The present invention relates to a method and a device for irradiating ions in a ion cyclotron resonance (ICR) trap with photons and/or electrons. For electron irradn. a hollow electron emitter is used, through the hole of which a light beam can be sent into the ICR trap. The emitter generates a hollow, tubular electron beam. In a special application low energy ions within the tubular electron beam are irradiated with photons. The ions can be cyclotron-excited mass selectively, by which they enter the electron beam and interact with electrons. [on SciFinder (R)]