EXTENSIVE DOUBLE-EXCITATION STATES IN ATOMIC HELIUM

High-resolution photoionization studies of He have revealed more than 50 states below the N = 2-7 thresholds of He+, including sixteen (sp,2n+) and five (sp,2n-) states in the N = 2 series. With a resolving power of E/DELTA-E congruent-to 10 000, states as narrow as 0.1 meV could be observed and linewidths were determined with an accuracy up to +/- 0.5 meV. Interchannel interferences, evident through effects on positions, shapes, and intensities of Rydberg lines, were interpreted within the framework of the multichannel quantum-defect theory.