High-resolution study of 1Po double-excitation states in helium.

Abstract

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