The Nature of Autoionized States

Autoionized states are Rydberg states converging to an ionization limit that lies above the lowest ionization threshold. The general formula for the energy of Rydberg states is 

The quantum defect is about 1.0 for user orbitals of molecules composed of atoms between Li and N, since these orbitals penetrate into the molecular core (cf. Mulliken, 1964) and consequently are strongly modified by the presence of inner electrons of the same sa symmetry. The quantum defect is of the order 0.7 for npa and npn orbitals, which are less modified by the core, and nearly zero for ndcr, ndir, and ndS orbitals, which have no precursors in the core. For a given molecule this quantum defect is roughly independent of the ion state to which the Rydberg series converges (see, for example, for O2, Table 1 of Wu, 1987). 

---