The observation of “forbidden” transitions in He II photoelectron spectra

Anomalous bands are reported in the He II spectra of diatomic and triatomic molecules which are attributed to excitation of a second electron along with the primary ionization. This dual process is made possible by the mixing of ionized states of the same symmetry but different configurations i.e. by configuration interaction between ionized states. The close relationship between the observed bands and those reported in X-ray spectra which result from the so-called shake-up process is discussed.