A study of autoionizing decay routes of doubly excited states in helium using two-dimensional photoelectron spectroscopy

The autoionizing decay routes of doubly excited, 1Po, Rydberg states in helium have been investigated using two-dimensional photoelectron spectroscopy. Constant ionic state spectra for the He+ (N = 4, 5 and 6) states have been extracted from two-dimensional photoelectron spectra obtained in the photon energy region 75.5-79 eV. A preference was observed for autoionizing decays in which the change in the principle quantum number, N, of the electron which remains bound was minimized. This is in accordance with propensity rules determined from theoretical studies. The energies of the lower members of the most prominent Rydberg series (N, N -2n) converging on each of the He+ (N = 5) and He+ (N = 6) thresholds have been determined. (author)