Vibronic coupling in dimer – A convenient approximation revisited

Analysis of the S2←S0 vibronic spectrum of the ortho-cyanophenol dimer using a multimode vibronic coupling approach

“…The electronic excitonic coupling V AB is assumed to be independent of the nuclear coordinate. The corresponding Hamiltonian for the simplified case of a single pair of modes is given by 14,15,30 …”

Polariton Effects in Molecular Crystals Studied by Classical and Quantum-Electrodynamic Approaches

“…It is readily seen that virtually all the intensity transfer to high energy regions is already present in the excitonic spectra; hence, it cannot result from exciton-photon coupling. In fact, it is a consequence of vibronic interactions, merely approaching the standard Born-Oppenheimer result [9]; this was only to be expected for large transition dipoles where the exciton-resonance interaction dominates over vibronic coupling. The true polaritonic effects (revealed by com- paring the broken curve with the dotted curve) turn out to be rather small even for the strongest transitions considered here; they just give rise to some broadening of the absorption signal and to a minor blue shift.…”

“…It also bridges the gap between the dielectric and Hamiltonian formulation of exciton theory [14], enabling one to solve the problem by standard methods of matrix diagonalization. Although this is most easily done in the Heitler-London approximation which introduces some inherent errors, these errors are of quantitative rather than qualitative nature, in contrast to the strong--vibronic-coupling approximation where even the qualitative picture is substantially distorted [9]. Moreover, the errors may be partially corrected by perturbation theory.…”

“…Yet, the observed vibronic absorption spectra still turned out to exhibit some peculiar features that might potentially be attributable to polaritons, the most dramatic being a massive intensity transfer to higher energies (with respect to the spectrum of the isolated molecule). A recent study [9] shows that the effect is of vibronic provenance: owing to the strong resonance interactions characteristic of intense transitions, oligothiophenes represent the case of strong intermolecular (weak vibronic) coupling. The blue shift of the dominant absorption band merely reconstitutes the result that is standard in the situation approaching the adiabatic Born-Oppenheimer limit, and seems to be puzzling only as long as the interpretation is couched in terms of the strong-vibronic-coupling basis set, evidently inapplicable in this context but traditionally used in molecular crystal spectroscopy.…”

“…(1) couples the states with different number of phonons, it is tantamount to the modified strong-coupling (MSC) approximation of Ref. [9].…”