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 ${\mathrm{He}}^{+}$, including sixteen (sp,2n+) and five (sp,2n-) states in the N=2 series. With a resolving power of E/ΔE≃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.