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 matrial 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.

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.