High-resolution study of1Podouble-excitation states in helium

We describe extensive results on the ${}_{}{}^1{}_{}{}^{}$ ${\mathit{P}}^{\mathit{o}}$ double-excitation resonances of He below the N=2–9 ionization thresholds obtained recently by high-resolution photoionization measurements using synchrotron radiation from the SX700/II monochromator at the Berliner Elektronenspeicherring für Synchrotronstrahlung (BESSY). The resonance energies, linewidths, Fano-q parameters, and quantum defects of the various Rydberg resonances, derived from the experimental data, are compared in detail with the results of ab initio calculations employing the complex-rotation method. All three optically allowed Rydberg series below the N=2 ionization threshold were identified. For 3≤N≤8, the two most intense Rydberg series were resolved below each threshold, and for N=3, one state of a third series could be identified. For the higher Rydberg series (N≥5), the two neighboring series, N and N+1, generally interfere, which has a strong influence on the transition intensities, linewidths, and quantum defects of the principal series. All together, states of 18 different Rydberg series in the ${}_{}{}^1{}_{}{}^{}$ ${\mathit{P}}^{\mathit{o}}$ double-excitation resonance region of He were observed. © 1996 The American Physical Society.