How can a monochromatic X-Ray tube produce a spectrum in XPS? I thought that if we use monochromatic source, we can only get one peak, if it exists.
Because Photoelectric Effect allows only the electron that have the corresponding frequency(energy level) that can be excited.
appreciate your help to rectify my understanding.
 A: Because the X-rays, in the Photoelectron spectroscopy device, excite the electrons from the individual core levels out and as we know the electrons from the individual core levels reside in different energy levels. Thus the amount of energy left after the electrons overcome the individual energy barrier of each level is different since they derive all their energy from a single source (The single source). The equation goes like this
$$KE=h\nu -E_f-E_b$$
Where $E_b$ is the binding energy of every electronic level under consideration and $E_f$ is the Fermi level height of the material under scrutiny with respect to vacuum. Since $E_b$ is different for different electronic levels, hence $KE$ of the electrons are different which result in different peaks.
A: 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.
