Vibrational-Relaxation And Dephasing Of Fermi Resonance States In Molecular-Crystals

The vibrational relaxation of Fermi resonance bound states in molecular crystals is discussed. The anharmonic Hamiltonian includes two parts. The first, considered exactly, includes the single site cubic Fermi coupling term and gives rise to the formation of bound states. The second part, treated as a perturbation, includes cubic and quartic terms coupling the internal to the lattice phonons. These terms give rise to depopulation and dephasing processes of the bound states. Explicit expressions are obtained for the cubic and quartic contributions to the linewidths and frequency shifts. Model calculations are reported for a linear molecular crystal. On the basis of the theory and model calculations of the present paper, available experimental data on the CO2, N2O, CS2, NH4Cl, KClO4, and RbClO4 crystals are reviewed.