Monochromatic X-rays produce electrons from a multiplicity of orbitals,
producing a range of electron kinetic energies. One can analyze the outgoing electron
energy to produce a spectrum. It's a spectrum of Xray photon energy
minus binding energy of the electron, so for a fixed Xray energy, it's
the electron binding energy that is represented.
Over a small range of energies, you can give monochromatic excitation and analyze the output electrons' individual energy. Over a larger range, the
'monochromatic excitation' from a synchrotron source can be swept through
a range of frequencies. There is a significant improvement in Xray
absorption efficiency for electron binding energies at or slightly above the
incident Xray energy, so sweeping the Xray source energy is useful.