for probing a particular core level. The high photon flux, in addition, makes it possible to perform  XPS experiments also from low density atomic species, such as molecular and atomic adsorbates.
  One of the synchrotron facilities that allows XPS measurement is Max IV synchrotron in Lund, Sweden. The Hippie beam line of this facility also allows to perform in operando Ambient Pressure X-Ray Photoelectron Spectroscopy (AP-XPS9. This latter technique allows to measure samples in ambient conditions, rather than in vacuum.