When a beam of electrons of energy $\mathrm{E}_0$ is incident on a metal surface kept in vacuum or evacuated chamber so electrons can be emitted with maximum energy $\mathrm{E}_0$ (due to elastic collision) and with any energy less than $\mathrm{E}_0$, when part of incident energy of electron is used in liberating the electrons from the surface of metal. So maximum energy of emitted electrons can be $\mathrm{E}_0$.