Electronic structure and the mechanism of autoionization for doubly excited states

Abstract

Apart from pure phenomenology, the rigorous and quantitative study of many-electron autoionizing states presents intriguing questions as regards their structure and dynamics. In this paper we present an analysis of such states within astate specific theory with application to five low-lying doubly excited states (DES) of He. The zeroth order description is multiconfigurational and is obtained numerically at the MCHF level. In this way, major radial and angular correlations are accounted for accurately, and reliable predictions can be made without the requirement of large computations. The additionallocalized correlation is obtained by optimizing variationally analytic virtual orbitals. Core orbitals corresponding to the open channels are projected out of the MCHF solutions, as well as out of the correlation functions. Theasymptotic correlation, which gives rise to autoionization, is computed from a multichannel reaction matrix approach. It is shown that when calculating the width, the Rydberg series should not be part of the localized correlation if the frozen-core Hartree-Fock scheme is used for the generation of bound and scattering orbitals of each channel. Our calculations of the width of the first three1 S DES of He include the effects of coupling of resonances via the continuum. The wavefunctions for the 2s 2 and 2p 2 1 S DES, together with a Hartree-Fock scattering orbital in the field of He+ 1s are employed for the calculation of two quantities introduced by Rehmus, Ezra and Berry to the study of doubly excited states of He. The “conditional probability density”, ρ, and the “differential transition amplitude”,D. As regards ρ, our approach yields results which agree with those of Rehmus et al. However, as regardsD our results contradict their conclusions. In particular, we are forced to disagree with their proposal about the “generalizedS N − 2 mechanism” for atomic autoionization. Instead, we propose a mechanism which is based on the interelectronic repulsion and the heavy overlap of bound and scattering orbitals in small regions of space. This mechanism is in harmony with the generally observed slower rates of autoionization for triplets as compared to those of singlet states of the same orbital angular momentum.