Top image: Four methods of transferring energy. (a) The Franck-Hertz experiment of 1914, which demonstrated that electrons passing through a vapor of mercury atoms could excite the atoms to higher energies. (b) The Latysheff-Leipunsky experiment of 1930 showing the inverse process: excited atoms could transfer some of their energy to passing electrons. (c) The laser process by which an excited atom can be stimulated to emit some of its energy as a photon in concert with a similar photon already moving through the resonant cavity. (d) The PASER process in which excited atoms transfer energy not into creating photons but into accelerating electrons. (Images courtesy AIP)
Bottom image: Experimental PASER setup. Electrons from an accelerator are modulated into short bursts by a co-moving CO2 laser and by the use of a "wiggler" magnet array, through which the electron bursts move. Later the electron microbursts acquire energy from the excited CO2 atoms in a special cavity.