The low lying states of ammonia; generalized valence bond and configuration interaction studies

The nine lower singlet and triplet states of ammonia were studied using ab initio generalized valence bond (GVB) and configuration interaction (CI) wavefunctions. The eight lowest (vertical) excited states were found to be Rydberg in nature. Vertical excitation energies were determined to be (in eV): 6.14 and 6.37 for n → 3s; 7.86 and 7.88 for n → 3p_x,y, and 7.87 and 8.15 for n → 3p_z (in each case the triplet energy is first). These results are in excellent agreement (0.06 eV average error) with the observed electron impact excitation energies (in eV), 6.39 (A ¹A″₂), 7.93 (B ¹E″), and 8.26 C ¹A′₁). The small singlet triplet energy splittings are consistent with those expected for Rydberg-like excited states. We find no vertical transition consistent with the small peak at 4.4 eV apparently observed in threshold electron impact experiments.