Le, Anh-ThuMorishita, ToruTong, X.-M.Lin, C. D.2023-12-072023-12-072005-09-30https://hdl.handle.net/2097/43686We examined nearest-neighbor spacing (NNS) statistics in doubly excited states of helium near the double ionization threshold. Using the Brody parameter q to measure the NNS distribution between the regular Poisson distribution (q=0) and the chaotic Wigner distribution (q=1), we showed that for levels near the N=20 threshold of He+, or at about 0.13eV below the double ionization threshold, the NNS distribution has q=0.66. The result shows the slow approach of the NNS of helium energy levels towards the Wigner distribution vs the excitation energy. Using an s-wave model where the angular momentum of each electron is restricted to zero, we also examined the NNS for levels up to the N=30 threshold of He+. We showed the gradual increase in q as the excitation energy is increased. To generate the theoretical data needed for the NNS analysis, we have used the hyperspherical close-coupling method, with the recently proposed diabatization of potential curves and the truncation of channels, to greatly reduce the complexity in the calculation. We also investigated the dependence of q vs the nuclear charge in the same scaled energy region, and for different symmetries, to assess their relation with the rate of approaching the q=1 Wigner distribution.© American Physical Society (APS). This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s).Text