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 dependent on the torsional level of BA, indicating the rotational constant changes 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. However, the pump-probe time-resolved fluorescence depletion (TRFD) detected the weak J-type transient. Those facts imply the loss of coherence in the BA vdW complexes due to the excited-state dynamics, which coincides with the analysis of the laser-induced fluorescence excitation and dispersed fluorescence 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 measurement with the help of a minimum energy structure calculation using atom-atom pairwise potentials. The rapid dephasing in the excited state was demonstrated by the magic angle TRFD detection near t=0. The dominant dephasing process for the rare-gas complexes is ascribed to intramolecular vibrational energy redistribution (IVR) which is accelerated by significant coupling between the torsional vibration and the low-lying vdW vibrations. IVR process for the H2O complex accompanies the rapid conversion to the charge-transfer state, which is also responsible for the loss of excited-state coherence. (C) 2000 American Institute of Physics. [S0021-9606(00)01534-8].