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A theoretical investigation of the excited electronic states of the equilateral triangular H3+ ion is reported. Ab initio calculations were performed for the lowest energy states of symmetries A1′3,A2″1,A2″3,E′1, and E′3, using the single-center expansion configuration-interaction method. For each state we have obtained the potential-energy curve in D3h symmetry. The calculated state energies are expected to be in error by at most 0.01 hartree (6 kcal/mole). Two of the excited states E′1 and E′3 were found to be purely repulsive with respect to the totally symmetric nuclear coordinate, while three excited states A2″1,A2″3, and A1′3 showed minima with respect to the totally symmetric nuclear coordinate. However, arguments based on group-theoretical correlation between our calculated D3h state energies and the energies of possible dissociation products suggest that none of the excited states considered achieves an absolute energy minimum in D3h symmetry. For the allowed electric dipole transitions A2″1 ← A1'1 and E'1 ← A1'1 we have calculated the vertical (Franck–Condon) transition energies, oscillator strengths, and polarizations. The nature of the excited-state energy curves, at the ground-state equilibrium geometry, indicates that in every case only continuum absorption from the A1'1 ground state would be observed experimentally.
Kawaoka et al. (Sat,) studied this question.