Observation of quasimonochromatic radiation in the vacuum ultraviolet region generated by $5.7$-MeV electrons in a multilayer mirror

Angular distributions of radiation in the vacuum ultraviolet region with a photon energy of about $70$ eV, which have been measured for several angles of incidence of $5.7$-MeV electrons on the surface of a periodic Mo/Si nanostructure of a multilayer mirror, have been analyzed. It has been shown experimentally that vacuum ultraviolet radiation in the direction of backward transition radiation has a quasimonochromatic component associated with coherent radiation of the periodic structure of the target.