Relaxation of Vibrational Excitons in Molecular-Ionic

Interest in vibrational relaxation in molecular solids has grown considerably since ultra-short puls ed laser sources made picosecond time-resolved experiments possible. Time resolved stimulated Raman (TRSR)1 and coherent antistokes Raman scattering (CARS)2 experiments are in fact useful techniques for direct measurement of the relaxation time 7: of coherently excited vibrational states. A limited number of internal vibron and external phonon vibrational bands in molecular crystals has been studied by time resolved techniquesš'". Recently, the same kind of information has been obtained in the frequency domain (linewidth 2r and relaxation time 7: are related by 2r = 1/(2nc7:)) by means of high-resolution spontaneous Raman experiments?"!'. Parallel to the experimental work, much effort has been devoted to the development of computational procedures for calculating phonon linewidths or relaxation times on the basis of anharmonic intermolecular potentials, using perturbative methods'v=. In spite of the experimental and theoretical interest of the subject, some relevant aspects of the problem are still open to discussion: i) therelative importance of pure dephasing (T2*) with respect to population decay processes (TI) in determining the observed relaxation times; ii) the role of the different multi-phonon scattering processes responsible for the linebroadening of vibrational transitions in molecular crystals.