Vibrational energy transfer and localization in disordered solids by picosecond CARS spectroscopy

The dephasing rate of the three longest lived vibrons of naphthalene-h ₈, (T₁ ∼ 100 ps) is investigated by picosecond CARS in disordered mixed crystals containing naphthalene-rf s. Vibrational energy transfer can either occur preferentially to one acceptor state or fairly equally to a number of acceptors. For two of the vibrons studied, the decay occurs preferentially to the lower energy d₈N vibron of the same normal coordinate. The other vibron studied appears to couple fairly equally to a large number of acceptors since its corresponding d₈N vibration is higher in energy and unavailable for decay. The coupling matrix elements are calculated for the specific decay routes and found to be ∼10 greater than the coupling to other acceptor states. For one of the vibrations, nonexponential decays are observed at high d₈N concentrations indicating a distribution of environments and hence localization. A fit to a Voight line shape gives an estimate of the inhomogeneous distribution of frequencies experienced by this localized vibration.