Interferences in photoexcited double-excitation series of He

The double-excitation ${}_{}{}^1{}_{}{}^{}$ ${\mathit{P}}^{\mathit{o}}$ Rydberg series of He, converging to the N=4 to 9 thresholds, were studied by high-resolution photoionization using synchrotron radiation. Both the principal series with correlation quantum numbers K=N-2 and the secondary series with K=N-4 could be resolved up to previously unobtained n values. This allows a substantially improved analysis of the series with respect to energies and linewidths, leading to an improved understanding of the interseries interferences. The data are in excellent agreement with calculations based on the complex-rotation method.