9,9 '-Bianthryl and its van der Waals complexes studied by rotational coherence spectroscopy: Structure and excited state dynamics

The structure and excited state dynamics of jet-cooled 9,9'-bianthryl (BA) and its 1:1 van der Waals (vdW) complexes with Ne, Ar, and H2O were studied using rotational coherence spectroscopy (RCS). For a free BA molecule, the magnitude and persistence of the recurrent transient appearing in the time -correlated single photon counting (TCSPC) measurement was found to be depe ndent on the torsional level of BA, indicating the rotational constant chan ges with the torsional energy level. The RCS-TCSPC measurement of the BA-Ar and BA-H2O complexes in the S-1 state showed no coherent transients. Howev er, the pump-probe time-resolved fluorescence depletion (TRFD) detected the weak J-type transient. Those facts imply the loss of coherence in the BA v dW complexes due to the excited-state dynamics, which coincides with the an alysis of the laser-induced fluorescence excitation and dispersed fluoresce nce spectra. The structure of the ground-state 1:1 BA complex with Ne, Ar, and H2O was determined based on the RCS transients observed in the TRFD mea surement with the help of a minimum energy structure calculation using atom -atom pairwise potentials. The rapid dephasing in the excited state was dem onstrated by the magic angle TRFD detection near t=0. The dominant dephasin g process for the rare-gas complexes is ascribed to intramolecular vibratio nal energy redistribution (IVR) which is accelerated by significant couplin g between the torsional vibration and the low-lying vdW vibrations. IVR pro cess for the H2O complex accompanies the rapid conversion to the charge-tra nsfer state, which is also responsible for the loss of excited-state cohere nce. (C) 2000 American Institute of Physics. [S0021-9606(00)01534-8].