Three-electron radiative transitions

A new type of many-electron radiative transitions involving three electrons is predicted. The results of the authors' investigation by many-body perturbation theory are presented. Spectral lines observed in the wavelength range of 37.5 to 54.0 nm by means of photon-induced fluorescence spectroscopy (PIFS) following the excitation of the Kr I 3d-1np resonances are reported and compared with the predictions.