Two-mode jahn-teller effect in nh⁺ ₃

The two-mode E ⊗ (ε{lunate} + ε{lunate}) Jahn-Teller effect in NH⁺ ₃ is investigated theoretically. Using ab initio coupling constants, it is shown that the inclusion of the interaction between the degenerate stretching and bending vibrational modes is essential for understanding the experimental findings. An effective single-mode Jann-Teller hamiltonian is introduced to simulate the more complicated two-mode problem. Although such a hamiltonian cannot be expected to reproduce the details of the vibrational structure, it is generally found to reproduce the band shape of unresolved spectra most accurately.