Vibrational dynamics of anthracene in liquid solution studied by picosecond IR/UV spectroscopy with polarization resolution

Two-step fluorescence excitation is carried out with tunable pulses in the infrared in the range 3000-3100 cm(-1) and in the near ultraviolet. The anthracene molecules are promoted close to the bottom of the first excited singlet state via excited vibrational levels in the electronic ground state, the symmetry properties and dynamics of which are studied. The polarization dependence of the excitation spectra is studied theoretically and experimentally to determine the relative orientation of the involved transition dipoles. Comparison with conventional absorption spectra yields information on the symmetry and assignment of the vibrational levels, not accessible in previous work without polarization resolution. Evidence is presented for the prominent role of overtone levels, populated by weak IR absorption and also energy redistribution from CH-stretching modes. The polarization dependence is also applied to separate reorientational and vibrational relaxation and to study the vibrational dynamics unambiguously. The time-resolved measurements provide two vibrational time constants with only the longer component depending on solvent. The shorter time constant of approximately 4 ps is suggested to represent intramolecular energy redistribution, corresponding to an estimated temperature increase of the vibrational manifold of anthracene of approximately 180 K. The subsequent slower decay with 8-15 ps may be interpreted as a vibrational cooling process involving energy transfer to solvent molecules.