Abstract: A device and method for obtaining high resolution of electron energy in an electron beam is described. The device has a resonance chamber containing a gas which exhibits a narrow scattering resonance at a specific electron energy value. The device utilizes the narrow resonance property of the gas to filter the electron energy spectrum at that energy value. A preferred embodiment is a spectrometer having an electron accelerator, an electromagnetic filter, a resonance chamber containing helium, and a trapped electron detector device. The electrons in the beam are accelerated and the beam is passed through an electromagnetic filter centered at approximately 20.614 eV. The filtered beam passes into a resonance chamber where the electrons have inelastic collisions with the helium atoms to produce the He2.sup.1 S excited state. The He2.sup.1 S scattering resonance has a narrow width of less than 0.001 eV at its energy threshold of 20.614 eV and serves as a filter.