X-rays from X-ray tube are most common light source for XPS. Another choice is X-rays from synchrotron radiation. X-rays from Ga droplet and free electron laser are less common, but recently emerged promising technologies.
Generation of X-ray in X-ray tube is based on the irradiation of an electron beam to metal targets.
To excite core electrons above the vacuum level, photon wih the energy more than 10 eV is needed.