Vibrational relaxation in molecular crystals

Abstract

The relaxation processes responsible for the finite lifetime of excited vibrational states in molecular crystals are discussed on the basis of recent experimental and theoretical developments. Experimental information on the lifetime of optically active phonons is obtained in the time domain by time-resolved CARS spectroscopy or in the frequency domain by high-resolution infrared and Raman spectroscopy. The experimental results are then interpreted in terms of many-body perturbation techniques or of molecular dynamics simulation.

The elementary mechanisms involved in vibrational energy relaxation processes are discussed in detail. These can be summarized as: (a) depopulation processes of phonon states via energy transfer to other phonon states; (b) pure dephasing processes due to interaction with thermal bath phonons; and (c) depopulation or scattering processes due to impurities and crystal defects.

The contributions to the phonon lifetime and band profile of these different mechanisms are analysed. The available experimental evidence is critically reviewed for lattice phonons and for internal vibrons in separate sections. For internal vibrons the specific case of the relaxation of two-phonon states is considered and different practical situations, ranging from the occurrence of sharp bound states outside the two-phonon continuum to the complete amalgamation of resonances in the continuum, are discussed.