Coherent interaction of a beam of particles with matter

It is shown that the collective interaction of an ordered beam of particles with matter can give rise to coherent resonance effects (excitation, stimulated emission, ionization, etc.) if one of the eigenfrequencies of the beam, associated with its periodic structure, is equal to the frequency of a quantum transition in matter. The energy transferred to matter by one particle in the beam may be much less than the energy of this quantum transition.