Supermultiplet classification of higher intrashell doubly excited states of H- and He

We compute new estimates of energy levels of doubly excited states with two electrons in the same shell, for all principal quantum numbers N< or =5 and angular momentum values 0< or =L< or =2N-2 in H- and He. We investigate the structure of these intrashell spectra using a recent ''supermultiplet'' approach which originates in the O(4) shell structure of one-electron atoms. We provide new interpretations of the electron correlation underlying the O(4) states and supermultiplets of two-electron atoms; this accounts qualitatively for approximate separability of rotationlike and vibrationlike progressions of levels found in the computed spectra. Certain diamond-shaped patterns of supermultiplet energies are found to contain near degeneracies of levels with ΔL= +- 2; this accounts in part for level clustering we find in the unresolved spectra. Our investigation of scaling of the apparent rotational and vibrational parts of the energy with higher N shows consistency with a simple model of the atom which has electrons on the surface of a spherical shell, with radius Rapprox. =N2. We investigate the model group theoretically, and find two different O(4) groups which describe angular electron correlation in limiting cases of large or small shell radius for each principal quantum number N. A third O(4) group is related to intrashell radiative transitions