A strong S1-S2 vibronic coupling effect is observed in the time-resolved coherent anti-Stokes Raman spectroscopy (CARS) spectra originating from the first excited singlet state of diphenylhexatriene. As determined from picosecond CARS measurements, the excited state spectrum appears on a subpicosecond time scale. An extremely high excited state hyperpolarizability |γ|exicted state ≃ 3 × 10-31 is derived from a CARS line shape analysis and is attributed to the increased delocalization after excitation in accordance with semiempirical calculations of bond lengths. We observe two strongly frequency-broadened vibrations being upshifted with respect to the C=C double bond stretching region of the ground state and assign them to the totally symmetric C=C stretching motion of the chain. Both frequencies depend on the solvent polarizability, giving evidence of strong S1-S2 vibronic coupling in the lowest excited singlet states. We discuss a model of S1-S2 vibronic coupling via an asymmetric low frequency mode. According to this model a double-well potential for the respective vibrational coordinate is generated in the first excited singlet state, resulting in two frequencies originating from the same type of vibration.
|Original language||English (US)|
|Number of pages||7|
|Journal||Journal of Physical Chemistry A|
|State||Published - May 11 2000|
ASJC Scopus subject areas
- Physical and Theoretical Chemistry