Molecular Rydberg states transition intensities

The tetrahedral sulfate ion (Td) exhibits two unoccupied antibonding molecular orbitals ai and h of 3s and 3p character, respectively (Sekiyama et al. 1986 Tyson et al. 1989 Hawthorne et al., 2000 Myneni 2000). The 15— ai transitions are dipole-forbidden (because of the 5-character of a orbitals), and the intense features in the NEXAFS spectra of sulfate correspond to the 15—orbitals (Fig. 27). The high-energy features above these intense bound state transitions correspond to the Rydberg-type transitions of 3d character or continuum state transitions (Table 4—in Appendix). 

Transitions in which the transition moment is at right angles to the internuclear axis are Q — N and the bands typically have low intensities. In molecular orbital terminology these are t - n transitions (9). Both V - N and Q - N transitions occur without a change in the principal quantum number and are valence shell transitions in contrast to Rydberg transitions, R - N where a change of the principal quantum number does occur. The symbol I describes a triplet state formed in a valence shell transition. 

In Tables 5 and 6 we compare the spectral intensities, in the form of oscillator strengths, for 3s-3p and 3p-3d transitions (in the united atom notation) between states of the same and different molecular symmetries within each corresponding molecular point group, in the Rydberg radicals CH5, H3O, H2F, and NeH, all ofwhich have sodium as the united atom limit. 

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