Spectroscopy and relaxation dynamics of photoexcited anisole and anisole-d3 molecules in a supersonic jet

The vibronic structures of the S1-S0 electronic transitions of jet-cooled anisole and anisole-d3 molecules have been investigated in detail using the laser induced fluorescence and single vibrational level dispersed fluorescence (DF) spectroscopy. Normal mode frequencies of the ground and excited states including the methyl and methoxy internal rotations were determined by analyses of dispersed fluorescence spectra and molecular orbital calculations. Strong vibrational mixing in the S1 state was observed in several DF spectra. Duschinsky rotation between 6a and b modes prominently appeared for both molecules and it was found that methyl deuteration depressed the second-order vibronic coupling of these modes. Another explicit Duschinsky rotation was found in 10b and 16a modes of both molecules. However, in the case of the deuterated molecule the mixing cannot be explained by only Duschinsky rotation. A Fermi resonance due to level proximity should be involved in the mixing scheme. Vibronic bands in the higher frequency regions exhibit broadened and structureless fluorescence due to intramolecular vibrational energy redistribution (IVR). The onset of the IVR process inferred from the DF spectra was found to be increased by methyl deuteration on the contrary to general propensity rule. The deuteration effect is characteristic of anisole molecules and indicates considerable decrease in the interaction with the dark bath modes. Fluorescence lifetime measurements suggest the enhancement of intersystem crossing in the levels with out-of-plane vibrational components. The observed broadening of DF spectra corresponded to non-radiative decay rates from levels with the in-plane vibrational modes. This suggests that the energy flow into out-of-plane bath modes through the IVR process should dominate non-radiative rates on initially excited in-plane vibrational levels. Our analysis clarified that the out-of-plane vibrations accompanying the methoxy motion make large contribution to the relaxation dynamics.