Effect of the breathing mode on a strongly coupled E otimes epsilon Jahn-Teller system

The theoretical absorption spectrum of a vibronic system exhibits two main peaks of different height and width, and, at very strong Jahn-Teller energy, other peaks at higher energies (cone resonances or Slonczewski resonances). Tetrameric cations of the group V elements can be pictured as strongly coupled Jahn-Teller systems; however, high resolution photoelectron spectra available in the literature exhibit only two broad spectral bands. To explain the disappearance of the cone resonances, we have considered a two-mode vibronic model, that, besides a strong linear interaction with the mode , also includes a linear interaction with the total symmetric mode (breathing mode); we have also considered separately off-diagonal second-order effects on the Jahn-Teller system. The analysis has been carried out in the framework of the recursion Lanczos technique, properly implemented. We have verified that the Slonczewski resonances are quenched by the breathing mode, even in the case of moderate coupling; we have then compared our model with the available experimental spectra of group V tetrameric clusters.