Analysis of vibrational correlations and couplings in the lowest two singlet states of pentacene by high resolution, fully resonant, coherent four-wave mixing spectroscopy

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Abstract
Coherent, fully resonant four-wave mixing (FRFWM) spectroscopy is used to probe the vibrational and vibronic state of the pentacene S0 and S1 electronic states. High resolution is obtained using mixed molecular crystals at cryogenic temperatures. The FRFWM spectra combined with conventional absorption and fluorescence spectra allow correlation of the S1 fundamentals to the corresponding ground state normal coordinates. Couplings between modes are indicated and may be related to Fermi resonance of the pentacene vibrations by means of host librational phonons.