Development of Orbitally Allowed Intensity along Vibronic Progressions of Linearly Inactive Doubly Degenerate Jahn-Teller Vibrations in A ↔ E Electronic Spectra

Abstract

We introduce a novel "interstate" vibronic mechanism which may lead to a strong development of orbitally allowed intensity along the 0-0, 0-1, 0-2, 0-3, etc., vibronic progressions of a doubly degenerate (e) Jahn-Teller vibration in the spectra of A ↔ E electronic transitions. The mechanism is operative even in the complete absence of linear Jahn-Teller activity in the radiatively involved doubly degenerate electronic state, designated E(I). It involves a linear Herzberg-Teller vibronic coupling via the pertinent e Jahn-Teller mode connecting E(I) and a second doubly degenerate electronic state, E(II). The E(II) system, in turn, is simultaneously involved in linear J-T activity via the relevant e mode. The novel mechanism is highly effective in formation of well-developed vibronic progressions in the Jahn-Teller vibrations even in the far off-resonance weak-moderate coupling regime. In typical cases, its capability is comparable to that manifested by the classic orbitally allowed A ↔ E ⊗ e J-T spectra, as discussed by Longuet-Higgins et al. The latter spectra ensue via the direct action of the linear Jahn-Teller activity in the radiatively engaged E system. Note that we are only interested in the investigation of the development of orbitally allowed intensity pertinent to the A ↔ E(I) system and not in any contributions of vibronically borrowed forbidden intensity which, in principle, may be transferred to the A ↔ E(I) system via its linear vibronic coupling with the A ↔ E(II) system.