Observation of the (2p,nd)1Podouble-excitation Rydberg series of helium

Abstract

Double-excitation Rydberg resonances of He below the N=2 threshold of ${\mathrm{He}}^{+}$ were studied with an energy resolution of ≃4 meV by photoionization using synchrotron radiation. All three Rydberg series of ${}_{}{}^1{}_{}{}^{}$ ${\mathit{P}}^{\mathit{o}}$ resonances accessible by photoabsorption, including the hitherto ‘‘missing’’ (2p,nd${)}^1$ ${\mathit{P}}^{\mathit{o}}$ 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.