ROLE OF THE BREATHING MODE IN A STRONGLY COUPLED E-CIRCLE-TIMES-EPSILON JAHN-TELLER SYSTEM

The theoretical absorption spectrum of a vibronic Ex epsilon system ex hibits two main peaks of different height and width, and, at very stro ng Jahn-Teller energy, other peaks at higher energies (cone resonances or Slonczewski resonances). Tetrameric cations of the group V element s (P-4(+), As-4(+), Sb-4(+)) can be pictured as strongly coupled Ex ep silon Jahn-Teller systems; however, high resolution photoelectron spec tra available in the literature exhibit only two broad spectral bands. To explain the disappearance of the cone resonances, we have consider ed a two-mode vibronic model, that, besides a strong linear interactio n with the mode epsilon, also includes a linear interaction with the t otal symmetric mode alpha(1) (breathing mode); we have also considered separately off-diagonal second-order effects on the Ex epsilon Jahn-T eller system. The analysis has been carried out in the framework of th e recursion Lanczos technique, properly implemented. We have verified that the Slonczewski resonances are quenched by the breathing mode, ev en in the case of moderate coupling; we have then compared our Ex(epsi lon + alpha(1)) model with the available experimental spectra of group V tetrameric clusters.