Evidence of strong vibronic coupling in the first excited singlet state ofdiphenylhexatriene by picosecond CARS spectroscopy

Applying the polarization-sensitive variant of coherent anti-Stokes raman s cattering (CARS) with picosecond time resolution solvent-dependent vibratio nal shifts and depolarization ratios of vibrational resonances of the first excited singlet state of diphenylhexatriene were investigated. The depolar ization ratios of all observed Raman lines (rho(R) similar to 0.72) near th e transient absorption of the excited state show the dominance of anisotrop ic scattering originating from vibrations of the same symmetry species. Vib rational frequencies were determined under polarization conditions avoiding spectral interference with the electronic background. Frequencies of the t wo broad C=C stretching bands in the spectral range above 1600 cm(-1) incre ase with decreasing polarizability of the solvent indicating strong vibroni c coupling in the first excited singlet state. (C) 1999 Elsevier Science B. V. All rights reserved.