VIBRONIC FISSION INTO 2-PARTICLE STATES AND OPTICAL-ABSORPTION PROFILES IN MOLECULAR-CRYSTALS

We account for the absorption continua, observed in the spectra of molecular crystals, by analysing the coupling of the optically excited collective state (the vibron) to the continuum of 2-particle states (one exciton and one intramolecular vibration). We assume the lattice to be rigid. In the adiabatic case, where electronic intermolecular exchange interactions are small compared to the intramolecular vibration energy, we propose an exact derivation of the optical absorption. Numerical calculations, performed for three types (3D, 2D, 1D) of simple lattices, allowed us :(i) to investigate respective linear and quadratic effects on the absorption profile; (ii) to account, by strong fission mechanisms, for stepwise absorption thresholds and broad vibronic resonances characterizing subpicosecond lifetimes. Comparison with experimental data recorded in our laboratory show that fission may be the dominant relaxation channel of the absorbed energy.