Vibrational dephasing in molecular mixed crystals: a picosecond time domain CARS study of pentacene in naphthalene and benzoic acid

Multiresonant time-domain coherent anti-Stokes Raman scattering (CARS) experiments have been employed in a study of the decay of vibrational coherences of pentacene doped into naphthalene and benzoic acid. In all cases, the CARS decay is found to be exponential, which indicates that the electronic and vibronic inhomogeneities in this system are strongly correlated. The temperature dependence of vibrational dephasing shows no effect of coupling to the lowest-frequency librational mode of pentacene that is known to dominate electronic dephasing. This surprising result can be understood on basis of a dephasing model where rapid coherence exchange exists between a cold vibrational transition and a corresponding near-resonant librationally hot one. For the 767 cm–1 vibrational transition, oscillations of the CARS signal as a function of delay are shown to arise from interference at the detector with a nearby naphthalene host signal. An inconsistency with a previously reported spontaneous Raman study is resolved by showing that the signal observed there is actually site-selected fluorescence.