Transfer of oscillator strength in regions of avoided crossings. Atomic spectra for non-integer values of Z. application to the NI 4S o- 4P sequences

Abstract

We report FOTOS calculations for the oscillator strengths of the N 1s 22s 22p 3 4S 0 → (1s 22s2p 4, 1s 22s 22p 23s) 4P iso-electronic sequences. Except for NeIV, excellent agreement with recent experimental values is found. Since this system exhibits a considerably localized valence-Rydberg (v-R) mixing, we have used it as a model for a study of the behavior of oscillator strengths in situations analogous to avoided crossings of molecular adiabatic curves. The two-state approximation (TSA) explains the observed spectroscopic data reasonably well. Our calculations extend to non-integer values of the nuclear charge for which we obtain Hartree-Fock as well as correlated wavefunctions. The notion introduced here, i.e. the use of non-integer Z to study the behavior of energy levels and oscillator strengths in atomic systems, is theoretically equivalent to the study of Born-Oppenheimer curves in diatomic molecules and constitutes a useful alternative for the rigorous study of avoided crossings.