Radiative decay of doubly excited states in helium below the He+ (N=2) ionization threshold

The radiative decay of P-1(0) doubly excited states in helium has been inve stigated using a novel apparatus in which metastable atoms and vacuum ultra violet photons are detected. The intensity ratio of the energetically narro w (sp,2n-)P-1(0) and (2p, nd)P-1(0) series to the broader (sp,2n+)P-1(0) se ries is strikingly enhanced in comparison with conventional photoabsorption , photoion, or photoelectron measurements using synchrotron radiation. The experimental approach is a new way forward for the study of energetically n arrow doubly excited states.