Electronic structure and oscillator strengths of highly excited states: Resonances in He−, Li, and Be

The calculation of energy levels and widths of certain highly excited states in atoms and molecules is an interesting theoretical problem. This is due primarily to peculiarities of their wavefunctions which result from the (near) degeneracies of such states with Rydberg and continuum series. In this paper we: (a) discuss certain aspects of this problem related to the computation of compact excited-state wavefunctions. The BeI″1s22p2″ 1S and NI 1s22s2p4 4P states serve as examples; (b) present results of a variational treatment of the positions of the 2s22p and ″2p3″ 2P0 resonances in He−(57.3 eV, 60.5 eV) and Li (141.7 eV, 148.5 eV) and an estimate for the He−″2s2p3d″ 2P0 resonances {[(2s2p) 3P03d] 2P0: 61.9 eV, [(2s2p) 1P03d] 2P0: 63.1 eV}. The 2s22p 2P0 levels are in good agreement with recent experimental values (57.2 eV, ∼141.5 eV). A many-body calculation of the Li 1s22s 2S→2P0 two and three electron excitation oscillator strengths yields f2s22p=0.00018, f2p 3=0.0007. These calculations were carried out according to our recently proposed first-order theory of oscillator strengths (FOTOS).