Observation of the (2p,nd)1Po double-excitation Rydberg series of helium.

Abstract

Double-excitation Rydberg resonances of He below the {ital N}=2 threshold of He{sup +} were studied with an energy resolution of {congruent}4 meV by photoionization using synchrotron radiation. All three Rydberg series of {sup 1}{ital P{ital o}} resonances accessible by photoabsorption, including the hitherto missing'' (2{ital p},{ital nd}){sup 1}{ital P{ital o}} series, were resolved. This settles a long-standing discrepancy between experiment and theoretical predictions and establishes a firm basis for testing the accuracy of atomic multiconfiguration calculations.