An electron source for generating a high intensity electron beam, the electrons of which have a limited number of discrete kinetic energies is provided. The electron source which could be used e.g. in an electron impact spectrometer, comprises a monoor di-chromatic light source, e.g., a neon plasma the light of which injected into a gas, preferably a noble gas, in order to generate photo electrons having a limited number of discrete kinetic energies.