Mechanism of the enhancement of some high-lying resonance series in the photoionization spectra of excited helium

Abstract

The hyperspherical close-coupling method is used to compare the detailed structures in the photoionization spectra of He(1 $^{1}$ S) and ${\mathrm{He}}^{\mathrm{*}}$(2 $^{1}$S) between the ${\mathrm{He}}^{+}$(n=4) and ${\mathrm{He}}^{+}$(n=6) thresholds; both the total photoionization cross section and the partial cross sections for the production of ${\mathrm{He}}^{+}$(n) are studied. Some doubly excited resonance series of excited angular modes that are either weak or unobservable in the ground-state spectrum are greatly enhanced in the metastable-state spectrum. This corroborates that previously derived propensity rules do not apply, in fact, to the photoionization of ${\mathrm{He}}^{\mathrm{*}}$(2 $^{1}$S). The dependence of the propensity rules on the initial state is interpreted in terms of the correlation patterns of its wave function in the region of the hyperradius that contributes dominantly to the photoionization cross section. This interpretation suggests a generality of the enhancement of some resonance series in excited-state spectra. Some overlapping resonances that are overlooked in the literature are uncovered and analyzed by using the Gailitis-average procedure.